... 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 multiple ways to diversify cropping systems, and there are good reasons to do so. One important ecological principle is that diversity contributes to stability and productivity (see discussion in Chapter 8). Diversity over time in a field, year to year, is achieved by using cover crops and by rotating a number of crops. You can also diversify spatially, or across your farm’s landscape, by planting different crops in different fields or in strips within fields. A well-planned crop rotation—for example, one that might use the same sequence of crops but staggered differently from one field to the next in a given year or season—provides diversification both over time and across the farm landscape. Crop diversification can also occur with less frequent rotations by using perennials. For example, dairy farmers may grow alfalfa for three to four years before it is rotated to corn. Agroforestry, the growing of tree species together with annual crops or other perennials, affords another way of adding habitat diversity to a farm. Integrating livestock and cropping brings yet another dimension of diversity by introducing animals onto the farm. Of course, all three—crop rotations, agroforestry and integrating livestock—can be practiced together. In this chapter we’ll discuss crop rotations and agroforestry. Integrating crops and livestock is discussed separately in the following chapter (Chapter 12).
Rotating crops usually means more income diversity and fewer problems with insects, parasitic nematodes, weeds and diseases caused by plant pathogens. Rotations that include nonhost plants are effective for controlling insects like corn rootworms, nematodes like the soybean cyst nematode, and diseases like root rot of field peas. In order to suppress specific soil diseases, the length of time between growing the same or a similar crop may vary from relatively short (one to two years for leaf blight of onions) to fairly long (seven years for clubroot of radishes or turnips). Crops that actually suppress a disease may do so by encouraging diversity of soil organisms that outcompete or consume plant pathogens. