agricultural technology

Introduction

      application of techniques to control the growth and harvesting of animal and vegetable products.

Soil preparation
      Mechanical processing of soil so that it is in the proper physical condition for planting is usually referred to as tilling; adding nutrients and trace elements is called fertilizing. Both processes are important in agricultural operations.

Tilling
       tillage is the manipulation of the soil into a desired condition by mechanical means; tools are employed to achieve some desired effect (such as pulverization, cutting, or movement). Soil is tilled to change its structure, to kill weeds, and to manage crop residues. Soil-structure modification is often necessary to facilitate the intake, storage, and transmission of water and to provide a good environment for seeds and roots. Elimination of weeds is important, because they compete for water, nutrients, and light. Crop residues on the surface must be managed in order to provide conditions suitable for seeding and cultivating a crop.

      Generally speaking, if the size of the soil aggregates or particles is satisfactory, preparation of the seedbed will consist only of removing weeds and the management of residues. Unfortunately, the practices associated with planting, cultivating, and harvesting usually cause destruction of soil structure. This leaves preparation of the seedbed as the best opportunity to create desirable structure, in which large and stable pores extend from the soil surface to the water table or drains, ensuring rapid infiltration and drainage of excess or free water and promoting aeration of the subsoil. When these large pores are interspersed with small ones, the soil will retain and store moisture also.

      Seedbed-preparation procedures depend on soil texture and the desired change in size of aggregates. In soils of coarse texture, tillage will increase aggregate size, provided it is done when only the small pores are just filled with water; tillage at other than this ideal moisture will make for smaller aggregates. By contrast, fine-textured soils form clods; these require breakage into smaller units by weathering or by machines. If too wet or too dry, the power requirements for shattering dry clods or cutting wet ones are prohibitive when using tillage alone. Thus, the farmer usually attempts tillage of such soils only after a slow rain has moistened the clods and made them friable.

      Some soils require deepening of the root zone to permit increased rate of water intake and improved storage. Unfavourable aeration in zones of poor drainage also limits root development and inhibits use of water in the subsoil.

      Tillage, particularly conventional plowing, may create a hardpan, or plow sole; that is, a compacted layer just below the zone disturbed by tillage. Such layers are more prevalent with increasing levels of mechanization; they reduce crop yields and must be shattered, allowing water to be stored in and below the shattered zone for later crops.

Primary tillage equipment (farm machinery)
      Equipment used to break and loosen soil for a depth of six to 36 inches (15 to 90 centimetres) may be called primary tillage equipment. It includes moldboard, disk, rotary, chisel, and subsoil plows (plow).

      The moldboard plow is adapted to the breaking of many soil types. It is well suited for turning under and covering crop residues. There are hundreds of different designs, each intended to function best in performing certain tasks in specified soils. The part that breaks the soil is called the bottom or base; it is composed of the share, the landside, and the moldboard.

      When a bottom turns the soil, it cuts a trench, or furrow, throwing to one side a ribbon of soil that is called the furrow slice. When plowing is started in the middle of a strip of land, a furrow is plowed across the field; on the return trip, a furrow slice is lapped over the first slice. This leaves a slightly higher ridge than the second, third, and other slices. The ridge is called a back furrow. When two strips of land are finished, the last furrows cut leave a trench about twice the width of one bottom, called a dead furrow. When land is broken by continuous lapping of furrows, it is called flat broken. If land is broken in alternate back furrows and dead furrows, it is said to be bedded or listed.

      Different soils require different-shaped moldboards in order to give the same degree of pulverization of the soil. Thus, moldboards are divided into several different classes, including stubble, general-purpose, general-purpose for clay and stiff-sod soil, slat, blackland, and chilled general-purpose. The blackland bottom is used, for example, in areas in which the soil does not scour easily; that is, where the soil does not leave the surface of the emerging plow clean and polished.

      The share is the cutting edge of the moldboard plow. Its configuration is related to soil type, particularly in the down suction, or concavity, of its lower surface. Generally, three degrees of down suction are recognized: regular for light soil, deep for ordinary dry soil, and double-deep for clay and gravelly soils. In addition, the share has horizontal suction, which is the amount its point is bent out of line with the landside. Down suction causes the plow to penetrate to proper depth when pulled forward, while horizontal suction causes the plow to create the desired width of furrow.

      Moldboard-plow bottom sizes refer to width between share wing and the landside. Tractor-plow (tractor) sizes generally range from 10 to 18 inches (25 to 45 centimetres), although larger, special-purpose types exist.

      On modern mechanized farms, plow bottoms are connected to tractors either as trailing implements or integrally. One or more bottoms may be so attached. They are found paired right and left occasionally (two-way), with the advantage of throwing the furrow slice in a constant direction as the turns are made. A variation is the middlebreaker, or lister, which is a bottom equipped with both right- and left-handed moldboards.

      The disk plow employs round, concave disks of hardened steel, sharpened and sometimes serrated on the edge, with diameters ranging from 20 to 38 inches (50 to 95 centimetres). It reduces friction by making a rolling bottom in place of a sliding one. Its draft is about the same as that of the moldboard plow. The disk plow works to advantage in situations where the moldboard will not, as in sticky nonscouring soils; in fields with a plow sole; in dry, hard ground; in peat soils; and for deep plowing. The disk-plow bottom is usually equipped with a scraper that aids in pulverizing the furrow slice. Disk plows are either trailed or mounted integrally on a tractor.

      The rotary plow's essential feature is a set of knives or tines rotated on a shaft by a power source. The knives chop the soil up and throw it against a hood that covers the knife set. These machines can create good seedbeds, but their high cost and extra power requirement have limited general adoption, except for the small garden tractor.

      The chisel plow is equipped with narrow, double-ended shovels, or chisel points, mounted on long shanks. These points rip through the soil and stir it but do not invert and pulverize as well as the moldboard and disk plows. The chisel plow is often used to loosen hard, dry soils prior to using regular plows; it is also useful for shattering plow sole.

      Subsoil plows are similar in principle but are much larger, since they are used to penetrate soil to depths of 20 to 36 inches (50 to 90 centimetres). Tractors of 60 to 85 horsepower are required to pull a single subsoil point through a hard soil at a depth of 36 inches. These plows are sometimes equipped with a torpedo-shaped attachment for making subsurface drainage channels.

Secondary tillage
      Secondary tillage, to improve the seedbed by increased soil pulverization, to conserve moisture through destruction of weeds, and to cut up crop residues, is accomplished by use of various types of harrows (harrow), rollers (roller), or pulverizers, and tools for mulching and fallowing. Used for stirring the soil at comparatively shallow depths, secondary-tillage equipment is generally employed after the deeper primary-tillage operations; some primary tillage tools, however, are usable for secondary tillage. There are five principal types of harrows: the disk, the spike-tooth, the spring-tooth, the rotary cross-harrow, and the soil surgeon. Rollers, or pulverizers, with V-shaped wheels make a firm and continuous seedbed while crushing clods. These tools often are combined with each other.

      When moisture is scarce and control of wind and water erosion necessary, tillage is sometimes carried out in such a way that crop residues are left on the surface. This system is called trash farming, stubble mulch, or subsurface tillage. Principal equipment for subsurface tillage consists of sweeps and rod weeders. Sweeps are V-shaped knives drawn below the surface with cutting planes horizontal. A mounted set of sweeps provided with power lift and depth regulation is often called a field cultivator.

      The typical rod weeder consists of a frame with several plowlike beams, each having a bearing at its point. Rods are extended through the bearings, which revolve slowly under power from a drive wheel. The revolving rod runs a few inches below the surface and pulls up vegetative growth; clearance of the growth from the rod is assisted by its rotation. Rod weeders are sometimes attached to chisel plows.

      Some control of weeds is obtained by tillage that leaves the middles between crop rows loose and cloddy. When a good seedbed is prepared only in the row, the seeded crop can become established ahead of the weeds. Plowing with the moldboard plow buries the weed seeds, retards their sprouting, and tends to reduce the operations needed to control them. If weed infestations become bad, they can be reduced somewhat by undercutting.

      Since rainfall amount and distribution seldom match crop needs, farmers usually prefer tillage methods that encourage soil-moisture storage at times when crops are not growing. From the soil-moisture standpoint, any tillage practice that does not control weeds and result in greater moisture intake and retention during the storage period is probably unnecessary or undesirable.

      The use of cropping systems with minimal tillage is usually desirable, because intensive tillage tends to break down soil structure. Techniques such as mulching also help prevent raindrops from injuring the surface structure. Excessive tillage leaves the soil susceptible to crusting, impedes water intake, increases runoff, and thus reduces water storage for crop use. Intensive vegetable production in warm climates where three crops per year may be grown on the same land may reduce the soil to a single-grain structure that facilitates surface cementation and poor aeration.

      The loosening and granulating actions of plowing may improve soil structure if the plowing is done when the moisture content is optimum; if not so timed, however, plowing can create unfavourable structure. The lifting and inversion of the furrow slice likewise may not always be desirable, because in many cases it is better to leave a trashy surface.

      The concept of minimum tillage has received much attention. One type of minimum tillage consists in seeding small grain in sod that has been relatively undisturbed. Narrow slits are cut in the sod and seed and fertilizer placed in the breaks thus formed. Soil normally subject to erosion can be planted to grain this way while still retaining the erosion resistance of the sod. The technique has been successful in preparing winter grazing in southeastern portions of the United States. In another type of minimum tillage, the land is broken and planted without further tillage in seedbed preparation. One approach involves breaking the land and planting seeds in the tractor tracks (wheel-track planting); the tractor weight crushes clods and leaves the seed surrounded by firm soil. Another method consists of mounting a planter behind the plow, thus planting without further traffic and leaving a loose seedbed that is satisfactory in areas where postplanting rains may be heavy. In some areas, where winter rain often comes after wheat is drilled, a rotation of wheat following peas has been successful. After the peas have been harvested, the field is rough plowed, and fall wheat is then drilled in directly. All these methods minimize expense and land preparation, tending to leave the soil rough, which reduces erosion and increases water intake. Somewhat similar systems are employed with row crops, where chemical weed control assists in reducing need for cultivation.

Mulch tillage
      Mulch tillage has been mentioned already; in this system, crop residues are left on the surface, and subsurface tillage leaves them relatively undisturbed. In dryland areas, a maximum amount of mulch is left on the surface; in more humid regions, however, some of the mulch is buried. Planting is accomplished with disk openers that go through several inches of mulch. Since mulch decomposition may deprive the crop of nitrogen, extra fertilizer is often placed below the mulch in humid areas. In rainy sections, intercropping extends the protection against erosion provided by mulches. Intercrops are typically small grains or sod crops such as alfalfa or clover grown between the rows of a field crop that reach maturity shortly after the field crop has been established and furnish mulch cover for a long time.

      If growth of the intercrop competes with the main crop for moisture and nutrients, that growth may be killed at seeding time or soon thereafter by undercutting with sweeps.

      Tillage in dry areas must make maximum use of scanty rainfall. The lister (double-mold board) plow, or middlebreaker, is here used to make water-impounding ridges that promote infiltration. The special problems of dryland farming will be considered below (see Regional variations in technique: Dryland farming (agricultural technology)).

Fertilizing and conditioning the soil
      Soil fertility is the quality of a soil that enables it to provide compounds in adequate amounts and proper balance to promote growth of plants when other factors (such as light, moisture, temperature, and soil structure) are favourable. Where fertility of a soil is not good, natural or manufactured materials may be added to supply the needed plant nutrients; these are called fertilizers (fertilizer), although the term is generally applied to largely inorganic materials other than lime or gypsum. Fertilizer grade is a conventional expression that indicates the percentage of plant nutrients in a fertilizer; thus, a 10–20–10 grade contains 10 percent nitrogen, 20 percent phosphoric oxide, and 10 percent potash. The green plant, however, requires more nutrients (nutrient) than these.

