Chapter 16: Tillage and cropping systems.
After completing this chapter, you should be able to:
* explain the reasons for and effects of tillage
* describe conventional and conservation tillage
* list several cropping systems
* briefly describe organic and sustainable agriculture
TERMS TO KNOW
To produce crops, a grower places seeds in contact with the soil, provides nutrients, controls pests, and manages soil water. These activities usually involve some form of tillage. There are many ways to work the soil and different situations require different methods. Each method has an effect on the crops and the soil. This chapter looks at some standard tillage and cropping systems.
Uses of Tillage
Tillage is working the soil to provide a favorable environment for seed placement and germination and crop growth. In the United States, mechanization and research have led to a variety of tillage systems. Regardless of the method of tillage used, a grower has three basic goals: (1) weed control, (2) alteration of physical soil conditions, and (3) management of crop residues.
Weed Control. Tillage for weed control can be divided into two time periods: before crop planting and after crop planting. Before planting, tillage prepares a weed-free seedbed that greatly simplifies weed control during the growing season. Tillage destroys young seedlings, and repeated tillage operations may also weaken perennial weeds. After planting, cultivation continues to destroy or bury emerging seedlings. However, deep cultivation or cultivation late in the season may sever crop roots and reduce yields.
The importance of tillage for weed control has declined with increases in both herbicide use and tillage systems designed around herbicide use. Some herbicides require incorporation into the soil by shallow tillage. However, with increased interest in organic agriculture, renewed emphasis on mechanical tillage may be expected.
Physical Soil Conditions. Tillage alters physical soil properties, such as structure, moisture, and temperature. Tillage during seedbed preparation stirs and loosens soil, improves aeration, and creates a suitable medium for growth. Deep tillage and subsoiling may temporarily break up subsoil compaction.
However, tillage causes a long-term decline in physical structure. The decline is partly due to losses of soil organic matter that result from tillage. Repeated tillage operations crush some soil aggregates. Wheel traffic compacts the soil, especially wet soils, and tillage pans may form. Soil aggregates on the surface of bare soil shatter from raindrop impact, causing crusts that hinder seed germination and shed water. The bare soil resulting from many forms of tillage erodes easily. Recent changes in tillage aim to reduce these adverse side effects.
Tillage also affects the moisture level and temperature of soil. Tilled soil usually warms up earlier in the spring, allowing earlier seeding and better germination. In areas where soil tends to be wet or cold in the spring, crops may be planted on ridges created by tillage. The ridges warm and dry faster than the rest of the soil.
Shallow cultivation of crust-forming soils may improve crop yield even where herbicides are used to control weeds. By breaking up crusts, cultivation improves water infiltration and reduces runoff. Such cultivation should be just deep enough to break the crust.
Crop Residue Management. After most crops are harvested, residues like stalks or leaves remain in the field. The amount of residue depends on the type of crop, how well it grew, and how it is harvested. For example, corn leaves about 8,500 pounds of residue per acre for a 150-bushel corn crop, and about 5,600 pounds of residue for a 100-bushel crop. If the corn is harvested for silage rather than grain, little residue is left in the field. Figure 16-1 lists residues for several crops.
There are several ways growers manage crop residues, depending on objectives. Moldboard plowing buries crop residues, resulting in a clean field that is easy to plant and cultivate. In semiarid grain-growing areas, special tillage tools, including rod weeders and sweeps, till under the surface to kill weeds but leave residues on the surface to protect against wind erosion. Conservation tillage in more humid climates leaves residues on the surface to protect against water erosion. Figure 16-2 lists the amounts of residue left on the soil surface from various tillage tools.
In addition to crop residues, tillage may also incorporate phosphates, potash, and lime into the root zone. Tillage also incorporates sewage sludge, manures, and nitrogen sources like urea that volatilize if left on the soil surface.
Seedbed Preparation. The three reasons for tillage come together in preparing a seedbed to ensure that the soil meets the needs of germinating seeds. Seeds need a moist soil at the right temperature with sufficient air for seed respiration. The soil should be loose enough for good aeration, but compact enough around the seed for good soil/seed contact. It should be free of clods that prevent proper seed/soil contact and seedling emergence (figure 16-3).
Seedbed smoothness and the amount of allowable crop residues depend upon seed size and type of planter. Large seeds, like corn and soybeans, germinate in a fairly rough, cloddy seedbed. Very small seeds, like alfalfa seed, germinate best in a very fine, firm seedbed. A seedbed free of crop residue is easiest to plant in, but conservation tillage demands that crop residues be left on the surface to control erosion. Most older seed planters only operate on a fairly smooth, clean seedbed. Many modern planters can plant through crop residues and clods, preparing correct soil conditions near the seed.
