Building Soils for Better Crops, 2nd Edition

	Crop Rotations ... with methods of farming in which grasses form an important part of the rotation, especially those that leave a large residue of roots and culms, the decline of the productive power is much slower than when crops like wheat, cotton, or potatoes, which leave little residue on the soil, are grown continuously. Henry Snyder, 1896 There are very good reasons to rotate crops. Rotating crops usually means fewer problems with insects, parasitic nematodes, weeds, and diseases caused by bacteria, viruses, and fungi. Rotations are effective for controlling insects like the corn rootworm, nematodes like the soybean cyst nematode, and diseases like root rot of field peas. In addition, rotations that include legumes supply nitrogen to succeeding crops. Growing sod-type forage grasses, legumes, and grass-legume mixes as part of the rotation also increases soil organic matter. When you alternate two crops, such as corn and soybeans, you have a very simple rotation. More complex rotations require three or more crops and a five- to 10-year (or more) cycle to complete. Rotations are an important part of any sustainable agricultural system. Yields of crops grown in rotations are frequently about 10 percent higher than when grown in monoculture. When you grow a grain or vegetable crop following a legume, the extra supply of nitrogen certainly helps. However, yields of crops grown in rotation are often higher than in monoculture, even when both are supplied with plentiful amounts of nitrogen. In addition, following a non-legume crop with another nonlegume also produces higher yields than a monoculture. For example, when you grow corn following grass hay, or cotton following corn, you get higher yields than when corn or cotton are grown year after year. This yield benefit from rotations is sometimes called a rotation effect. Another important benefit of rotations is that growing a variety of crops in a given year spreads out labor needs and reduces risk caused by climate or market conditions. Rotations Influence Soil Organic Matter Levels You might think you're doing pretty well if soil organic matter remains the same under a particular cropping system. However, if you are working soils with depleted organic matter, you need to build up levels to counter the effects of previous practices. Maintaining an inadequately low level of organic matter won't do! The types of crops you grow, their yields, the amount of roots produced, the portion of the crop that is harvested, and how you treat crop residues will all affect soil organic matter. Soil fertility itself influences the amount of organic residues returned, because more fertile soils grow higher-yielding crops, with more residues. The decrease in organic matter levels when row crops are planted on a virgin forest or prairie soil is very rapid for the first five to 10 years, but eventually, a plateau or equilibrium is reached. After that, soil organic matter levels remain stable, as long as production practices aren't changed. An example of what can occur during 25 years of continuously grown corn is given in figure 11.1. Soil organic matter levels increase when the cropping system is changed from a cultivated crop to a grass or mixed grass-legume sod. However, the increase is usually much slower than the decrease that occurred under continuous tillage. Figure 11.1 Organic matter changes in the plow layer during long-term cultivation followed by hay-crop establishment.   A long-term cropping experiment in Missouri compared continuous corn to continuous sod and various rotations. More than 9 inches of topsoil was lost during 60 years of continuous corn. The amount of soil lost each year from the continuous corn plots was equivalent to 21 tons per acre. After 60 years, soil under continuous corn had only 44 percent as much topsoil as that under continuous timothy sod. A six-year rotation consisting of corn, oats, wheat, clover, and two years of timothy resulted in about 70 percent as much topsoil as found in the timothy soil, a much better result than with continuous corn. Differences in erosion and organic matter decomposition resulted in soil organic matter levels of 2.2 percent for the unfertilized timothy and 1.2 percent for the continuous corn plots. Two things happen when perennial forages (hay-type crops) are part of the rotation and remain in place for some years during a rotation. First, the rate of decomposition of soil organic matter decreases, because the soil is not continually being disturbed. (This also happens when using no-till planting, even for non-sod-type crops, such as corn.) Second, grass and legume sods develop extensive root systems, part of which will naturally die each year, adding new organic matter to the soil. Crops with extensive root systems stimulate high levels of soil biological activity. The roots of a healthy grass or legume-grass sod return more organic matter to the soil than roots of most other crops. Older roots of grasses die, even during the growing season, and provide sources of fresh, active organic matter. Roots of plants also continually give off, or exude, a variety of chemicals that nourish nearby microorganisms. We are not only interested in total soil organic matter we want a wide variety of different types of organisms living in the soil. We also want to have a good amount of active organic matter and high levels of well decomposed soil organic matter, or humus, in the soil. Although