Essential plant (plant development) nutrients
      In total, the plant has need of at least 16 elements (chemical element), of which the most important are carbon, hydrogen, oxygen, nitrogen, phosphorus, sulfur, potassium, calcium, and magnesium.

      The plant obtains carbon and hydrogen dioxide from the atmosphere; other nutrients are taken up from the soil. Although the plant contains sodium, iodine, and cobalt, these are apparently not essential. This is also true of silicon and aluminum.

      Overall chemical analyses indicate that the total supply of nutrients in soils is usually high in comparison with the requirements of crop plants. Much of this potential supply, however, is bound tightly in forms that are not released to crops fast enough to give satisfactory growth. Because of this, the farmer is interested in measuring the available nutrient supply as contrasted to the total quantities. This point will be considered later.

      The solid content of soils is broadly classified as organic and inorganic. Materials of organic origin range from fresh plant tissue to the more or less stable black or brown degradation product (humus) formed by biological decay. The organic matter is a potential source of nitrogen, phosphorus, and sulfur; it contains more than 95 percent of the total nitrogen, 5 to 60 percent of the total phosphorus, and 10 to 80 percent of the total sulfur. These three elements are cycled through the entire environment of living things (the biosphere). The soil organic matter can be considered as one of the storage points in these cycles. Where nonlegumes are grown in the absence of fertilizer or manures, the crop must gain its nitrogen supply from the organic matter; only a part, however, of the needed phosphorus and sulfur is so supplied.

      The inorganic or mineral fraction, which comprises the bulk of most soils, is derived from rocks and their degradation products. The power to supply plant nutrients is much greater in the larger particles, sand and silt, than in the fine particles, or clay. The minerals that comprise the sand and silt in soil contain most of the elements essential for plant growth as a part of their structure. The difficulty is that these minerals decompose so slowly in soil that the rate of supply of the nutrient elements is usually insufficient for good growth of plants.

      When the available supply of a given nutrient becomes depleted, its absence becomes a limiting factor in plant growth, and the addition of this nutrient to the soil will increase yields of dry matter. Excessive quantities of some nutrients may cause decrease in yield, however.

Determining nutrient needs
      Determination of a crop's nutrient needs is an essential aspect of fertilizer technology. The appearance of a growing crop may indicate need of fertilizer; in some plants, however, the need for more or different nutrients may not be easily observable. If such a problem exists, its nature must be diagnosed, the degree of deficiency must be determined, and the amount and kind of fertilizer needed for a given yield must be found. There is no substitute for detailed examination of plants and soil conditions in the field, followed by simple fertilizer tests, quick tests of plant tissues, and analysis of soils and plants.

      Sometimes plants show symptoms of poor nutrition. chlorosis (general yellow or pale-green colour), for example, indicates lack of sulfur and nitrogen. Iron deficiency produces white or pale-yellow tissue. Symptoms can be misinterpreted, however. Plant disease can produce appearances resembling mineral deficiency, as can various organisms. Drought or improper cultivation or fertilizer application each may create deficiency symptoms.

      After field diagnosis, the conclusions may be confirmed by experiments in a greenhouse or by making strip tests in the field. In strip tests, the fertilizer elements suspected of being deficient are added, singly or in combination, and the resulting plant growth observed. Next, it is necessary to determine the extent of the deficiency.

      An experiment in the field can be conducted by adding nutrients to the crop at various rates. The resulting response of yield in relation to amount of nutrient supplied will indicate the supplying power of the unfertilized soil in terms of bushels or tons of produce. If the increase in yield is large, this practice will show that the soil has too little of a given nutrient. Such field experiments may not be practical, because they can cost too much in time and money. Soil-testing laboratories are available in most areas; they conduct chemical soil tests to estimate the availability of nutrients. Commercial soil-testing kits give results that may be very inaccurate, depending on techniques and interpretation. Actually, the most accurate system consists of laboratory analysis of the nutrient content of plant parts, such as the leaf. The results, when correlated with yield response to fertilizer application in field experiments, can give the best estimate of deficiency. Further development of remote sensing techniques, such as infrared photography, are under study and may ultimately become the most valuable technique for such estimates.

The economics of fertilizers
      The practical goal is to determine how much nutrient material to add. Since the farmer wants to know how much profit to expect if he buys extra fertilizer, the tests are interpreted as an estimation of increased crop production that will result from nutrient additions. Cost of nutrients must be balanced against value of crop or even against alternative procedures, such as investing the money in something else with a greater potential return. The law of diminishing returns is well exemplified in fertilizer technology. Past a certain point, equal inputs of chemicals produce less and less yield increase. The goal of the farmer is to use fertilizer in such a way that the most profitable rate is employed.

      Fertilizers can aid in making profitable changes in farming. Operators can reduce costs per unit of production and increase the margin of return over total cost by increasing rates of application of fertilizer on principal cash and feed crops. They are then in a position to invest in soil conservation and other improvements that are needed when shifting acreage from surplus crops to other uses.

      Among sources of organic matter and plant nutrients, farm manure has been of major importance in past years. Manure is understood to mean the refuse from stables and barnyards, including both excreta and straw or other bedding material, while the term fertilizer refers to chemicals. Large amounts of manure are produced by livestock; such manure has value in maintaining and improving soil because of the plant nutrients, humus, and organic substances contained in it.

      As manure must be managed carefully in order to derive the most benefit from it, some farmers may be unwilling to expend the necessary time and effort. Manure must be carefully stored to minimize loss of nutrients, particularly nitrogen. It must be applied to the right kind of crop at the proper time. Also, additional fertilizer may be needed, such as phosphoric oxide, in order to gain full value of the nitrogen and potash that are contained in manure.

      Manure is fertilizer graded as approximately 0.5–0.25–0.5 (percentages of nitrogen, phosphoric oxide, and potash), with at least two-thirds of the nitrogen in slow-acting forms. Commercial fertilizer equivalent to one ton (900 kilograms) of average manure can be purchased at a fairly low price. Furthermore, the expense of applying 100 pounds (45 kilograms) of 10–5–10 fertilizer is much less than the cost of applying 20 times as much manure. On properly tilled soils, the returns from fertilizer usually will be greater than from an equivalent amount of manure. The application of manure to a crop cannot be controlled as readily as can granulated fertilizer. In general, manure does not provide all the plant nutrients needed and fails to provide any that cannot be supplied by artificial fertilizers. Thus, there is a tendency to discount the value of manure as fertilizer. In underdeveloped countries, however, where artificial fertilizer may be costly or unavailable and where labour is relatively cheap, manure is attractive as a fertilizer.

      The main benefits of manure are indirect. It supplies humus, which improves the soil's physical character by increasing its capacity to absorb and store water, by enhancement of aeration, and by favouring the activities of lower organisms. Manure incorporated into the topsoil will help prevent erosion from heavy rain and slow down evaporation of water from the surface. In effect, the value of manure as a mulching material may be greater than is its value as a source of essential plant nutrients.

Green manuring
      In reasonably humid areas, the practice of green manuring can improve yield and soil qualities. A green-manure crop is grown and plowed under for its beneficial effects, although during its growth it may be grazed. These green crops are usually annuals, either grasses or legumes, whose roots bear nodule bacteria capable of fixing atmospheric nitrogen. Among the advantages of green-manure crops are the addition of nitrogen to the soil, increase in general fertility level, reduction of erosion, improvement of physical condition, and reduction of nutrient loss from leaching. Disadvantages include the chance of not obtaining a satisfactory growth; the possibility that the cost of growing the manure crop may exceed the cost of applying commercial nitrogen; possible increases in disease, insects, and nematodes (parasitic worms); and possible exhaustion of soil moisture by the crop.

      Green-manure crops are usually planted in the fall and turned under in the spring before the summer crop is sown. Their value as a source of nitrogen, particularly that of the legumes, is unquestioned for certain crops such as potatoes, cotton, and corn (maize); for other crops, such as peanuts (groundnuts; themselves legumes), the practice is questionable. Farmers are gradually turning away from growing green-manure crops except where the crop may also serve as winter cover for the land.

      Compost, peat, and sludge are used in agriculture and gardening as soil amendments rather than as fertilizers, because they have a low content of plant nutrients. They may be incorporated into the soil or mulched on the surface. Heavy rates of application are common.

      Compost, or synthetic manure, is basically a mass of rotted organic matter made from waste-plant residues. Addition of nitrogen during decomposition is usually advisable. The result is a crumbly material that when added to soil does not compete with the crop for nitrogen. When properly prepared, it is free of obnoxious odours. Composts commonly contain about 2 percent nitrogen, 0.5 to 1 percent phosphorus, and about 2 percent potassium; if phosphate and potash are added while composting, those values are higher. The nitrogen of compost becomes available slowly and never approaches that available from inorganic sources. This slow release of nitrogen reduces leaching and extends availability over the whole growing season. Composts are essentially fertilizers with low nutrient content, which explains why large amounts are applied. The maximum benefits of composts on soil structure (better aggregation, pore spacing, and water storage) and on crop yield usually occur after several years of use.

      In practical farming, the use of composted plant residues must be compared to the use of fresh residues. More beneficial soil effects usually accrue with less labour by simply turning under fresh residues; also, since one-half the organic matter is lost in composting, fresh residues applied at the same rate will cover twice the area that composted residues would cover. In areas where commercial fertilizers are expensive, labour is cheap, and implements are simple, however, composting meets the needs and is a logical practice.

       peat, composed of prehistoric plant remains that have accumulated under airless conditions in bogs, is a widely used organic soil amendment. peat moss, the remains of sphagnum plants, is probably its most common form; it contains less than 1 percent nitrogen, with phosphorus (pH) and potassium below 0.1 percent. It is highly acid, with pH between 3 and 4.5 (a pH value of 7 is neutral and one above 7 basic). Peat improves the water-storage capability of soils and gives better structure to fine soils. Heavy applications of peat is usually the practice. It is used mostly by specialty-crop producers and on lawns and gardens.

      Sewage sludge is the solid material remaining from the treatment of sewage. Its value for soil improvement depends on the method used for treating the sewage. Activated sludge (activated-sludge method), which results from aerobic (oxygen) treatment, contains 5 to 6 percent nitrogen and 1 to 3.5 percent of phosphorus. After suitable processing, it is sold as fertilizer and soil amendment for use on lawns, parks, and golf courses. It is rarely used in farming.

Liming
      Liming to reduce soil acidity is practiced extensively in humid areas where rainfall leaches calcium and magnesium from the soil, thus creating an acid condition. Calcium and magnesium are major plant nutrients supplied by liming materials. Ground limestone is widely used for this purpose; its active agent, calcium carbonate, reacts with the soil to reduce its acidity. The calcium is then available for plant use. The typical limestones, especially dolomitic, contain magnesium carbonate as well, thus also supplying magnesium to the plant.

      Another liming material is basic slag, a by-product of steel manufacture; its active ingredient is calcium silicate. Marl and chalk are soft, impure forms of limestone and are sometimes used as liming materials, as are oyster shells. Calcium sulfate (gypsum) and calcium chloride, however, are unsuitable for liming, for, although their calcium is readily soluble, they leave behind a residue that is harmful.

      Lime is applied by mixing it uniformly with the surface layer of the soil. It may be applied at any time of the year on land plowed for spring crops or winter grain or on permanent pasture. After application, plowing, disking, or harrowing will mix it with the soil. Such tillage is usually necessary, because calcium migrates slowly downward in most soils. Lime is usually applied by trucks specially equipped and owned by custom operators.