Conventional tillage, the primary form of tillage since invention of the moldboard plow, involves two stages. First, primary tillage breaks up the soil and buries crop residues. Primary tillage is often accomplished with an inverting implement, like the plow or lister plow, that inverts or tips over the top few inches of soil. Secondary tillage produces a fine seedbed by a series of operations that break up the soil into smaller and smaller chunks. Secondary tillage involves mixing implements like harrows. The following discussion describes these operations in more detail.
[FIGURE 16-3 OMITTED]
Plowing. The traditional primary plowing tool is the moldboard plow (figure 16-4). The moldboard shears off a section of soil, tips it upside down, and fractures it along several planes. In the process, any organic material on the soil surface is buried. The moldboard plow leaves the surface very rough with a series of ridges and furrows.
Moldboard plows work best in moist soil; in wet or dry soil the operation uses more power and the results are poor. For wet or dry soils, a disc plow works better. A series of three to ten large (two to two and one-half feet) discs are mounted on a frame at an angle to the direction of travel. The discs cut into the soil as they rotate and roll the soil over.
Subsoilers like the one shown in figure 16-5 are used to shatter tillage pans or natural soil pans. Subsoiling should be done when the soil is dry, because if pans are moist, they do not shatter. Deep plowing can temporarily help water infiltration and root penetration into the subsoil. Usually, however, compacted layers reform as soil is exposed to further wheel traffic and tillage.
Harrowing. Harrowing is usually a two-step process. In the first stage, ridges left from plowing are smoothed out and large clods broken up. Then smaller lumps are pulverized and a fine seedbed is produced.
[FIGURE 16-4 OMITTED]
[FIGURE 16-5 OMITTED]
[FIGURE 16-6 OMITTED]
Growers commonly begin the operation with a disc harrow (figure 16-6). The typical tandem disc has four gangs of discs set like the four arms of an X. The front two gangs turn the soil inward, and the back two turn it back out. A disc tends to compound compaction problems because it shatters soil aggregates but does not dig deep enough to loosen compaction. Spring-tooth harrows and field cultivators (figure 16-7) may be used rather than the disc. A long, springy C-shaped tooth and a spear point or broad shovel digs into the soil, dragging clods to the surface and breaking them up.
A finishing harrow, or drag, completes the job of pulverizing the soil. Figure 16-8 shows a drag being pulled behind a spring-tooth harrow.
The steps just described are often modified. If the soil has good tilth, deep tillage by plowing may not always be needed. In such cases, the tandem disc shown in figure 16-6 is heavy enough to be used alone. Growers often combine operations, hitching several tillage tools behind the tractor. Any time a grower can eliminate a pass through the field, compaction is reduced, and time and fuel are saved.
[FIGURE 16-7 OMITTED]
[FIGURE 16-8 OMITTED]
Lister Plowing. Lister plows are equipped with two moldboards mounted back to back, resulting in a pattern of ten-inch-high ridges and furrows across the field (figure 16-9). In humid regions, crops may be planted on the warmer, drier ridges. In arid areas, they may be planted in the moist soil of the lister furrow. Listing can also help protect the soil from wind erosion. Listing on the contour captures water to improve water use and reduce water erosion.
PREPARATION OF FURROW-IRRIGATED FIELDS. Additional steps are needed to prepare furrow-irrigated fields. After the standard primary and secondary tillage, the grower carefully levels the field with a blade to ensure the proper grade for flow of surface-applied water. Then the field is listed with a special tool to create ridges and furrows.
Timing and Depth of Plowing. Farmers in the eastern United States can plow in either fall or spring. Fall plowing gives the farmer a head start on spring planting by warming and drying the soil. Freezing and thawing on fine-textured soil breaks up large lumps, making it easier to develop a good seedbed. The benefits of fall plowing are especially important with fine-textured soils with somewhat poor drainage.
Spring plowing leaves crop stubble in the field over winter to capture snow and reduce erosion. To conserve soil, one plows in the spring unless there are overriding reasons for fall plowing.
[FIGURE 16-9 OMITTED]
In the western United States, where moisture preservation is critical, plowing immediately after harvest gives more time for the soil to store moisture if weeds are controlled. However, leaving soil bare increases the risk of erosion.
The standard plow depth of seven to eight inches gives the best results. Shallow plowing results in a poor seedbed, and deeper plowing takes more power without noticeably improving yields.