most experiments have compared soil organic matter changes under different cropping systems, few experiments have looked at the effects of rotations on soil ecology. The more residues your crops leave in the field, the greater the populations of soil microorganisms. Experiments in a semiarid region in Oregon found that the total amount of microorganisms in a two-year wheat-fallow system was only about 25 percent of the amount found under pasture. Conventional moldboard plow tillage systems are known to decrease the populations of earthworms, as well as other soil organisms. More complex rotations increase soil biological diversity. Including perennial forages in the rotation enhances this effect. Residue Availability As pointed out in chapters 3, 5, and 8, more residues are left in the field after some crops than others. High residue-producing crops should be incorporated into rotations whenever possible. Species Richness and Active Rooting Periods In addition to the quantity of residues remaining following harvest, a variety of types of residues is also important. The goal should be a minimum of three different species in a rotation, with more if possible. The percent of the time that living roots are present during a rotation is also important. The period that active roots are present varies considerably, ranging from 32 percent of a corn-soybeans rotation to 57 percent of the time for a beans-wheat rotation to 76 percent of the time for a 3-year beans-wheat-corn rotation (table 11.1). Farm Labor and Economics Before discussing appropriate rotations, let's consider some of the possible effects on farm labor and finances. If you grow only one or two row crops, you must work incredibly long hours during planting and harvesting seasons. Including forage hay crops and early harvested crops, along with those that are traditionally harvested in the fall, allows farmers to spread their labor over the growing season, making the farm more easily managed by family labor alone. In addition, when you grow a more diversified group of crops, you are less affected by price fluctuations of one or two crops. This may provide more year-to-year financial stability. Although, as pointed out above, there are many possible benefits of rotations, there are also some costs or complicating factors. It is critically important to carefully consider the farm family's labor and management capacity when exploring diversification opportunities. You may need more equipment to grow a number of different crops. There may be conflicts between labor needs for different crops; cultivation and sidedressing nitrogen fertilizer for corn in some locations might occur at the same time as harvesting hay. In addition, the more diversified the farm, the less chance for time to relax. General Principles Try to consider the following principles when you're thinking about a new rotation: 1. Follow a legume forage crop, such as clover or alfalfa, with a high nitrogen-demanding crop, such as corn, to take advantage of the nitrogen supply. 2. Grow less nitrogen-demanding crops, such as oats, barley, or wheat, in the second or third year after a legume sod. 3. Grow the same annual crop for only one year, if possible, to decrease the likelihood of insects, diseases, and nematodes becoming a problem. 4. Don't follow one crop with another closely related species, since insect, disease, and nematode problems are frequently shared by members of closely related crops. 5. Use crop sequences that promote healthier crops. Some crops seem to do well following a particular crop (for example, cabbage family crops following onions, or potatoes following corn). Other crop sequences may have adverse effects, as when potatoes have more scab following peas or oats. 6. Use crop sequences that aid in controlling weeds. Small grains compete strongly against weeds and may inhibit germination of weed seeds, row crops permit mid-season cultivation, and sod crops that are mowed regularly or intensively grazed help control annual weeds. 7. Use longer periods of perennial crops, such as a forage legume, on sloping land and on highly erosive soils. Using sound conservation practices, such as no-till planting, extensive cover cropping, or strip-cropping (a practice that combines the benefits of rotations and erosion control), may lessen the need to follow this guideline. 8. Try to grow a deep-rooted crop, such as alfalfa, safflower, or sunflower, as part of the rotation. These crops scavenge the subsoil for nutrients and water, and channels left from decayed roots can promote water infiltration. 9. Grow some crops that will leave a significant amount of residue, like sorghum or corn harvested for grain, to help maintain organic matter levels. 10. When growing a wide mix of crops as is done on many direct marketing vegetable farms try grouping into blocks according to plant family, timing of crops (all early season crops together, for example), type of crop (root vs. fruit vs. leaf), or crops with similar cultural practices (irrigated, using plastic mulch).   