Methods of application
      Fertilizers may be added to soil in solid, liquid, or gaseous forms, the choice depending on many factors. Generally, the farmer tries to obtain satisfactory yield at minimum cost in money and labour.

      Manure can be applied as a liquid or a solid. When accumulated as a liquid from livestock areas, it may be stored in tanks until needed and then pumped into a distributing machine or into a sprinkler irrigation system. The method reduces labour, but the noxious odours are objectionable. The solid-manure spreader conveys the material to the field, shreds it, and spreads it uniformly over the land. The process can be carried out during convenient times, including winter, but rarely when the crop is growing.

      Application of granulated or pelleted solid fertilizer has been aided by improved equipment design. Such devices, depending on design, can deposit fertilizer at the time of planting, side-dress a growing crop, or broadcast the material. Fertilizer attachments are available for most tractor-mounted planters and cultivators and for grain drills and some types of plows. They deposit fertilizer with the seed when planted, without damage to the seed, yet the nutrient is readily available during early growth. Placement of the fertilizer varies according to the types of crops; some crops require banding above the seed, while others are more successful when the fertilizer band is below the seed.

      The use of liquid and ammonia fertilizers is growing, particularly of anhydrous ammonia, which is handled as a liquid under pressure but changes to gas when released to atmospheric pressure. Anhydrous ammonia, however, is highly corrosive, inflammable, and rather dangerous if not handled properly; thus, application equipment is quite specialized. Typically, the applicator is a chisel-shaped blade with a pipe mounted on its rear side to conduct the ammonia five to six inches (13 to 15 centimetres) below the surface. Pipes are fed from a pressure tank mounted above. Mixed liquid fertilizers containing nitrogen, phosphorus, and potassium may be applied directly to the surface—by field sprayers where close-growing crops are raised. Large areas can be covered rapidly by use of aircraft, which can distribute both liquid and dry fertilizer.

The future for fertilizers
      Future trends in fertilizer technology may be predicted by extrapolating from current developments. Mixtures and materials with high percentages of plant nutrients will dominate the field. Better ways of providing nitrogen, the most expensive of the three major nutrients, will be forthcoming, including increased use of anhydrous ammonia, ammonium nitrate, and urea. Nonleachable nitrogen, for example, can be obtained through the urea–formaldehyde (ureaform) reaction, and ammonium metaphosphate offers a concentrated liquid product. Micronutrients, or trace elements, specific to particular geographical areas will come into increasing use, as will custom mixing and bulk selling of mixtures containing several nutrients based on reliable soil and plant data.

      “Complete environment” seeding in which seed, fertilizer, and water are incorporated in a biodegradable (decomposable in the soil) tape may come into use; with the tape planted, no further fertilizer or water will be needed until growth is well established. Such techniques using biodegradable tapes have already been developed on a small scale for use by home gardeners. Finally, larger and more precise fertilizing machines will be developed and adopted.

Factors in cropping

Cropping systems
      The kind and sequence of crops grown over a period of time on a given area of soil can be described as the cropping system. It may be a pattern of regular rotation of different crops or one of growing only one crop year after year on the same area.

      Early agricultural experiments showed the value of crop rotations that included a legume sod crop in the regular sequence. Such a system generally maintains productivity, aids in keeping soil structure favourable, and tends to reduce erosion. Alfalfa, sweet clover, red clover, and Ladino clover are considered effective for building up nitrogen. Some legumes, however, do not leave nitrogen behind in the soil because it is deposited as protein in the harvested seed; soybeans are an example. Turning under the top growth of a legume aids in adding nitrogen. Though yields of grains are higher when they are rotated with legumes, it is difficult to determine how much of the improvement depends on the nitrogen added by the legume and how much on improved soil structure or fewer insects and disease.

      The determination of the best rotation depends upon whether the crops compete with each other (i.e., if growing one crop lowers the yield of its successor) or complement each other; and the output of one crop on a given acreage leads to increased output of the other. This desirable complementary relationship exists only when one crop or soil-management practice concurrent with it provides nutrient or conditions required by the other crop. In this circumstance, grasses and legumes may complement grains or row crops by furnishing nitrogen, controlling erosion and pests, and improving soil structure to such an extent that greater production is achieved. The reverse can also occur; in certain prairie soils, continuous growing of deep-rooted legumes depletes soil moisture, and subsequent forage yield is improved by frequent plowing of the sod and planting of corn. In high-rainfall or irrigated areas, forage stands deteriorate from winter killing, disease, or grazing, to a point where a year of grain in the rotation allows an improved stand of forage later. Fallow (idle) land is complementary to wheat and other small grains in subhumid areas such as the Great Plains of the United States; such rotation is quite beneficial to wheat yield. Complementary relationships between crops can be terminated by the application of the physical law of diminishing returns, however, and give way to competition.

      Both long-range and short-range profits motivate the farmer as cropping systems are examined in relationship to soil erosion. Excessive loss of soil to streams, rivers, and reservoirs is unacceptable to public policy as well as economically damaging to the farmer, and crop rotations that promote erosion are minimized. Soil losses are least from fields in continuous sod and most from continuous row crops. If row crops are grown in rotation with sod, the erosive susceptibility of row crops is reduced over a period of time. Peanuts (groundnuts), potatoes, tobacco, cotton, sugar beets, and some vegetables, and similar row crops that require frequent cultivation (intertillage) and leave minimal post-harvest residue are most likely to permit serious erosion. Less erosive are row crops such as corn (maize), sugarcane, and grain sorghum, which require less cultivation and leave more residue. Small grains such as wheat, oats, barley, and rye usually permit less erosion than the row crops. Among sod crops, grasses or grass–legume mixtures are less erosive than pure stands of legumes such as alfalfa. Fortunately, cropping systems that tend to control soil erosion usually tend also to give better yields than systems that promote excessive erosion. This results from increased availability of water to the plants and increased amounts of nutrients, which in erosive systems are washed away and lost.

      The practice of growing the same crop each year on a given acreage, monoculture, has not been generally successful in the past, because nonlegume crops usually exhaust the nitrogen in the soil, with a resulting reduction in yields; this is particularly true in humid regions. The advent of low-cost nitrogen fertilizers, however, has induced reconsideration of the possible advantages of monoculture. These advantages can best be discussed in terms of a hypothetical general farm where it may be desirable to produce several different kinds of crops: the question to be answered is whether monoculture can do better than rotational systems in producing these crops while still maintaining productivity.

Advantages of monoculture
      First, if different kinds of soil exist on the farm, a monoculture system may permit each crop to be grown on the soil best suited to it. Forage crops, for example, could be confined to steep land to minimize erosion; intertilled crops could be planted on the better soils with gentle slopes. Wet areas could be used continuously for crops not requiring early-spring field operations, while dry soils could be used for drought-resistant crops such as sorghums or winter small grains.

      Second, the fertility level of the soil can be adjusted to fit one crop more precisely than it can be adjusted to fit all the crops in a rotation.

      Third, where continuous cropping is practiced and perennial forage crops are used, regular reseedings are avoided. This is an advantage, because each seeding is accompanied by the possibility of failure.

      Fourth, systems based on monoculture usually offer greater flexibility in planning the system to meet year to year changes in the need for various crops. Part of the acreage can be shifted from one crop to another without upsetting the total farm cropping plan.

Disadvantages of monoculture
      On the other hand, requirements for successful monoculture are more demanding of management skill than are sod-based rotations. The entire nitrogen need of nonlegume crops must be met by purchased fertilizers or by use of manure. Closer attention to soil erosion is necessary, except for perennial sod. Soil-structure problems can become severe where intertilled crops are grown continuously. In monoculture, the farmer is completely dependent on chemical insecticides, disease-resistant plant varieties, soil fumigation, and similar methods of controlling insects and diseases that are usually controlled by crop rotation.

      Thus, the choices of cropping systems that maintain good productivity, minimize soil losses, and are in harmony with demand and desired business organization are not easily made. The growing use of systems analysis will undoubtedly aid in rational selection among the bewildering array of possibilities.

Crop protection
      Crops are vulnerable to attack, damage, and competition. Insects (insect), plant disease, nematodes, rodents, weeds, and air pollution are among the many enemies that can reduce crop yields and deny man the use of some of his farm-stored crops.

      Insects, for example, can destroy a crop in a relatively short time. Control measures for many years have engaged the attention of farmer and scientist, yet full success has not been achieved, and the battle continues. The problem is further complicated by the fact that control measures not only kill unwanted insects, but also may harm honey bees as well as the parasites and predators that destroy insect pests.

      At least 10,000 species of insects are classed as unwanted. Of these, several hundred species are particularly destructive and require some degree of control. They destroy food as well as the forage, pasture, and grain needed to produce livestock; and, in addition, they carry and transmit many diseases of plants and animals.

Chemical control of insects
      Insecticides generally are effective, cheap, and safe if handled correctly; the good derived from them, however, can be partly offset by adverse effects. Chlorinated hydrocarbon insecticides such as DDT, for example, may leave residues toxic to beneficial insects, fish, and other wildlife; the insecticides may be found in meat and milk, or they may persist in the soil. Another problem is that some species of insects build up resistance to chlorinated hydrocarbon, organic phosphate, and carbamate insecticides. These disadvantages can be overcome only by persistent search for new and safer insecticides accompanied by wide use of nonchemical insect control.

      A wide range of organophosphate and carbamate materials is now available. These can be applied to avoid most of the problems related to residues. Malathion and carbaryl, for example, are used to control insects in areas where persistent materials might appear later in meat or milk and can also be applied in areas where fish and wildlife might be affected. Those two chemicals offer a broad range of toxicity to insect pests. Unlike chlorinated hydrocarbons, they can be applied up to within a day or so of harvest without harm to many crops; they are dangerous, however, to those who apply them and must be handled with care.

      Some insecticides are effective in very small amounts. This fact has stimulated development of ultralow-volume technology, where special equipment permits dispersal of low volumes of undiluted chemicals, which offers cost advantages as well as drastic reduction of the chemicals in the environment. For example, six to 16 ounces (170 to 450 grams) per acre of Malathion may be effective against grasshoppers, boll weevil, cereal-leaf beetle (Oulema melanopus), mosquitoes, and the beet leafhopper (Circulifer tenellus). Formulation of chemicals in granules rather than sprays offers some advantages in use and applications; among others, it reduces the amount needed and also lessens the chance of adverse effects on beneficial insects and wildlife.

      Certain insects that attack cotton, vegetables, and forage crops may be controlled by chemicals absorbed by the plant. Called systemics, they are placed with the seed at planting time. The chemical is taken up by the plant, and insects die when they attempt to feed on the leaf or stem. Beneficial insects that do not feed on the plant remain unharmed.

Nonchemical control of insects

Mechanical and cultural controls
      Light traps that give off radiation that attracts insects have been under test for many years. They have been somewhat successful in controlling the codling moth (Carpocapsa pomonella) and the tobacco hornworm (Protoparce sexta).

      Use of reflective aluminum strips, placed like a mulch in vegetable fields, has reduced or prevented aphid attack and thus protected cucumbers, squash, and watermelons from mosaic diseases. This technique may supplant insecticides, which frequently do not kill aphids quickly enough to prevent crop losses from virus transmitted by them.

      For stored products, heat or cold can control many insects that frequent such places. Also, changing the proportions of oxygen, nitrogen, and carbon dioxide in the storage atmosphere can provide control.

      It has been discovered that, if adult Indian-meal moths (Plodia interpunctella) were exposed to certain wavelengths of sound during the egg-laying period, their reproduction was reduced by 75 percent. The sound waves had a similar effect on flour beetles (Tribolium species). Further development is needed, but this method offers potential as a nonchemical control. Other types of physical energy can also kill insects. Light waves, high-frequency electric fields, high-intensity radio frequencies, and gamma radiation have been investigated; some offer promise.