Conservation tillage is a program of crop residue management aimed at reducing erosion (figure 16-10). Rather than plowing under crop residues, some or all of the residue is left on the soil surface. The definition of conservation tillage has required that, at planting, 30 percent or more of the soil surface be covered with crop residues. USDA 1994 conservation standards specify instead that enough residue remain on the surface to reduce soil losses below a tolerable level as calculated by current erosion prediction methods. Erosion prediction and soil loss tolerance are described in chapter 18.
[FIGURE 16-10 OMITTED]
Conservation tillage reduces water and wind erosion by at least 40 percent to 50 percent. In areas where moisture can be limiting, conservation tillage increases soil moisture by improving infiltration and reducing runoff, reducing evaporation, and trapping snow. Because of reduced runoff, fewer pesticides and nutrients leave the field. Conservation tillage also improves organic matter content near the soil surface, as described in chapter 6. Conservation tillage, therefore, is one of the most important Best Management Practices for soil and water conservation.
Conservation-tilled soil tends to be cooler than clean-tilled soil because of light reflection off the mulch and increased soil moisture. In warm climates, cooler soil benefits production, but may hinder growth in northern states. Diffusion and mass flow of nutrients improves in the moister soil, increasing nutrient uptake.
[FIGURE 16-11 OMITTED]
Other benefits of conservation tillage are obtained from fewer trips across the field. These benefits include less time in field work and lower fuel costs. At times, compaction is reduced because of less wheel traffic. Conservation tillage may also require fewer implements, thus reducing equipment costs per acre. Conservation tillage provides better habitat for pheasants and other wildlife. For instance, no-till fields have been shown to provide improved habitat for nesting ducks in North Dakota and nesting bobwhite quail in Tennessee.
Because of soil conservation and economic benefits of conservation tillage, its use has spread rapidly in recent decades. The USDA reports that in 1995 about 35 percent of United States agricultural land was under conservation tillage, another 25 percent under other reduced tillage methods, and 40 percent under conventional tillage. As technology improves, conservation-tillage use is expected to grow.
Conservation tillage covers several different tillage methods:
Mulch-Till or Chisel-Plow. A chisel plow (figure 16-11), which loosens the soil but does not invert it, is used for primary tillage. Chisel plowing to eight inches leaves the soil rough with about 50 percent to 80 percent residue cover (figure 16-12). Light discing reduces residues to between 30 percent and 50 percent. Seeds are then planted through the remaining residues. After planting, cultivation and herbicides control weeds.
[FIGURE 16-12 OMITTED]
Strip-Till. With no primary tillage, a specialized implement tills a band of soil and plants seeds into the band. Another type of implement sweeps residues off a strip into the middle of the rows. The planting operation bares about one-fourth of the soil surface, leaving about 50 percent of crop residues.
Ridge-Till. The ridge-till system excels in cool, moist conditions. Seed is planted on six-inch ridges (figure 16-13) with crop residues swept into the shallow furrows. About two-thirds of crop residues remain after planting. Cultivation with special tools minimizes residue burial and rebuilds ridges for the coming year.
The ridges in this system warm up and dry more quickly than soil in other tillage systems. In addition, if oriented across the slope, they further reduce runoff and erosion. If oriented perpendicular to prevailing winds, they further reduce wind erosion and help to trap snow. The roots of plants on the ridges also grow separate from the compacted zone between the ridges where wheel-traffic occurs.
[FIGURE 16-13 OMITTED]
No-Till. In this method soil is barely disturbed (figure 16-14). Specialized planters cut a slot through the residues, insert the seed and fertilizer, and close the slot. About 90 percent of the soil surface is untouched. Contact, systemic, and preemergent herbicides are used to control weeds with no cultivation.
Because no-till involves the least soil disturbance, it maximizes the benefits of conservation tillage. By not disturbing the soil surface, it preserves the tops of earthworm and other channels at the surface, greatly improving water infiltration. No-till best preserves soil organic matter, and organic matter content actually rises in the soil near the surface.
[FIGURE 16-14 OMITTED]
A number of general principles apply to conservation tillage. Crop residues should not be burned, and baling or grazing of crop residues should not deplete residues below the acceptable level. Tillage and planting implements should be used properly to preserve residue cover.
Residue levels can be predicted from data on crop residues left by crops (figure 16-1) and the amount of residue remaining from each pass with an implement (figure 16-2). It can also be measured directly in the field by a number of methods.
For moisture-preserving purposes, residue levels should exceed 50 percent. For trapping snow, stubble should be left standing at least six inches tall over winter.
Differences Between Conventional and Conservation Tillage
Conventional and conservation tillage differ in more than the obvious ways. Most growers ask first about yields. Current research shows equivalent yields from conventional and conservation tillage if known technology is applied to each tillage system and the systems are matched to crops and soil types. Figure 16-15 provides some comparisons of the systems.
Equipment. Conservation tillage places some requirements on equipment. For example, residues should be spread evenly behind harvest equipment. Planters must penetrate the residues, place the seed, cover the seed, and ensure seed/soil contact in a rough seedbed.
Fertility. Because there is less soil mixing in conservation tillage (especially in no-till), the form and placement of fertilizers are affected. Lime, phosphates, and potash tend to stay near the soil surface. However, because residues provide a mulch, soil near the surface tends to remain moist, promoting root growth near the surface and improving uptake from that layer. Conservation tillage, especially no-till, can reduce nitrogen availability by increasing nitrogen tie-up in surface layers, increasing leaching and volatization, and by reducing average soil temperatures. The injection of nitrogen deeper into the soil and nitrification inhibitors will reduce these problems.
The pH of the top two inches of soil tends to drop rapidly, especially in no-till, affecting seed germination, crop growth, and herbicide activity. Careful testing for pH is required for this layer, followed by a topdressing of lime if needed.
Matching Tillage to Soil Type. Soils tend to be cooler and wetter in conservation tillage, especially with the no-till method. On fine-textured soils in northern states, cooler soil may delay planting and hamper seed germination. No-till is a poor choice on cold, poorly drained fine-textured soils; for these conditions, the ridge-till method is a better choice. Nor does no-till work well in highly compactable soils. On excessively drained coarse soils, no-till can improve yields by preserving moisture. Local extension agents can provide advice on the best system for each grower.
Weed Control. With less tillage, there is greater reliance on herbicides for weed control. Tillage will kill any weed seedling, but herbicides are more selective. This makes weed identification and herbicide selection more critical. Also, surface-applied chemicals are more suitable for conservation tillage than those needing to be incorporated into the soil.
Pest Control. Conservation tillage, especially no-till, alters the environment presented to pest organisms. Diseases that overwinter on crop residues, like small grain leaf diseases, can be a greater problem in conservation tillage compared with conventional tillage, where plowing buries infected residues. This factor increases the need to select resistant crop varieties and to rotate crops.
Some researchers state that insects will be a greater problem in conservation tillage, because overwintering insects are not buried by plowing. Others state that research has not proved this claim. Growers should follow the advice of local Cooperative Extension agents.
Growers employ a number of cropping systems; several are described here. A grower's choice depends on climate, economics and market demand, government programs, and grower preference. Each system requires different soil-management techniques and has different effects on the soil.
Continuous Cropping. In continuous cropping, a farmer grows the same crop each year. Continuous cropping is favored by many farmers because they can grow the most profitable crop. This method also allows the grower to specialize in the crop best suited to local soil or climate conditions. In general, however, yields often decline with continuous cropping. At the same time, expenses for fertilizer, herbicides, and pesticides tend to rise compared with expenses for a crop rotation system.
Crop Rotation. Crop rotation means that a series of different crops is planted on the same piece of ground in a repeating system (figure 16-16). Many farmers do not rotate because it means planting some less-profitable crops. Often a farmer has no use for certain crops in common rotations. For instance, a farmer who feeds no animals has little use for hay unless a buyer can be found for it.
[FIGURE 16-16 OMITTED]
However, crop rotation has important benefits for those who practice it. Crop rotation:
* Aids the control of diseases and insects that rely on one plant host, reducing a grower's pesticide bill.
* Helps control weeds. Many weed species grow best in certain crop types, so alternating crops suppresses the weeds. Some rotations suppress weeds by allelopathy, where one plant emits chemicals from the roots that suppress growth of other plants. For instance, soybeans planted into wheat residues suffer fewer weed problems because of allelopathic effects of the wheat.
* Supplies nitrogen if certain legumes like alfalfa are in the rotation. This can lower a farmer's fertilizer bill.
* Improves soil organic matter and tilth. Deep-rooted crops like alfalfa also improve subsoil conditions.
* Reduces erosion compared with continuous row crops, as long as the rotation includes small grains or hay. This topic is covered in more detail in chapter 18.
Generally, crop rotations involve some combination of three kinds of crops: row crops, small grains, and forages. The specific crops and crop sequence vary from place to place.
Row Crops. Row crops, where adapted, are usually the most profitable. Row crops are planted in wide rows and cultivated for weed control, with the help of herbicides. The crops are fertilized by broadcasting, banding, and sidedressing. Fertigation is often used in irrigated fields. Row crops usually leave the soil bare, making it erosion-prone. As a result, row crops are best suited to fairly level ground. Conservation tillage, crop rotation, and other conservation practices greatly reduce erosion from row crops (see chapter 18). Common row crops include corn, sorghum, soybeans, and cotton.
Small Grains. Small grains, like oats or wheat, are planted in closely spaced rows seven or eight inches apart. As a result, they quickly cover the soil surface. Land planted to small grains loses less soil to erosion. Small grains also leave a large amount of residue that controls erosion in conservation-tillage systems. The dense growth of small grains competes with weeds that infest row crops; and several suppress weeds by allelopathy.
Soil nitrogen and potash must be properly balanced for good grain yields. A good supply of nitrogen promotes growth and improves protein content. Excess nitrogen or low potassium, especially in moist soils, increases lodging. Fertilization is usually carried out by preplant broadcasting and may be followed by topdressing of the growing grain. Banding has become popular, especially in conservation-tillage systems.
Perennial Forage. Forages are harvested for their green matter and fed to animals. They may be harvested as hay or used for grazing in pasture or range. Forages improve soil tilth, add organic matter, and control erosion. Taprooted plants, like alfalfa, help break up soil pans. Legume forages also fix nitrogen that can be used by later crops. Examples include legumes such as alfalfa and a wide array of forage grasses.
Double Cropping. Double cropping is the practice of harvesting two crops from the same piece of ground in one year. A common example is planting soybeans into winter wheat stubble (figure 16-14). Double cropping is easiest in warm climates with long growing seasons. The use of double cropping has grown with the use of no-till systems. The second crop can be planted right behind harvest of the first crop, omitting time-consuming seedbed preparations.
Multiple cropping keeps the soil covered with vegetation for a larger part of the year. Better erosion control results. Two crops grow more green matter than one, so the practice helps maintain organic matter in the soil. Where one crop is a legume, the nitrogen addition is welcome. Two crops also draw more heavily on soil nutrients and water, so fertilizer and water must be more carefully managed.
Cover Cropping. Like double cropping, cover cropping involves two crops in one year on the same piece of ground. However, the cover crop is grown as a conservation tool and is usually not harvested. Several cover-cropping methods exist. In one variation, a cover crop is interseeded between rows of a taller row crop that is already growing. In a second variation, a cover crop is grown until it covers the ground, then the main crop is planted right into that first crop. The cover crop may be killed or treated in some way to reduce competition. Cover crops may consist of grasses like rye grass or legumes like various vetches. The cover crop is sometimes termed a "living mulch." A 1994 study of cover cropping in corn in mid-Atlantic states exemplifies the value of cover cropping. (1) This study concluded that of the systems studied, including clean cultivation, the most profitable was corn planted no-till in a killed winter cover of hairy vetch. The hairy vetch also contributed nitrogen and organic matter to the soil.
Cover cropping is a potent BMP for a number of problems. The thick cover reduces runoff and erosion and loss of chemicals in that runoff. The cover crop often sops up fertilizers, especially nitrates, left over from the last crop, to lessen nitrate seepage into ground water. Cover cropping can increase soil organic matter content, lessen weed growth, and suppress some crop pests. In some areas, the standing mulch protects seedlings from blowing sand, and if a legume, adds nitrogen to the soil.
The term dryland farming is applied to farming in low-rainfall areas without irrigation. In the United States, dryland farming is practiced in states west of a line from western Minnesota to eastern Texas, following the ninety-sixth meridian. We will discuss two characteristic dryland farming systems: small grain-summer fallow rotation and rangeland grazing.
Summer Fallow. Many dry areas lack enough water to produce good crops each year. As a result, crop rotation of small grain-summer fallow is used. In the crop year of the rotation, small grains are grown because they are relatively tolerant of low-moisture conditions. After the grain is harvested, the soil is left fallow for the next year. More complex systems include a three-year rotation of winter wheat-fallow-sorghum, common on the southern Great Plains.
Summer fallow is the practice of leaving the soil crop- and weed-free to store moisture. During the fallow period, weeds are controlled by cultivation or herbicides. By controlling weeds, no moisture is lost from the soil because of transpiration. Some water is lost by evaporation, but not all. After a rain, water seeps into the soil. As the soil dries, some moisture moves to the surface by capillary rise, where it evaporates. However, after the surface dries, it seals the rest of the water in the soil. After the next rain, more water is sealed in the soil. Generally about 25 percent of the rainfall on a fallow field will be stored for the following crop.
The effectiveness of summer fallow can be improved by reducing water runoff. On slopes, contour tillage helps reduce runoff. Using tillage tools that leave crop residues on the surface also helps prevent runoff. Chemical fallow, which leaves grain stubble standing and crop residues undisturbed, saves an additional one-half to two inches of moisture.
[FIGURE 16-17 OMITTED]
Three problems arise from the practice of summer fallow. During the fallow year, wind erosion can be quite serious. This problem will be discussed in detail in chapter 18. Second, crop-fallow rotations lead to a long-term decline in soil organic matter. The other problem is the development of saline seeps.
Saline seeps (figure 16-17) appear in almost two million acres of the Great Plains of the United States and Canada. Saline seeps appear where glacial till overlays an impermeable layer. During fallow, increased percolation picks up salts and carries them deeper into the soil. When salty water reaches the tight layer, it spreads out sideways, flowing downslope. Finally, the salty water seeps to the surface on a lower field. The water evaporates, leaving salt on the soil surface.
Reclaiming a saline seep begins by finding the origin of the salty water--the recharge area. Shallow ditches, land leveling, or contouring can divert excess water from the recharge area. Growers may seed the recharge area and the soil around the seep to alfalfa. With its high water demand and deep root system, alfalfa lowers the water table. During the reclamation period, salt-tolerant barley may be grown if needed.
Researchers have studied ways to reduce fallow problems by devising annual cropping systems. For instance, in the northern Great Plains about a quarter of the annual precipitation falls as snow--if all this were captured rather than allowed to blowoff, it could equal the moisture saved during a fallow year. Studies using tall wheatgrass strips to capture snow--as shown in figure 8-11--have shown improved profitability for annual wheat cropping with reduced problems like wind erosion. (2)
Rangeland. Range is an uncultivated area used for livestock grazing, particularly in the western United States. Rangeland is particularly important because it occupies such a large proportion of the land surface of the United States: up to half may be rangeland ecosystems. Of federally owned lands in the western United States, 85 percent are grazed by livestock. This land provides food and important wildlife habitat.
Grazing is the best agricultural use of range because it is too dry, too rocky, or too infertile for other agricultural uses. Most rangeland, if cultivated, would erode badly. Generally, little is done with rangeland because water shortages make improvement unprofitable.
Care is needed, however, to keep range healthy, profitable, and acceptable as wildlife habitat. A 1994 report by the National Academy of Sciences (3) noted that because range receives few inputs like irrigation or fertilizer, the health and productivity of rangeland rely heavily on natural processes. That report goes on to say that "Rangeland health should be defined as the degree to which the integrity of the soil and ecological processes of rangeland are sustained." It also defines soil stability and natural nutrient cycles as one of the prime criteria for rangeland health.
Grazing patterns strongly affect soil properties and cover on rangeland. A North Dakota study compared long-term effects of no, moderate, and heavy grazing on native mixed-grass range and a fertilized crested wheat-grass plot. (4) The study found that soil was most compacted in the heavily grazed site, and diversity of plant cover was best maintained by moderate grazing. The authors concluded that heavy grazing degraded the soil and plant cover resource, while both moderate grazing of native grasses and wheatgrass plots could sustain grazing long-term.
Ill health of rangeland tends to result from changes in fire patterns, which alters the type of vegetation; invasion by alien weeds; and overgrazing by livestock and wild animals. Overgrazing exposes bare soil to erosion and changes the vegetation as it shifts to plants not favored by the grazers, often brush and weeds. The 1992 National Resource Inventory identified 17 percent of public rangeland with serious brush or weed problems, 23 percent with accelerated erosion, and 18 percent with multiple problems.
Controlled grazing is key to preventing erosion. The number of grazing animals should be only as large as the land can safely carry. Animals should not occupy a single range for a long period, and vegetation should be given a chance to recover before livestock is returned. Animals also should not feed on specific rangeland during periods of slow plant growth.
Increasing concern for long-term farm productivity and the effect of agricultural practices on the environment led to the concept of sustainable agriculture. The American Society of Agronomy in 1989 declared that "a sustainable agriculture is one that, over the long-term, enhances environmental quality and the resource base on which agriculture depends; provides for basic human food and fiber needs; is economically viable; and enhances the quality of life for farmers and society as a whole." (5)
Those who research or practice sustainable agriculture have several concerns. There is concern that agriculture's resource base is being depleted: declining soil productivity due to erosion and loss of organic matter and nutrients; depletion of fertilizer sources like phosphate rock; and cost and availability of energy. A feared consequence of agriculture is a degraded environment: pollution of water by agricultural chemicals, nutrients, and siltation. Stability of the farm economy and community further motivates sustainable agriculture.
Sustainable agriculture, then, is a philosophy and collection of practices that seeks to protect resources while ensuring adequate productivity. It strives to minimize off-farm inputs like fertilizers and pesticides and to maximize on-farm resources like nitrogen fixation by legumes. Top yields are less a goal than optimum and profitable yields based on reduced input costs.
Soil and water management are central components of sustainable agriculture. Techniques include crop rotation, conservation tillage, cover cropping, nutrient management, and others. A recent mid-Atlantic study (6,7) comparing plastic mulch and vegetative mulch, (produced by shredding a standing cover crop of hairy vetch) on a tomato crop illustrates research on sustainable methods. Researchers found the vetch mulch greatly reduced runoff, erosion, and sediment and pesticide transport off the field compared to polyethylene mulch, a standard practice for growing tomatoes. The runoff from polyethylene mulched plots was found to be more toxic to aquatic organisms. Half the nitrogen was used in the vetch plots, of interest in light of fertilizer nitrogen issues discussed in chapter 15. The study suggests that the killed vetch mulch was a more sustainable practice than conventional plastic mulch for tomato production.
Organic Farming. Organic farming is a type of sustainable agriculture that also prohibits the use of synthetic substances, including inorganic fertilizers and pesticides. A major theme shared by organic farms is promoting a healthy soil by controlling erosion and keeping organic matter levels high. Buyers believe organic products to be safer, more nutritious or flavorful, or support the process of organic farming.
Organic growers add nitrogen by the use of manures, composts, legumes, and organic nitrogen fertilizers. Phosphorus and potassium come from manures and mineral fertilizers such as rock phosphate. Organic farms tend to rely more on natural nutrient cycles than do conventional farms. Crop rotations figure centrally in many organic operations. Weed control tends to rely on crop rotation, cultivation, and sometimes flaming or mulches.
According to a 1980 United States Department of Agriculture study, successful organic farms come in all sizes and crops. This and other studies point to soil benefits including reduced erosion, increased soil organic matter content, higher populations of earthworms, richer soil flora, and others. In a 2000 review of studies comparing conventional, sustainable, and organic systems in horticultural crops, their comparative production and profit were highly variable; results depended greatly on the specific sites and practices. Profitability for organic production tended to rely on the higher prices offered for organic produce. (8)
State-sponsored programs to certify organic production have grown in recent years, and in 1990 the Organic Foods Production Act directed the USDA to set up a federal program. The final rules for that program were published in 2000. The rules set certain production standards, prohibit the use of many substances on organic land including sewage sludge, and provide a list of allowed and disallowed synthetic materials. It also sets labeling requirements. These and state standards define what can be sold as organic and help the consumer purchase organically grown foods.
Tillage has three goals: weed control, alteration of physical soil conditions, and management of crop residues. Tillage also has a number of side effects, however, especially an increase in erosion, compaction, and reduced soil permeability.
Conventional tillage buries crop residues to produce a smooth, residue-free seedbed. Conservation tillage leaves residues on the soil surface to prevent erosion and preserve soil water.
Three cropping systems are used by growers: continuous cropping, crop rotation, and multiple cropping. Continuous cropping (or a simple corn-soybean rotation) allows a farmer to grow the most profitable crops yearly. Crop rotation, on the other hand, is better for the soil and helps control erosion.
In low-rainfall areas, small grains are grown in rotation with summer fallow. During fallow, weeds are controlled by cultivation or weed killers to store moisture for the following crop. Problems with summer fallow include erosion and saline seeps.
Animal grazing is the most practical agricultural use of dry, steep, or rocky land in the West. Controlled grazing, seeding of improved grasses, and sometimes fertilization and water management keep range in good condition.
Organic farming replaces chemical fertilizers and pesticides with crop rotation, manuring, cultivation, and mineral fertilizers. Organic farmers focus on having a "healthy" soil. Sustainable agriculture aims to reduce some of the problems of standard agriculture by using techniques that lower off-farm inputs, increase use of resources found on the farm, and the use of Best Management Practices.
1. What is conservation tillage and what are criteria for determining if a practice qualifies as conservation tillage?
2. Compare conservation-tillage methods most suitable for areas with soils that tend to be cold and damp in the spring to those that would be warmer and drier.
3. Some people think of sustainable agriculture as a return to "old-time" methods. Do you agree or disagree? Explain your answer.
4. What influence do you think widespread adoption of conservation tillage could have on global climate change?
5. Describe the purposes of tillage.
6. After the last cultivation, we plant a fast-growing legume between the rows of a corn crop. What do we call this practice? What might be benefits and drawbacks?
7. Why does summer fallow store some water in the soil for next year's crop? Review the discussion of capillary water movement in chapter 7 if necessary. How efficient is this practice?
8. Discuss the degree to which rangeland grazing, conventional row-crop agriculture, and organic farming utilize natural nutrient cycles.
9. Would you guess that organic farms consume directly and indirectly less or more energy than conventional ones? Think about fertilizer sources as well as other factors.
10. Using charts in this chapter, estimate the amount of residue left on the soil surface in these three practices:
a. 150 bushel/acre corn after moldboard plow and discing to six inches.
b. 150 bushel/acre corn after a single chisel plow with spear point.
c. 50 bushel/acre soybeans treated as in a and b above.
Which crop leaves the most residue? Which system is likely to have the least erosion?
1. If you are not already familiar with the tillage tools described in this chapter, visit an equipment dealer to look at them.
2. Survey and visit local farms to find out what tillage and cropping systems they use.
3. Study organic production certification rules for your state or the rules for the federal program (Federal Register, 7 CFR Part 205: National Organic Program; Final Rule. December 21, 2000). Typing "organic certification and [name of state]" into your browser should find information.
4. An excellent source of information on cover crops is at <http://www.attra.org/attra-pub/covercrop.html>.
(1) Lichtenberg, E., et al. (1994). Profitability of legume cover crops in the Mid-Atlantic Region. J. Soil & Water Conservation 49(6):582-585.
(2) B. G. McConkey et al. (1990). "Perennial Grass Windbreaks for Continuous Wheat Production on the Canadian Prairies." Journal of Soil and Water Conservation, 45, (4): 482-485.
(3) National Academy of Sciences. 1994. Rangeland health: New methods to classify, inventory, and monitor rangelands. National Academy Press.
(4) ASA (1989). "Decisions Reached on Sustainable Agriculture. Agron. News, Jan 1996, p. 10.
(5) Wienhold, B., Hendrickson, J., & Karn, J. 2001. Pasture management influences on soil properties in the northern Great Plains. J. Soil Water Cons. 56(1): 27-31.
(6) Rice, P. et al. (2001). Runoff loss of pesticides and soil: A comparison between vegetative mulch and plastic mulch in vegetable production systems. J. Environmental Quality 30(5): 1808-1821.
(7) Rice, P. et al. (2002). Comparison of copper levels in runoff from freshmarket vegetable production using polyethylene mulch or a vegetative mulch. Environmental Toxicology and Chemistry 21(1): 24-30.
(8) Brumfield, R. 2000. An examination of the economics of sustainable and conventional horticulture. Hort Technology 20(4):687-691.
FIGURE 16-1 Crop residues in pounds per acre for several crops. To obtain these values, the number of pounds residue per bushel of grain is multiplied by the per-acre yield. Sample yields may not represent yields in your area. Approximate Residue per Bushel Sample Yield Sample Residue Crop Grain (lb/acre) (bu/acre) (lb/acre) Barley 80 x 50 = 4,000 Corn 56 125 7,000 Flax 80 15 1,200 Oats 60 32 4,300 Rye 100 30 3,000 Sorghum 60 50 3,000 Soybeans 50 40 2,000 Wheat 100 40 4,000 FIGURE 16-2 Percentage residue remaining after one pass over the field for various tillage tools. If two or more tillage operations are practiced, each operation after the first uncovers about half the amount it covers. Estimated Percentage of Residue Remaining Implement After Each Operation Inverting tools Moldboard plow 5 Lister plow 20 Mixing tools Field cultivator 80 Chisel plow, spear point 80 Chisel plow, twisted point 50 Rototill to 6 inches 25 Rototill to 3 inches 50 Tandem disc to 6 inches 25 Tandem disc to 3 inches 50 Spring-tooth harrow 60 Spike-tooth harrow 70 Subsurface tools Blades or sweeps 90 Rodweeders 90 FIGURE 16-15 Average corn yield after tillage treatment for continuous corn in Ohio. The soil is a silt loam. On this soil, no-till gave the best results. (Source: Ohio Agricultural Research and Development Center) Tillage System Corn Yield (bu/acre) Conventional tillage 142 Chisel plow 138 No-till, after harvest for silage 142 No-till, after harvest for grain 144