Rotation Examples It's impossible to recommend specific rotations for a wide variety of situations. Every farm has its own unique combination of soil and climate and of human, animal, and machine resources. The economic conditions and needs are also different on each farm. You may get useful ideas by considering a number of rotations with historical or current importance. A five- to seven- year rotation was common in the mixed livestock-crop farms of the northern Midwest and Northeast during the first half of the 20th century. An example of this rotation is the following: Year 1. Corn Year 2. Oats (mixed legume/grass hay seeded) Years 3, 4, and 5. Mixed grass-legume hay Years 7 and 8. Pasture The most nitrogen-demanding crop, corn, followed the pasture, and grain was harvested only two of every five to seven years. A less nitrogen-demanding crop, oats, was planted in the second year as a "nurse crop" when the grass-legume hay was seeded. The grain was harvested as animal feed and oat straw was harvested to be used as cattle bedding; both eventually were returned to the soil as animal manure. This rotation maintained soil organic matter in many situations, or at least didn't cause it to decrease too much. On prairie soils, with their very high original contents of organic matter, levels still probably decreased with this rotation. For many years, the western corn rootworm was effectively controlled by alternating between corn and soybeans. Recently, populations of the rootworm with a longer resting period have developed and they are able to survive the very simple rotation. In the corn belt region of the Midwest, a change in rotations occurred as pesticides and fertilizers became readily available and animals were fed in large feedlots, instead of on crop-producing farms. Once the mixed livestock farms became grain-crop farms or crop-hog farms, there was little reason to grow sod crops. In addition, government commodity price support programs unintentionally encouraged farmers to narrow production to just two feed grains. The two-year corn-soybean rotation is better than monoculture, but it has a number of problems, including erosion, groundwater pollution with nitrate and herbicides, depletion of soil organic matter, and increased insect problems (see box). Research indicates that with high yields of corn grain there may be sufficient residues to maintain organic matter. With soybeans, residues are minimal. The Thompson mixed crop-livestock (hogs and beef) farm in Iowa practices an alternate seven-year corn belt rotation similar to the first rotation we described. For fields that are convenient for pasturing beef cows, the Thompson rotation is as follows: Year 1. Corn Year 2. Soybeans Year 3. Corn Year 4. Oats (mixed legume/grass hay seeded) Years 5, 6, and 7. Mixed grass-legume hay Organic matter is maintained through a combination of practices that include the use of manures and municipal sewage sludge, green manure crops (oats and rye following soybeans and hairy vetch between corn and soybeans), crop residues, and sod crops. These practices have resulted in a porous soil that has significantly lower erosion, higher organic matter content, and more earthworms than neighbors' fields A four-year rotation under investigation in Virginia uses mainly no-till practices as follows: Year 1. Corn, winter wheat no-till planted into corn stubble. Year 2. Winter wheat grazed by cattle, foxtail millet no-till planted into wheat stubble and hayed or grazed, alfalfa no-till planted in fall. Year 3. Alfalfa harvested and/or grazed. Year 4. Alfalfa is harvested and/or grazed as usual until fall, then heavily stocked with animals to weaken it so that corn can be planted the next year. This rotation follows many of the principles discussed earlier in this chapter. It was designed by researchers, extension specialists, and farmers and is very similar to the older rotation described earlier. A few differences exist this rotation is shorter, alfalfa is used instead of clover or clover-grass mixtures, and there is a special effort to minimize pesticide use under no-till practices. Weed-control problems occurred when going from alfalfa (fourth year) back to corn. This caused the investigators to use fall tillage followed by a cover crop mixture of winter rye and hairy vetch. Some success was achieved suppressing the cover crop in the spring by just rolling over it with a disk harrow and planting corn through the surface residues with a modified no-till planter. The heavy cover crop residues on the surface provided excellent weed control for the corn. Traditional wheat-cropping patterns for the semi-arid regions of the Great Plains and the Northwest commonly include a fallow year to allow storage of water and more mineralization of nitrogen from organic matter for use by the next wheat crop. However, the wheat-fallow system has several problems. Because no crop residues are returned during the fallow year, soil organic matter decreases unless manure or other organic materials are provided from off the field. Water infiltrating below the root zone during the fallow year moves salts through the soil to the low parts of fields. Shallow groundwater can come to the surface in these low spots and create "saline seeps," where yields will be decreased. Increased soil erosion, caused by either wind or water, commonly occurs during fallow years and organic matter decreases (at about 2 percent per year, in one experiment). In this wheat monoculture system, the build-up of grassy weed populations, such as jointed goat grass and downy brome, also indicates that crop diversification is essential. Farmers in this region who are trying to develop more sustainable cropping systems should consider using a number of species, including deeper-rooted crops, in a more diversified rotation. This would increase the amount of residues returned to the soil, reduce tillage, and lessen or eliminate the fallow