      Certain cultural practices can prevent or reduce insect crop damage. These include destruction of crop residues, deep plowing, crop rotation, use of fertilizers, strip-cropping, irrigation, and scheduled planting operations. Such practices are useful but cannot be relied upon entirely to eliminate severe infestations.

Biological controls (biological control)
      The question of using biological controls has always been of considerable public interest. The control agents include parasites, predators, diseases, protozoa, and nematodes that attack the insect pests. Biological controls cannot replace insecticides entirely, because nature provides for survival of both beneficial and destructive insects. Before the population of a parasite or predator can expand, a high population of the host species must also be present. Sometimes the control agents are far outnumbered by the pest insect. Parasites and predators have furnished good control of the Japanese beetle (Popillia japonica), European corn borer (Pyrausta nubilalis), alfalfa aphid (Therioaphis maculata), alfalfa weevil (Hypera postica), and several others.

      Microbial agents can be used for control. There exist about 1,100 viruses, bacteria, fungi, protozoa, rickettsiae, and nematodes that parasitize insects. Many pathogens are specific to a particular insect but are harmless to man and domestic animals. It is a possibility that insect pathogens can be produced, packaged, distributed, and applied in much the same way as insecticides.

      The ideal solution to insect-control problems is to plant crop varieties that are resistant to attack. The only difficulty is that such varieties are not universally available, and development entails a very long process.

       sterilization of male insects by gamma radiation (gamma ray) and their release into a population of wild insects is a promising approach. It has proved successful in control of screwworms and fruit flies (fruit fly), replacing chemicals in some areas. Chemical attractants, which lure insects into contact with small amounts of insecticide or a sterilant, also offer much promise.

      All aspects of insect control considered, it is possible that “integrated control,” coordinated employment of more than one method, may be the answer. Combining resistant varieties with a systemic insecticide that leaves the parasites and predators unharmed, for example, has been successful in combatting the spotted alfalfa aphid in California. Preliminary reduction of heavy infestation by chemical spray combined with bait, followed by the sterile-insect technique, provides another example of integrated control. Use of sex attractant in light traps, plus special management of postharvest residues, has controlled the tobacco hornworm. Other examples might be cited, but the principal value of such control methods lies in using less insecticide and thus contributing to maintenance of a good environment.

Control of plant diseases (plant disease) and nematodes (nematode)
      Insects, of course, are not the only agents hazardous to crops. Plant diseases and the microscopic worms called nematodes have the potential of creating wholesale destruction of crops, especially those grown in regions of wide weather fluctuation. In fact, these plant pests sometimes limit the kinds and varieties of crops that can be grown. The damage they cause may sometimes be mistaken for that caused by unfavourable weather. Epidemics may destroy crops completely.

      As with insects, control of plant diseases and nematodes covers a broad spectrum of measures: use of chemicals, resistant varieties, quarantine, forecasting and warning, cultural practices, heat treatment, and others. Furthermore, most plant virus diseases are transmitted by insect carriers, so control of insects is linked to control of disease.

      Nematodes and plant disease can at times be controlled fairly well by crop rotation, deep plowing, and burning of stubble and debris that remain after harvest. Though burning destroys aboveground organisms and permits economical control by chemicals, it contributes to air pollution and destroys organic matter. In another technique, propane-gas flame is applied to living plants as well as stubble to kill disease spores. A virus disease of sugarcane is controlled by heating diseased cuttings in hot-air ovens. Stem rot disease of peanuts (groundnuts) can be controlled by plowing under dead plant debris or by planting the seed on a raised bed followed by application of a preemergence weed killer.

      Successful control of epidemic plant disease may depend on prompt application of chemicals before the disease outbreak. Many governments operate plant-disease forecasting and warning services for farmers. The service is based primarily on analysis of temperature, rainfall, humidity, and dew—all factors that can create conditions favourable to disease outbreaks.

Weed control
      Weed control is vital to agriculture, because weeds decrease yields, increase production costs, interfere with harvest, and lower product quality. Weeds also impede irrigation water-flow, interfere with pesticide application, and harbour disease organisms.

      Early methods of weed control included mowing, flooding, cultivating, smothering, burning, and crop rotation. Though these methods are still important, other means are perhaps more typical today, particularly the use of herbicide (plant-killing) chemicals. Another technique is to introduce insects that attack only the unwanted plant and destroy it while leaving the crop plants unharmed.

      The inadequacy of the cultural, mechanical, and biological control systems, however, stimulated the rapid development of chemical usage since World War II. Herbicides (herbicide) have had an impact on crop production, changing many cultural and mechanical agricultural operations.

      Herbicides are formulated as wettable powders, granular materials, emulsions, and solutions. Any of them may be applied as a spot treatment, broadcast, placed in bands, or put directly on a specific plant part. When formulated as solutions or emulsions, the chemical is mixed with water or oil.

      Spraying (spraying and dusting) is the most common method, permitting extremely small amounts to be applied uniformly because of dilution. Sprays can be accurately directed underneath growing plants, and calibration and rate control are easier with spray machines than with granular applicators. Granular formulations have advantages under some conditions, however. The use of herbicides must be integrated into the overall farm program because the optimum date and application rate depend on the crop stage, the weed stage, weather conditions, and other factors.

      Careful use of herbicides in farm production lowers cost, resulting in a more economical product for the consumer. Herbicides cut the costs of raising cotton, for example, by reducing labour requirements for weed control up to 60 percent. Herbicides replace hand labour in growing crops, labour that is no longer available in developed nations at costs the farmer can afford. Machines for chemical application are widely available.

      When used as directed, herbicides are generally safe, not only for the operator but also for wildlife and livestock. The greatest difficulty lies in accidental injury to crop plants resulting from drift and from residues in the soil, particularly if residues enter water courses.

      The future of chemical pesticides and herbicides is under debate by those who manufacture, sell, and use them and by those who are concerned about environmental quality. The value of an assured food and fibre supply at reasonable cost is undeniable, and chemicals contribute much toward this. These substances also cause undesirable effects upon the environment, however, and indeed can be toxic to a wide range of organisms. This fact will demand an increasing amount of care in using chemicals, perhaps enforced by law, along with increasing use of nonchemical control techniques.

Harvesting and crop processing

Harvesting machinery (farm machinery)
      Harvesting machinery is generally classified by crop: reapers (reaper) for cutting cereal grains and threshers (thresher) for separating the seed from the plant. The more modern combine cuts, threshes, and cleans the grain in one operation. Corn (corn harvester) (maize) harvesting is performed by mechanical corn pickers that snap the ears from the stalk so that only the grain and cobs are harvested. Corn shelling may be done mechanically in the field, after or with picking. Stripper-type cotton harvesters (cotton harvester), which strip the entire plant of both open and unopened bolls, work best late in the season after frost has killed the green vegetative growth. Hay and forage machines include mowers, crushers, windrowers, field choppers, balers, and some machines that press the hay into wafers or pellets.

      Grass, legumes, corn (maize), and other crops are often put into silos to keep them in a succulent and fermented state rather than stored dry as hay. To make silage, the crops must be cut up to permit tight packing in the silo, producing anaerobic fermentation and preventing formation of mold. Almost all silage crops are cut in the field with a forage harvester that cuts and chops the crop immediately or picks up and chops a windrow that has been cut and raked earlier.

      Root crops are harvested with diggers and digger-pickers, which often pull up clods, stones, and vines with the crop. Though some machines carry workers who manually sort out extraneous material, this task is increasingly being performed mechanically. Modern sugar-beet (sugar beet) harvesters lift the whole root from the ground, clean the earth from it, and deliver it to a bin or wagon. Sometimes the beet tops are removed before harvest of the roots and used for cattle feed. Peanuts (groundnuts) are lifted, vines and all, and allowed to dry before removal of the pods.

      Tobacco-harvesting (tobacco) aids may be classified in three principal ways, according to the harvesting and curing methods used, which depend on the type of tobacco and its use. Flue-cured tobacco, a large plant that may stand three to four feet (90 to 120 centimetres) high, is harvested with machines that carry several workers who ride the lower platforms of the machines, cut the leaves, and place them on conveyor belts, where the leaves are tied mechanically or by hand. Burley tobacco has usually been harvested by workers using a machete-type knife. After cutting, the large end of the stalk is fixed onto the sharpened end of a stick, which—when loaded with a number of stalks—is hung by hand in a tobacco barn for curing. Researchers are attempting to mechanize the cutting, impaling, and hanging of burley tobacco. Little has been done, however, toward the mechanization of the harvesting of the small aromatic tobacco leaves, which are grown in the shade, picked by hand, tied with a string, then hung for curing.

      Tree-crop harvesting is accomplished by hand or with mechanical shakers. vegetable crops such as asparagus, lettuce, and cabbage are still harvested largely by hand, though scarcity and high cost of field labour has led to some mechanization in this area, notably with tomatoes.

Crop-processing machinery
      Machinery is widely used to prepare crops for convenient transportation, for safe storage, for the market, and for feeding to livestock. Advances in such machines have been rapid, particularly with new crops, increased yields, multiple-crop practices, and changing techniques.

      In the most common method of crop drying, the crop, usually grain, is spread on floors or mats and stirred frequently while exposed to the sun. Such systems, though extremely common in the underdeveloped countries, are very slow and dependent on the weather. Forced-air-drying systems allow the farmer much more freedom in choosing grain varieties and harvest time. Fairly simple in operation, these systems have been gaining popularity in the tropics. Heat is often added to increase air temperatures during the drying period.

      In a process called dryeration, wet corn (maize) is placed in a batch or continuous dryer. After losing 10 to 12 percent of its moisture, the hot corn is transferred to the dryeration cooling bin, in which it is tempered for six to 10 hours and then slowly cooled by ventilation for 10 hours. This process reduces kernel damage and increases dryer output.

      High moisture in stored hay not only causes rapid deterioration of its value as feed but often results in spontaneous combustion. When hay is first cut, it usually contains 70 percent or more moisture. It wilts and quickly dries to a moisture content of about 40 percent. At this stage, it can be dried to a safe storage condition, about 15 percent moisture, by blowing air through it, sometimes with supplemental heat.

      Feed-processing mills, often referred to as feed grinders, are used principally for milling cereals for livestock feed, which aids digestion. The ground material is usually fairly coarse and at times may only be crushed. Modern mills frequently are designed to allow the farmer to grind the grain and to mix in various other ingredients in desired quantities.

      Other types of crop-processing machinery include machines that separate desirable seed from weed seed, stems and leaves, and dirt; grading machinery to classify seed by width, length, or thickness; fruit graders and separators; and cotton gins, which separate cotton seeds from the fibres.

Regional variations in technique

Dryland farming (dry farming)
      Dryland farming refers to production of crops without irrigation in regions where annual precipitation is less than 20 inches (500 millimetres). Where rainfall is less than 15 inches (400 millimetres) per year, winter wheat is the most favoured crop, although spring wheat is planted in some areas where severe winter killing may occur. (Grain sorghum is another crop grown in these areas.) Where some summer rainfall occurs, dry beans are an important crop. All dryland crop yield is mainly dependent on precipitation, but practices of soil management exert great influence on moisture availability and nutrient supply.

      Where rainfall exceeds 15 inches (380 millimetres), the variety of crop possibilities is increased. In areas of favourable soils and moisture, seed alfalfa is grown, as is barley. Some grass seed may be grown, particularly crested wheat grass of various types.

Fallow system and tillage techniques
      Dryland farming is made possible mainly by the fallow system of farming, a practice dating from ancient times. Basically, the term fallow refers to land that is plowed and tilled but left unseeded during a growing season. The practice of alternating wheat and fallow assumes that by clean cultivation the moisture received during the fallow period is stored for use during the crop season. Available soil nitrogen increases and weeds are controlled during the fallow period. One risk lies in the exposure of soil while fallow, leaving it susceptible to wind and water erosion. Modern power machinery has tended to reduce this risk.

      Procedures and kinds of tillage that are comparatively new have proved effective in controlling erosion and improving water intake. Moldboard and disk plows are being replaced with chisels, sweeps, and other tools that stir and loosen the soil but leave the straw on the surface. Where the amount of straw or residue remaining from the previous crop is not excessive, this trashy fallow system works well, and tillage implements are designed to increase its effectiveness.

      Contour (contour farming) tillage helps to prevent excessive runoff on moderate slopes. Broad terraces can aid in such moisture conservation. Steeper slopes are planted to permanent cover.

      Compacted zones at a depth of five to eight inches (13 to 20 centimetres) can be caused by tillage. As such zones interfere with storage of moisture, they can be controlled by growing deep-rooted alfalfa at intervals, or the compacted zone can be broken by fall tillage with chisels or sweeps set to a depth just below the zone of compaction. Such deep tillage will result in reduced runoff and deeper moisture penetration.

      When using power machinery in dryland farming, the timing of operations is important. The soil is broken in the fall or early spring before weeds or volunteer grain can deplete the moisture. Use of a rod weeder or similar equipment during fallow can control the weeds. Planting is timed to occur during the short period in fall or spring when temperature and moisture are favourable.

      Fertilizer is an important component of dryland technology. For example, 20 pounds per acre (22 kilograms per hectare) of nitrogen are recommended where rainfall is less than 13 inches (330 millimetres), ranging up to 60 pounds per acre (67 kilograms per hectare) where more rain is available; those figures refer to the production of wheat, but they are applicable to other dryland-farming areas. Where average annual precipitation is less than 12 inches (300 millimetres), the use of nitrogen is limited to years where moisture outlook is exceptionally favourable. Nitrogen fertilizer can be applied either in fall or spring. Band placement or broadcast techniques are utilized. Good results are obtained from broadcast spring application of nitrate fertilizer, and fall application of ammonia has also been successful. Local climates and rainfall patterns also determine choice of fertilizer and time of application.

Crops and planting methods
       alfalfa grown for seed on drylands is planted in rows, usually two to three feet (60 to 90 centimetres) apart; cultivation between rows is required during the first year. Alfalfa is also grown for forage where favourable. This practice builds nitrogen and organic matter, while improving soil structure. These legumes can be rotated with wheat if rain is between 16 and 18 inches (400 and 450 millimetres) and will increase the yield of wheat.

      Other crops, such as cotton, peanuts (groundnuts), and grain sorghum, can be grown successfully in dryland agriculture. Where those crops are produced on sandy soils, special techniques are necessary to reduce soil blowing and drifting. Cotton and peanuts do not produce sufficient crop residues for protection from wind erosion, while sorghum does. For this reason, many farmers in such areas use various row combinations of cotton or peanuts with grain sorghum; two rows of grain sorghum and four to eight rows of peanuts in alternating strips is a popular technique. Another is to use a two-year rotation of cotton and grain sorghum, in which two rows are cropped and two rows are fallow. These systems not only afford protection from wind erosion but also promote effective use of soil moisture.

Tropical farming
      The area of the world bounded roughly on the north by the Tropic of Cancer and on the south by the Tropic of Capricorn, a vast land that embraces large parts of Latin America, Africa, India, Australia, and Southeast Asia, contains climates less favourable to agriculture and human settlement than those of the temperate zones. Within this Equator-centred area occur the climates known as tropical, which are characterized by two general types: warm and wet, and warm with partly deficient rainfall. In either, the total precipitation is usually quite heavy, which leaches the tropical soils of nutrients. The area also has high temperatures with little variation the year round. The combination of high temperature and high rainfall causes organic matter to decompose quickly, leaving the soil deficient in humus. Vegetation flourishes in the tropics, along with weeds, insects, and disease organisms. Important climatic variations occur, depending upon land elevation.

      Tropical crops include coconut, palm oil, rice, sugar, pineapple, sisal, cocoa, tea, coffee, jute, rubber, pepper, banana, and many others. In certain highland tropical areas, however, the crops common to temperate-climate agriculture can also be grown. The amount of tropical land well-suited to agriculture, however, is limited.

Plant-pest problems
      The abundance of plant pests in the tropics, including weeds and disease, makes agriculture successful mainly in the plantation system, where needed control measures can be financed. The alternative is to move from deteriorated land to newer fields; this practice of shifting agriculture has also been common, because tropical soils lose their productive capacity so rapidly. The practice probably cannot be continued indefinitely, however, because of increasing population pressure.

      The largest quantities of commercial tropical products originate in plantations, where skilled management is combined with sufficient capital to provide mechanized equipment. This is particularly true in the production of coffee, cocoa, rubber, coconut, banana, pineapple, sugarcane, and others. Much rice is produced in the Asian tropics and Indonesia, however, on small farms with intensive hand labour and simple tools, where the prime mover is likely to be the ox or the water buffalo, not the tractor.

Water management
       drainage, irrigation (irrigation and drainage), and other special techniques of water management are important in tropical agriculture. An example is the cultivation of rice and sugarcane in the fertile coastal areas of Guyana. Originally through private enterprise and later by government efforts, large coastal areas were “empoldered” (diked) to keep back the sea in front and floods from the rivers in the rear. With a mean annual rainfall of 90 inches (2,300 millimetres), drainage is a critical factor; in fact, the system cannot discharge all possible floodwater, and so the crops must tolerate occasional drowning. With gravity drainage effective only at low tide, the drainage gates are opened on the ebbing tide and closed on the rising tide. Great difficulty is encountered in keeping the outlets unclogged by the heavy sediment discharge. Since rain does not always fall when it is needed, many fields are irrigated. Most of the rice soil is specially tilled after plowing in order to create a better seedbed under the water, using tractors operating in water four to six inches (10 to 15 centimetres) deep. After this special tillage, the seeds are broadcast in one to two inches (2.5 to five centimetres) of water. Though maintenance and operation of such an intricate water-control system are not simple, Guyana rice production has been doubled through its use.

Mechanical problems (farm machinery)
      Mechanization faces many obstacles before wide adoption is possible in tropical regions. Difficult soils, stones, stumps, abundant labour, resistance from farmers, lack of incentives, lack of skills, lack of capital, low wages, high cost of machines, lack of dealer service, fragmented land ownership, all contribute to slow development of mechanization. Tropical soils differ markedly from those in the countries that manufacture land-preparation machinery, making adaptation of new design necessary. The encountering of stones, wood, trash, and termite mounds causes machines to break down. Depressing climatic conditions reduce the performance of the machine operators. Tropical farm regions are notoriously irregular or mountainous, impeding intensive machine culture. The best soils in Brazil require special erosion controls, reducing the potential for large-scale mechanization. One of the greatest overall impediments to mechanization is the fear that unemployment might result from it, a failure to understand that economic development and higher living standards depend partly on increasing the productivity of labour.

      As an example of the problems encountered in mechanizing tropical crops, the harvesting experience of a large sugarcane plantation in Trinidad (Trinidad and Tobago) is illuminating. On flatland of some 30,000 acres (12,000 hectares), the cane is grown on heavy clay soil in a climate with 50 inches (1,300 millimetres) of rain during the seven-month wet season and 10 inches during the five-month dry season. By 1960 the rising wage rate made harvest mechanization imperative. First, the traditional “bed” system, which functioned to remove floodwater, was changed to ridge planting; this made it possible for machines to operate and was a remarkable change in itself. Then it was decided to harvest with the cane combine, which tops, cuts, chops, and loads the chopped cane into transport vehicles. Although the combine is complicated and requires considerable power, it was deemed better than mechanical half-measures.

      By 1969 the combines, however, were harvesting only 12.8 percent of the flatland crop, indicating that mechanization was far from complete. Three factors were responsible: first, the cane combines required extensive maintenance plus very expensive replacement parts. Second, it was difficult to mobilize a transport system to receive the output of the combines with any degree of economy. Third, the social problem of displaced workers had to be considered. The combines increased labour productivity sixfold over hand harvesting; thus, their introduction had to be slowed until surplus workers could be accommodated elsewhere. The limited success of this mechanization project indicates how complicated such a process really is.

      Taking the largest view of possibilities for improving tropical agriculture, the most promising inputs of technology are improved crop varieties and increased use of fertilizers.

Other specialized techniques
      The term hydroponics denotes soilless culture of plants. The possibilities of this technique have received considerable attention in recent years. In hydroponics, an outgrowth of laboratory techniques long used by scientists, plants are grown with their roots immersed in a water solution containing necessary minerals or rooted in a sand medium kept moistened by such a solution. Soilless culture of plants is similar in principle but larger in scale. A typical hydroponics technique has plants supported in a bed of peat, wood fibre, or similar material, on a wire screen with the roots dipping into the solution below. Aeration of the solution is provided. In another method, the plants are rooted in a medium of sand or gravel contained in a shallow tank into which the solution is pumped at intervals by automatic control. Between pumpings, the solution drains slowly down into a reservoir tank. Hydroponic techniques are practiced on a small scale both out-of-doors and in greenhouses.

      Of the elements known to be necessary for plant growth, carbon, oxygen, and hydrogen are obtained by the plant from atmospheric gases or from soil water. The others are all obtained as mineral salts from the soil. The elements absorbed as salts—iron, manganese, boron, copper, zinc, and molybdenum—are required in minute quantities and are called the micronutrients. The principal elements that must be provided as dissolved salts in hydroponic techniques are nitrogen, phosphorus, sulfur, potassium, calcium, and magnesium. Numerous solutions have been devised to fulfill these requirements.

      Crop yields of some plants can be obtained fully equal to those obtained on fertile soils. Wide-scale crop production by hydroponics, however, would be economic only for certain intensive types of agriculture or under special conditions. Some greenhouse crops, both vegetables and flowers, are grown by this method. In regions having no soil or extemely infertile soil but with favourable climate, hydroponic techniques have been very useful; for example, on some of the coral islands of the Pacific.

Greenhouses (greenhouse)
      The greenhouse is typically a structure whose roof and sides are transparent or translucent, permitting a sufficient quality and quantity of solar radiation to enter the structure for photosynthesis (see below Photosynthesis (agricultural technology)). It allows the growing of crops independently of the outside climate, since its interior temperature and humidity can be controlled. Greenhouses vary in size and complexity from small home or hobby structures to large commercial units covering an acre or more of land. An even smaller greenhouse might be termed the hot bed, a glass-topped box containing fermenting organic matter; the fermentation process yields heat, allowing the gardener to start plants from seed in early spring for later transplanting.

      The basic construction of a greenhouse consists of a light but sturdy frame capable of resisting winds and other loads. Conventional foundations usually support vertical walls; the roof may be gabled, trussed, or arched. The conventional greenhouse is fitted with glass panes, but plastic-film or fibre-glass panels often supplant glass.

      Maintenance of temperature within the greenhouse (environment) is difficult because of fluctuating outside conditions. When the sun shines brightly, little heat is needed, and the heating system must be controlled in some way to prevent injury to the crop. Hot water, steam, electric cable, or warm-air furnaces provide the heat, which is usually controlled by thermostat. Temperatures in greenhouses are regulated to suit the crop. Typical ranges are from 40° F (4° C) for lettuce, violets, carnations, and sweet peas to 70° F (21° C) for cucumbers, tomatoes, and orchids.

      Cooling is often required during summer days in warm climates. Ventilation is the simplest technique, reducing inside temperature to near that of the outdoors. Additional cooling by refrigeration may be required; in dry regions, the evaporative cooler is efficient and also increases the relative humidity within the structure. Another form of environmental control consists of adding extra carbon dioxide to the air if the crop requires it for extra photosynthetic efficiency.

      The commercial-greenhouse operator usually grows vegetables or ornamental plants. Such production makes more demands on the grower, because he must assume many of the tasks normally handled by nature in the open fields. He must regulate the temperature, ventilate, adjust the amount of entering sunlight, provide soil moisture, fertilize, and even facilitate pollination. During the off-season, the structure must be cleaned and fumigated, its soil restructured, and mechanical equipment checked. Mechanization of greenhouse operations has lagged far behind the pattern of agriculture in general. Disease is a particularly serious hazard in greenhouse farming, requiring constant attention and use of chemicals.

The factor of weather

Weather information
      The interaction of weather and living systems is a basic aspect of agriculture. Although great strides in technology have resulted in massive production increases and improved quality, weather remains an important limiting factor. Though man is not yet able to change the weather (weather forecasting), except on a very small scale, he is capable of adjusting agricultural practices to fit the climate. Thus, weather information is of utmost importance when combined with other factors, such as knowledge of crop or livestock response to weather factors; the farmer's capability to act on alternative decisions based on available weather information; existence of two-way communication by which specific weather forecasts and allied information can be requested and distributed; and the climatic probability of occurrence of influential weather elements and the ability of the meteorologist to predict their occurrence.

Other weather-research benefits
      Apart from the many applications of weather forecasting to current problems, meteorological research may benefit agriculture in at least three other ways: (1) improved planning of widescale land usage depends partly on detailed knowledge of plant-climate interactions; radiation, evapotranspiration, diurnal temperature range, water balance, and other parameters are measured and analyzed before a plan realizing maximum economic benefit for a given area is prepared; (2) agronomic experiments are combined with climatological documentation to obtain the greatest scientific and technological return; (3) problems of irrigation, row spacing, timing of fertilizer application, variety selection, and transplanting can best be solved with the aid of climatic environmental data; cultural practices related to artificial modification of microclimates should be based on research knowledge rather than personal judgment.

Observing climatic elements
      The climatic elements the observation of which is valuable for agricultural purposes can be approached on an idealized threefold scale: (1) microscale observations of small areas for research designed to elucidate basic physical processes; (2) mesoscale climatic networks designed for practicing farmers to improve their operations; and (3) macroscale regional networks intended for weather forecasting and for gathering basic climatic data (see also weather forecasting: Meteorological measurement and weather forecasting (weather forecasting)). Macroscale stations can be further divided into first-order and second-order stations, the number and type of observations different for each. Micrometeorology demands the most elaborate array of measuring devices, while a second-order macroscale station requires the least; in fact, the latter station will measure only five elements: air temperature, rain, snow, humidity, and surface wind. A first-order macrostation will be equipped to measure 16 elements: global radiation, sunshine hours, clouds, net radiation, air temperature, soil temperature, rain, snow, hail, dew, fog, humidity, pan evaporation, pressure, upper air wind, and surface wind. Mesoscale measurements include 10 elements and microscale 27 (three of which are derived from others).

      The World Meteorological Organization and the various national weather services are concerned with establishment and improvement of macroscale regional climatic stations, both first-class and second-class. Spaced at least 10 miles (16 kilometres) apart, their value for daily agricultural operations is limited, but they are useful for long-range planning and forecasting. Most parts of North America, Europe, and Australia have adequate networks of these stations, but wide gaps exist in the tropics, polar regions, and arid lands.

The degree day
      One weather characteristic of agricultural value is the degree day. This concept holds that the growth of a plant is dependent on the total amount of heat to which it is subjected during its lifetime, accumulated as degree days. Common practice is to use 50° F (10° C) as a base. Thus, if the mean daily temperature for a particular day is 60° F (16° C), then 10 degree days are accumulated for that day on the Fahrenheit scale. The total number of growing degree days required for maturity varies with crop variety as well as plant species. Also, the minimum threshold temperature (the temperature below which the plant is damaged or unable to grow) varies with plants; e.g., 40° F (4° C) for peas, 50° F (10° C) for corn (maize), and 55° F (13° C) for citrus fruits. Where studies have established the number of degree days required for maturity of a given crop, the planting dates can be scheduled for orderly harvest and processing. The system is helpful in selecting crop varieties appropriate to different geographical areas; it also has value in scheduling spray programs and predicting insect emergence.

      The growing-degree-day concept has certain weaknesses: (1) it assumes that the relationship between growth and temperature is linear (actually it is not); (2) it makes no allowance for changing threshold temperatures with advancing crop development; (3) too much weight is given to temperatures above 80° F (27° C), which may be detrimental; and (4) no account is taken of the diurnal temperature range, which is often more significant than the mean daily value.

Weather effects
      The essence of the weather–agriculture interaction for the farmer lies in wise adaptation of operations to the local climate and in techniques for manipulating or modifying the local environment ( microclimate) to minimize weather stresses on plants and animals. Many of these techniques have been practiced for centuries: seeding and cultivation, irrigation, frost protection, animal shelters, windbreaks, and others are methods of altering the microclimate. The climatic factors and their relation to plant growth in terms of protective techniques are important.

      Solar radiation is the ultimate source for all physical and biological processes of the earth. Agriculture itself is a strategy for exploitation of solar energy, made possible by water and nutrients. During daytime hours, solar radiation is delivered both directly and by diffused sky reflection. The incoming radiation that is not reflected by the surface or reradiated to outer space is the net radiation, which is the energy available for maintaining the earth's surface temperature. At night the net radiation is negative; that is, energy is lost to outer space by long-wave radiation, and none is gained. The net radiation balance varies widely throughout the world, setting limits on basic agricultural possibilities.

      Photosynthesis is the process by which higher plants manufacture dry matter through the aid of chlorophyll pigment, which uses solar energy to produce carbohydrates out of water and carbon dioxide. The overall efficiency of this critical process is somewhat low, and its mechanics are extremely complex. It is related to light intensity, wavelength, temperature, carbon dioxide concentration in the air, and the respiration rate of the plant. The distribution of solar energy within the plant community is affected by the leaf canopy's density, height, and capacity to transmit the energy; these therefore affect photosynthesis. The leaf-foliage density is characterized by the leaf-area index, the total leaf area of a plant over a given area of land. The optimum leaf-area index will vary between summer and winter and between temperate and tropical regions, but it represents a key factor in the search for better crop management based on improved photosynthesis. The efficiency of radiation utilization by field crops has been measured, showing that an ordinary crop converts less than 1 percent of available solar energy into organic matter.

      Photoperiodism is another attribute of plants that may be changed or manipulated in the microclimate. The length of a day is a photoperiod, and the responses of the plant development to a photoperiod are called photoperiodism. Response to the photoperiod is different for different plants; long-day plants flower only under day lengths longer than 14 hours; in short-day plants, flowering is induced by photoperiods of less than 10 hours; day-neutral plants form buds under any period of illumination. There are exceptions and variations in photoperiodic response; also, it is argued that the truly critical factor is actually the amount of exposure to darkness rather than to daylight. Temperature is intimately related to photoperiodism, tending to modify reactions to daylength. Photoperiodism is one determining factor in natural distribution of plants throughout the world.

      The phenomenon has many practical applications. Selection of a plant or a variety for a given locality requires knowledge of its interaction with the photoclimate. Artificial illumination is used to control flowering seasons and to increase production of greenhouse crops. In plant breeding, such stimulation of flowering has greatly reduced the time span from germination to maturity, shortening the time necessary to develop new varieties. In sowing field crops, photoperiodism can be used to select the date of sowing to produce optimum harvest size. Crop yield is reduced both by planting in a season that will cause plants to flower early and by planting at a time that will cause very late flowering. In Sri Lanka (formerly Ceylon), certain rice varieties with a vegetative period of five to six months may extend their life to more than a year when planted in the wrong season, causing almost complete loss of yield. Cowpeas in Nigeria will flower early and produce many seeds only when planted in daylengths of 12 hours or less.

Weather conditions and controls
      Regardless of how favourable light and moisture conditions may be, plant growth ceases when the air and leaf temperature drops below a certain minimum or exceeds a certain maximum value. Between these limits, there is an optimum temperature at which growth proceeds with greatest rapidity. These three temperature points are the cardinal temperatures for a given plant; the cardinal temperatures are known for most plant species, at least approximately. Cool-season crops (oats, rye, wheat, and barley) have low cardinal temperatures: minimum 32° to 41° F (0° to 5° C), optimum 77° to 88° F (25° το 31° C), and maximum 88° το 99° F (31° to 37° C). For hot-season crops, such as melons and sorghum, the span of cardinal temperatures is much higher. The cardinal temperatures may vary with stage of development. For example, cold treatment near 32° F (0° C) of germinated seeds before sowing can transform winter rye into the spring type; such treatment, called vernalization, has practical application in cold-climate plants.

      The range of diurnal temperature variation is also important; the best net photosynthesis is related to a large diurnal temperature range, or high daytime and low nighttime temperatures. Knowledge of the difference between leaf and air temperatures aids farmers in adopting protective measures. In middle and high latitudes, frost often occurs before the air temperature drops to freezing; in summer, heat injury to plants might be much more serious than that suggested by the air temperature alone. Because of this factor, farmers in Taiwan shade the pineapple fruit to prevent heat damage.

      Soil temperature sometimes is of greater ecological significance to plant life than air temperature. Germination of seed, root function, rate of plant growth, and occurrence and severity of plant diseases all are affected by soil temperature. Since an unfavourable soil temperature during the growing season can retard or ruin a crop, techniques have been developed for modifying the temperature. The two most important methods are (1) regulation of the energy exchange and (2) altering the thermal properties of the ground. Incoming energy can be regulated by an insulation layer on or near the ground surface, such as paper, straw, plastic, or trees; the outgoing radiation can be reduced by insulation materials or by generating smoke or fog in the air. Thermal properties of the ground can be modified by cultivation or irrigation, increasing the soil's ability to absorb radiation, or by varying the rate of evaporation. Mulching is a common technique for soil temperature control. Carbon black or white material can change the soil's ability to absorb radiation. In the Soviet Union, for example, it was reported that 100 pounds of coal dust per acre (112 kilograms per hectare) caused a one-month advance in the maturity date of cotton.

      Another aspect of temperature control is frost protection. Likelihood of damage from freezing temperature depends upon the plant species, the season, the manner of temperature change, the physiological state of the plant, and other factors. Orchards can be located so as to minimize the chances of frost damage.

      Two types of frost are recognized: (1) radiation frost, which occurs on clear nights with little or no wind when the outgoing radiation is excessive and the air temperature is not necessarily at the freezing point, and (2) wind, or advection, frost, which occurs at any time, day or night, regardless of cloud cover, when wind moves air in from cold regions. Both types may occur simultaneously. Most frost-protection techniques can raise the temperature only a few degrees, while some are effective only against radiation frost.

       heating is probably the best known and most effective frost-protection measure. It is most effective on nights with a strong temperature inversion, a condition in which the air temperature increases markedly from the ground up to as high as 40 or 50 feet (12 or 15 metres). The depth of air to be heated is thus rather shallow, and the area over which a given temperature rise can be produced increases linearly with the strength of the inversion. Lacking a temperature inversion, heaters protect by radiating heat to the plants and the ground surface, and by emitting a layer of humid smoke that reduces the net outgoing loss from the ground.