period. A four-year wheat-corn-millet-fallow rotation under evaluation in Colorado was found to be better than the traditional wheat-fallow system. Wheat yields have been higher in this rotation than wheat grown in monoculture. The extra residues from the corn and millet also are helping to increase soil organic matter. Many producers are also including sunflower, a deep-rooting crop, in a wheat-corn-sunflower-fallow rotation. Sunflower is also being evaluated in Oregon as part of a wheat cropping sequence. Vegetable farmers who grow a large selection of crops find it best to rotate in large blocks with each containing crops from the same families or having similar production schedules or cultural practices. Many farmers are now using cover crops to help "grow their own nitrogen," utilize extra nitrogen that might be there at the end of the season, and add organic matter to the soil. A four- to five-year vegetable rotation might be as follows: Year 1. Sweet corn followed by a hairy vetch/winter rye cover crop. Year 2. Pumpkins, winter squash, summer squash followed by a rye or oats cover crop. Year 3. Tomatoes, potatoes, peppers followed by a vetch/rye cover crop. Year 4. Crucifers, greens, legumes, carrots, onions, and miscellaneous vegetables followed by a rye cover crop. Year 5. (If land is available) Oats and red clover or buckwheat followed by a vetch/rye cover crop. Another rotation for vegetable growers uses a two- to three-year alfalfa sod as part of a six- to eight-year cycle. In this case, the crops following the alfalfa are high-nitrogen-demanding crops, such as corn or squash, followed by cabbage or tomatoes, and, in the last two years, crops needing a fine seedbed, such as lettuce, onions, or carrots. Annual weeds in this rotation are controlled by the harvesting of alfalfa a number of times each year. Perennial weed populations can be decreased by cultivation during the row-crop phase of the rotation. Most vegetable farmers do not have enough land or the markets to have a multi-year hay crop on a significant portion of their land. Aggressive use of cover crops will help to maintain organic matter in this situation. Manures, composts, or other sources of organic materials, such as leaves, should also be applied every year or two to help maintain soil organic matter. Sources Anderson, S.H., C.J. Gantzer, and J.R. Brown. 1990. Soil physical properties after 100 years of continuous cultivation. Journal of Soil and Water Conservation 45:117­121. Baldock, J.0., and R.B. Musgrave. 1980. Manure and mineral fertilizer effects in continuous and rotational crop sequences in central New York. Agronomy Journal 72:511­518. Barber, S.A. 1979. Corn residue management and soil organic matter. Agronomy Journal 71:625­627. Michigan Field Crop Ecology: Managing Biological Processes for Productivity and Environmental Quality. 1998. Cavigelli, M. A., S. R. Deming, L. K. Probyn, and R. R. Harwood (eds.). Michigan State University Extension Bulletin E-2646. East Lansing, MI. Coleman, E. 1989. The New Organic Grower. Chelsea Green. Chelsea, VT. See this reference for the vegetable rotation. Francis, C.A., and M.D. Clegg. 1990. Crop rotations in sustainable production systems. In Sustainable Agricultural Systems (C.A. Edwards, R. Lal, P. Madden, R.H. Miller, and G. House, eds.). Soil and Water Conservation Society. Ankeny, IA. Gantzer, C.J., S.H. Anderson, A.L. Thompson, and J.R. Brown. 1991. Evaluation of soil loss after 100 years of soil and crop management. Agronomy Journal 83:74­77. This source describes the long-term cropping experiment in Missouri. Grubinger, V.P. 1999. Sustainable Vegetable Production: From Start-Up to Market. Natural Resource and Agricultural Engineering Service, Ithaca, NY. Havlin, J.L., D.E. Kissel, L.D. Maddux, M.M. Claassen, and J.H. Long. 1990. Crop rotation and tillage effects on soil organic carbon and nitrogen. Soil Science Society of America Journal 54:448­452. Luna, J.M., V.G. Allen, W.L. Daniels, J.F. Fontenot, P.G. Sullivan, C.A. Lamb, N.D. Stone, D.V. Vaughan, E.S. Hagood, and D.B. Taylor. 1991. Low-input crop and livestock systems in the southeastern United States. pp. 183-205. In Sustainable Agriculture Research and Education in the Field. Proceedings of a conference, April 3­4, 1990. Board on Agriculture, National Research Council. Washington, D.C.: National Academy Press. This is the reference for the rotation experiment in Virginia. National Research Council. 1989. Alternative Agriculture. National Academy Press. Washington, D.C. This is the reference for the rotation used on the Thompson farm. Peterson, G.A., and D.G. Westfall. 1990. Sustainable dryland agroecosystems. In Conservation Tillage. Proceedings of the Great Plains Conservation Tillage System Symposium, August 21­23, 1990, Bismark, ND. Great Plains Agricultural Council Bulletin No. 131. See this reference for the wheat-corn-millet-fallow rotation under evaluation in Colorado. Rasmussen, P.E., H.P. Collins, and R.W. Smiley. 1989. Long-Term Management Effects on Soil Productivity and Crop Yield in Semi-Arid Regions of Eastern Oregon. USDA-Agricultural Research Service and Oregon State University Agricultural Experiment Station, Columbia Basin Agricultural Research Center, Pendleton, OR. This describes the Oregon study of sunflowers as part of a wheat cropping sequence. Werner, M.R., and D.L. Dindal. 1990. Effects of conversion to organic agricultural practices on soil biota. American Journal of Alternative Agriculture 5(1): 24­32.     Top