      In general, a large number of small heaters is most effective; large heaters set up convection currents that break up the warm ceiling and draw in cold air. For radiation-frost protection, the heaters are placed in “view” of the plants or trees, but for advective frost the heavier concentration is placed along the upwind border. Common fuels for the heaters include oil, coal, briquettes, and wood. Oil is most effective, because it can be ignited rapidly and extinguished easily. Heating is a costly technique; a few growers who tried it in England soon gave up the practice, and, even in places such as California, heating is becoming less common and is mostly restricted to a few high-value crops such as citrus fruits.

      The wind machine is popular for frost protection; although it affords less reliable results, its operating cost is much lower than that for heaters. These machines, which are like fans or propellers, break up the nocturnal temperature inversion by mechanically mixing the air, returning heat to the ground that was lifted during the day. The stronger the temperature inversion, the more effective is the wind machine, which is ineffective, however, against a daytime freeze or cold soil. Even under the best circumstances, ground-surface temperatures will rise very little; therefore, some operators install both heaters and wind machines, using the latter for strong-inversion nights and the former for wind-frost protection.

      Flooding and sprinkling with water prevent excessive ground cooling by increasing the heat conductivity and heat capacity of the soil and releasing latent heat of fusion, or the heat given off when the water freezes. The temperature of the plant will not fall below the freezing point so long as the change of state from water to ice is taking place. Flooding has the disadvantage of retarding increase in soil warmth during the day; thus, it can be used effectively for only one or two nights. Sprinkling creates water particles in the air that reduce outgoing radiation, but plant temperature declines immediately on cessation of sprinkling, and the ice formation may cause damage to the crop. In general, successful protection by flooding and sprinkling demands much skill and judgment from the operator.

      Brushing is a frost-protection technique in which shields of paper or aluminum foil are set up to reduce radiation loss to the sky; it has been used with fair success for tomato culture in California.

      Massachusetts cranberry growers add a thin layer of sand to the soil periodically. The sandy surface warms up easily and cools slowly by radiation; it also reduces evaporation of its low water content. Sanding can raise the temperature of loam, clay, and organic soils, thus diminishing frost hazard. Windbreaks can also function as frost protection by reducing inflow of cold air and by shielding plants from the total night sky.

      Spraying of harmless foams or gels on plants threatened by frost is a technique under investigation. The trapped air in the foam serves as insulative protection, while the foam can be designed to dissolve after any desired time interval. The technique has been explored for use on strawberries and other low-growing crops.

      Irrigation is probably the most common form of agricultural microclimatic control practiced by man. Also important are efforts to correct deficiencies in precipitation, the deficiencies that lead farmers to irrigate.

      Attempts to increase the amount of precipitation from clouds (cloud seeding) by seeding them with salt or silver iodide have been made for nearly three decades. Both aircraft and ground generators have been employed, but the techniques are typically beyond the means of an individual farmer. Results suggest that cloud modification is entirely possible, but the proof of increased rainfall at a level of statistical significance is a difficult problem. Success has been greatest under atmospheric conditions where natural rainfall is most probable. The prospect of modifying winter clouds to increase snowfall in mountain areas appears to be somewhat more promising, however.

      Most cloud-seeding efforts are expended in regions where precipitation is only marginal for agriculture. It is commonly assumed that at least 20 inches (500 millimetres) of rain per year, fairly well distributed, is required to maintain a stable farming community. Unfortunately, the years of large deficiencies in such areas are those with only limited opportunity for cloud seeding. Some observers believe that weather modification to increase precipitation may yet become practical and economically feasible; the legal, ethical, and ecological problems raised by the prospect will not be easily solved, however.

      The value of high humidity in the greenhouse is well known, but knowledge of humidity–plant interaction under field conditions is comparatively slight. Other things being equal, the evapotranspiration rate decreases with increasing humidity; thus, rate of water use is higher at low levels of humidity. The benefits of irrigation are apparently greater when the humidity is high, which simply means that the efficiency of water use increases with humidity.

      Wind affects plant growth in at least three significant ways: transpiration, carbon dioxide intake, and mechanical breakage. Transpiration (the loss of water mainly through the stomata of leaves) increases with wind speed, but the effect varies greatly among plant species; also, the effect is related to temperature and humidity of the air. In arid climates, dry and hot winds often cause rapid, harmful wilting. In winter, with frozen soil, the damaging effect of increased transpiration resulting from wind can be serious because the lost water cannot be readily replaced. By contrast, increasing wind promotes carbon dioxide intake within limits; this benefits the rate of photosynthesis. The effects of mechanical wind damage vary from species to species; some show a definite decrease in dry matter production with increasing wind, while others (usually short plants) are unaffected. Because of the long-recognized need, shelterbelts, massive plantings of trees that change the energy and moisture balance of the crop, are positioned to protect crops and to increase yields. A shelterbelt perpendicular to the prevailing wind reduces velocity on both sides. A medium-thick shelterbelt can reduce wind velocity by more than 10 percent to a distance of 20 times the tree height on the leeward side and three times the tree height to the windward. The length of the shelterbelt should be at least equal to that of the field to be protected. The sheltered area will suffer much less soil erosion and mechanical damage than unprotected areas. Other microclimatic effects of shelterbelts include: (1) small daytime temperature increases and nighttime decreases; (2) the occurrence of radiation frost in the leeside may be promoted; (3) rate of evaporation in the sheltered area is decreased, depending on wind velocity; (4) snow accumulates near the shelterbelt, causing increased moisture storage in dry farming.

      The overall effect of a shelterbelt is complicated but probably beneficial. There is much evidence that they increase efficiency of water use not only in subhumid and semi-arid regions but also in true deserts where oasis-type irrigation is practiced. The response to shelterbelts, however, depends on the species. Crops of low response to wind protection are the drought-hardy small grains and maize grown under dry farming conditions. Rice and forage crops such as alfalfa, lupine, and clover are moderately responsive. Crops that benefit most from wind protection are garden crops, such as lentils, potatoes, tomatoes, cucumbers, beets, strawberries, watermelons, deciduous and citrus fruits, and other tender crops, such as tobacco and tea. Some authorities assert that in strong wind areas shelterbelts will produce an average 20 percent yield increase, which is net gain of 15 percent when allowance is made for the land occupied by the belts themselves. Trees can be grown almost anywhere, even in the desert; tall plants such as corn (maize), sorghums, or even elephant grass can also be employed in arid regions by including them in the irrigation schedule. It would appear that windbreaks are among the most practical means of beneficial weather modification in agriculture.

The effects of pollution
      Practically all forms of technology exact a certain price in environmental damage; agriculture is no exception. Agriculture in turn is sometimes damaged by undesirable by-products of other technologies (see also pollution: The pollution of natural resources (pollution)).

      Air has physical properties and a chemical composition that are vital parameters of life for both plants and animals. Temperature, water vapour, movement, oxygen, and carbon dioxide in the atmosphere have a direct effect on food and fibre production. Air quality is changed by introduction of contaminants into it, and agricultural activities using such air may be affected adversely. Damage to plants by air pollutants is related to meteorological conditions, particularly temperature inversions in the atmosphere.

Air pollution
Air pollution damage to agriculture
      For more than a century air pollution has affected agriculture. Burning coal and petroleum produce sulfur oxides. Fluorides result from smelting and glass and ceramic manufacture. Rising levels of ammonia, chlorine, ethylene, mercaptans, carbon monoxide, and nitrogen oxides are found in the air. Motor vehicles and growing population produce photochemical air pollution affecting not only the urban concentrations but also the contiguous rural areas. The mixture of pollutants from all sources, including agriculture, has released a host of contaminants into the air, such as aldehydes, hydrocarbons, organic acids, ozone, peroxyacetyl nitrates, pesticides, and radionuclides. The effect of these pollutants on food, fibre, forage, and forest crops is variable, depending on concentration, geography, and weather conditions. Damage to crops by air pollution, of course, brings economic loss as well.

      The effects of air pollution on plants and animals may be measured by the following factors: (1) interference with enzyme systems; (2) change in cellular chemical constituents and physical structure; (3) retardation of growth and reduced production because of metabolic changes; (4) acute, immediate tissue degeneration. Pollutants that enter the air from sources other than agriculture and that produce plant response are classified as: (1) acid gases; (2) products of combustion; (3) products of reactions in the air; and (4) miscellaneous effluents.

Acid gases
      Acid gases include fluorides, sulfur dioxide, and chlorine. Hydrogen fluoride is extremely toxic to plants; some plants are injured by contact with concentrations of less than one part per billion. The damage apparently occurs initially to the chlorophyll, producing a mottled chlorosis and later killing the cells. Plants vary in degree of tolerance to hydrogen fluoride; usually the plants that accumulate fluoride readily are the most tolerant. Corn is more susceptible than tomato. All plants are most susceptible to fluoride injury during periods of rapid growth.

      Sulfur dioxide given off in combustion of oil and coal commonly causes necrosis (cell death) of the leaf. At certain concentrations, sulfur dioxide will affect plants if the stomata (minute pores in the epidermis of a leaf or stem) are open. High light intensity, favourable growth temperatures, high relative humidity, and adequate water supply are conducive to open stomata. Plants that close their stomata at night can tolerate sulfur dioxide much better during that period. Conifers are more susceptible in spring and early summer, when the new needles are elongating. The sulfur dioxide absorbed by the leaf cells unites with water to form a toxic sulfite, but this is slowly oxidized to a relatively harmless sulfate. The toxicity of sulfur dioxide thus is a function of the rate at which it is absorbed by the individual plant; rapid absorption will cause greater injury. Chlorine damage to plants is somewhat rare; its typical symptoms are bleaching and necrosis of the leaf.

Products of combustion
      The primary products of combustion are ethylene, acetylene, propylene, and carbon monoxide. Of these, ethylene is known to affect plants adversely; while the others may also do so, it would require higher concentrations of them than typically occur in polluted air. For many years it was observed that illuminating gas (3 percent ethylene) leaking from pipelines caused damage to nearby vegetation. Now, with the use of natural gas, ethylene in the air is derived mostly from certain chemical industries and from automobile exhaust. Greenhouse flowers in metropolitan areas are typically damaged by ethylene. Such injury appears to be caused by excessive speeding up of the life process, thus bringing on damage. Ethylene was first identified as affecting plant life over large areas in the field by its effects on cotton and other plants near a polyethylene factory.

      Ethylene, ozone, and peroxyacetyl nitrate are produced as reaction products in the air and are clearly implicated in plant injury. In addition, certain bisulfites and nitrogen dioxide are under suspicion; there are probably others. Ozone is a major air pollutant affecting agriculture. Damage has been identified in a number of field crops, including spinach, tobacco, fruits, vegetables, forest trees, and ornamentals. Symptoms of ozone toxicity appear as flecks, stipple, streaks, spots, tipburn, and premature yellowing of the foliage; these may be visible only on the upper leaf surface. Peroxyacetyl nitrate and its analogs produce symptoms called silver leaf and leaf banding, which have been observed in the Los Angeles area and elsewhere for many years.

      The adverse effects of airborne radioactive contaminants on the agricultural economy at the present time are small.

Air pollution by agriculture
      Contributions of agricultural technology to air pollution include pesticides, odours, smoke, dust, allergenic pollens, and trash. The widespread public concern about pesticides makes it imperative that pesticide technology be carefully controlled and that search for better methods be pursued vigorously.

Pesticides
      The problem of persistence in pesticides can be highlighted by noting that this attribute exists in a range from moderately persistent (a lifetime of one to 18 months—2,4-D, atrazine); persistent (lifetime up to 20 years—DDT, aldrin, dieldrin, endrin, heptachlor, toxaphene); or permanent (lead, mercury, and arsenic). Presumably, the less persistent types should be more desirable, other things being equal; but those that degrade rapidly, such as the organophosphate insecticides, are extremely toxic and nonselective, which encourages rapid emergence of resistant insects and destroys their natural enemies. Thus, it is apparently not possible to adopt chemicals that function without some drawback or disadvantage.

      Whether pesticides are applied by spraying or by surface application, air is the usual medium through which the chemicals move to their intended and unintended targets. Reliable data on how pesticides behave in air, such as distance travelled, are lacking, because adequate monitoring is unavailable. Their chemical and physical nature, method of application, and the atmospheric conditions will influence their concentration and ultimate fate. There is no doubt that pesticides may be transported long distances on dust particles. The rate of removal from air is difficult to predict, but in the long run the chemicals return to earth.

Odours (odour), pollen, and dust
      Odours from animal concentrations are recognized as being highly undesirable to air quality. Where these operations exist contiguous to urbanized areas, public reaction is usually unfavourable. Disposal of animal waste on the land may worsen the odour problem; in addition, high wind may move dry increments into the air. Smoke is emitted by operations designed to dispose of crop residues, or by controlled burning of weeds and brush. Air quality is also affected by transmission of allergenic pollen such as ragweed pollen, which can be blown for hundreds of miles.

      Improper land use and treatment can cause considerable deterioration in air quality. Practices that strip the soil of plant growth or crop residues for long periods contribute to wind erosion, particularly in dry-farming areas. Fortunately, the technology of preventing wind erosion is well understood and widely used. Trash related to agriculture is moved freely by wind and distributed in unwanted fashion. Hulls of rice and wheat and cotton-gin trash are examples of this kind of airborne nuisance.

      In contrast to most other technologies, however, the agricultural variety offers a major beneficial contribution to air quality. The photosynthesis of green plants removes carbon dioxide from the air and adds oxygen to it, thus helping to maintain the life-giving balance between these gases.

Soil and water pollution
Pollutants damaging to agriculture
      Soil and water pollutants that may adversely affect agricultural operations include sediment, plant nutrients, inorganic salts and minerals, organic wastes, infectious agents, industrial and agricultural chemicals, and heat.

      Sediment is a resource out of place whose dual effect is to deplete the land from which it came and impair the quality of the water it enters. Aside from filling stream channels, irrigation canals, farm ponds, and irrigation reservoirs, sedimentation increases cost of water clarification. Suspended sediment impairs the dissolved-oxygen balance in water. The recreational value of farm ponds is diminished by sediment, while soil depleted farmland is reduced in value.

Plant nutrients (nutrient)
      Nutrients of plants become resources out of place when they appear in groundwater and surface water; in fact, they become serious pollutants. Unwanted aquatic plants are nourished by plant nutrients derived from agricultural runoff, feedlots and barnyards, municipal and rural sewage, and industrial wastes. Aquatic plants clog irrigation and drainage structures, thus increasing maintenance cost and reducing capacity. Nitrates and nitrites in groundwater, which can poison human beings and livestock, result from both agricultural and industrial operations.

Inorganic salts and minerals
      Inorganic salts and minerals that impair the quality of soil and water are derived from natural deposits, acid mine drainage, industrial processes, and drainage flow from irrigated areas. Salt accumulation on irrigated soils causes the most damage and loss in this category. A high proportion of sodium in irrigation water supply affects plant life adversely (see below Salinity (agricultural technology)). More than just a trace of boron is highly toxic; therefore, water used in municipal and industrial processes involving borax may not be usable for agriculture.

Organic wastes
      Organic wastes emanating from municipal sewage, garbage, food-processing industries, pulp mills, and animal enterprises are attacked by aerobic bacteria. When this occurs in water, the oxygen content of the water is depleted or reduced to zero, at which point the anaerobic bacteria complete the process of reducing the wastes to inert material. This produces septic conditions that make the water unfit for recreational use, farmstead supply, or crop irrigation.

Infectious agents
      Where not carried by wind, infectious agents are transmitted mainly by water and soil. Bacterial and virus diseases of crops are spread by machines that move contaminated soil. Insects (insect) are prime carriers of these diseases. Weed seeds are spread by irrigation water, as are nematodes. Animal diseases transmitted by water and soil include leptospirosis, salmonellosis, hog cholera, mastitis, foot and mouth disease, tuberculosis, brucellosis, histoplasmosis, Newcastle disease, anthrax, coccidiosis, and many others. Mosquitoes breeding in stagnant water can transmit encephalitis. Most crops and livestock in the world are susceptible to one or more highly infectious disease that may be transported by soil or water. The cost of losses from these diseases is staggering.

Chemicals
      Organic chemicals in soil and water, such as detergents, insecticides, herbicides, fungicides, nematocides, rodenticides, growth regulators, and defoliants, can have adverse effects on agriculture. The application of persistent insecticides to potato lands has led to residues in sugar beets grown in the same soil the following year, for which there are no tolerances. Fish have been killed in farm ponds because of drainage of insecticide pollutants. Use of heptachlor (no longer recommended) to control alfalfa weevil led to soil contamination and uptake by hay; dairy cows that ate the hay produced milk containing heptachlor.

      Aerial and ground application of herbicides on nonagricultural lands (utility rights-of-way, roadsides, industrial sites) often cause damage to nontarget crops. Herbicide wastes may enter drainage or irrigation ditches and create trouble. The presence of chemical residues in agricultural commodities can cause serious problems ranging from confiscation to loss of public confidence. Practically all aspects of chemical usage are now regulated or restricted by government.

      Introduced into water by industrial processes, heat can have a detrimental effect on fish and other creatures in the water; damage to recreational value can result. But, though heat is a water pollutant, its effect is minor with respect to agriculture.

Pollutants from agriculture
      Some pollutants from agriculture have adverse effects on agriculture itself, as excesses of plant nutrients and salts from irrigation. These pollutants and others also affect the environment at large.

      Eutrophication occurs in a body of water when an increase of mineral and organic nutrients has reduced the dissolved oxygen, producing an environment that favours plant over animal life. The resulting algae and other water plants tend to choke other forms of life in the oxygen competition, especially where carbon and phosphorus are plentiful. Doubtless, much phosphorus in streams and lakes is delivered from agriculture, but primarily through soil erosion rather than runoff. Though the principal source of phosphorus is apparently municipal sewage-treatment plants, direct runoff from feedlots may also contain large amounts. The solution to the problem of phosphorus in surface water lies in using good soil-conservation practices and in minimizing runoff from animal concentrations and manure.

      In contrast, identification of nitrate sources in water supplies has suffered from conflicting evidence. Where nitrate is found in water, some have concluded it came from chemical fertilizers, while others have suggested it came from natural soil nitrification or nitrification of sewage effluent or animal wastes. The problem has serious aspects, because nitrate can cause serious illness in human beings. One difficulty in identifying nitrogen sources lies in the fact that it is present in soils for reasons other than fertilization; the growth of legumes, for example.

      Salinity is a major problem in irrigation agriculture. Through evapotranspiration, salts in the irrigation water become more concentrated in the drainage effluent. It is therefore claimed that water quality is seriously impaired by irrigation agriculture. Irrigation water always contains some salt, most of which is excluded by the plant roots; since the evaporated water is pure, the soil accumulates the residual salt, which is added to what the arid soils already have in abundance. This accumulation of salt must be removed if plants are to be grown at all, and it is removed by leaching with excess water.

      Survival of an irrigated area will depend, therefore, on a favourable salt balance: salt leaving the area must equal or exceed that received in the water supply. The irrigation farmer is not actually “producing” a contaminant but is transferring one in a more concentrated form. Future intensified use of limited irrigation water may add to the severity of this problem. Where the return flow is readily recoverable, as from tile drains or pumped wells, it could be purified by a desalination process, returning the purified fraction to the watercourses and disposing of the concentrated-salt fraction in such a way that usable groundwater is not affected.

Agricultural processing wastes
      The wastes from processing of agricultural products represent another pollution hazard. These include runoff or effluent from sawmilling, pulp manufacture, fruit and vegetable canning, cleaning of dairies, slaughtering of meat animals, tanning, manufacturing of cornstarch and soy protein, sugar refining, distilling, wool processing, and many others. The runoff from agricultural enterprises can contain disease organisms and other infectious agents. Insects associated with agriculture can transmit diseases. Plant diseases move from agriculture to lawns, gardens, parks, and golf courses.

Monitoring pesticides (pesticide)
      The monitoring of pesticides in water has been carried on in various areas since World War II. Some of the monitor networks, backed by analysis laboratories, are quite extensive. The accumulated data show how and when certain pesticides move from target areas into other parts of the environment. Ponds and catch basins sometimes show measurable amounts of pesticide residues from water leaving fields. Although most organic insecticides are hydrophobic and almost insoluble in water, they can become attached to materials suspended in water; but, after these materials settle, the remaining amounts of insecticide residues usually become negligible. This confirms the earlier supposition that the movement of chemicals from target areas is greatest when silt and organic loads are high in runoff water.

      Levels of pesticides in soils are constantly changing. So many variables and processes are involved, that rates of accumulation of even the most persistent insecticides are quite variable and difficult to determine. Soil monitoring programs are underway, however, and are providing much-needed information. The problem of accumulation in soils arises because the tiny organisms in soil are not capable of degrading many pesticides at rates sufficiently high to prevent soil and also water pollution. Thus, the persistent types, such as DDT and other chlorinated hydrocarbons, remain available for absorption by higher animals (including human beings) and for causing harm to nontarget organisms.

Robert E. Stewart Ed.

Additional Reading
Factors in soil preparation are analyzed by William R. Gill and Glen E. Vanden Berg, Soil Dynamics in Tillage and Traction (1967); Milton A. Sprague and Glover B. Triplett (eds.), No-Tillage and Surface-Tillage Agriculture (1986), a review of these alternatives to traditional plowing; Ronald E. Phillips and Shirley H. Phillips (eds.), No-Tillage Agriculture: Principles and Practices (1984); and Samuel L. Tisdale et al., Soil Fertility and Fertilizers, 5th ed. (1993).Various cropping systems are described in John Vandermeer, The Ecology of Intercropping (1989); and Charles A. Francis (ed.), Multiple Cropping Systems (1986). Hubert Martin and David Woodcock, The Scientific Principles of Crop Protection, 7th ed. (1983), focuses on pest control. Regional variations in farming technique are presented by K.G. Brengle, Principles and Practices of Dryland Farming (1982); Hans Ruthenberg et al., Farming Systems in the Tropics, 3rd ed. (1980); and L.V. Crowder and H.R. Chheda, Tropical Grassland Husbandry (1982). James Sholto Douglas, Advanced Guide to Hydroponics, new ed. (1985); and Howard M. Resh, Hydroponic Food Production, 4th ed. (1989), treat this specialized technique.Weather information is available in Weekly Weather and Crop Bulletin, published by the U.S. Dept. of Commerce, Weather Bureau. Studies of agricultural meteorology include Rudolf Geiger, The Climate Near the Ground (1965; originally published in German, 4th ed., 1961), a classic text; Jen-hu Chang, Climate and Agriculture (1968); Robert H. Shaw (ed.), Ground Level Climatology (1967); and Norman J. Rosenberg, Blaine L. Blad, and Shashi B. Verma, Microclimate, 2nd ed. (1983). George W. Cox and Michael D. Atkins, Agricultural Ecology (1979), analyzes world grain and vegetable production systems, with an emphasis on the influence of weather. David J. Briggs and Frank M. Courtney, Agriculture and the Environment (1985), describes temperate agricultural practices and systems and their impact on the environment, with examples from Britain. Mervyn L. Richardson, Chemistry, Agriculture, and the Environment (1991), focuses on pesticide and fertilizer pollution from both crop and livestock production. Pollution's effect on agriculture is reported in James J. MacKenzie and Mohamed T. El-Ashry (eds.), Air Pollution's Toll on Forests and Crops (1989).Robert E. Stewart Ed.

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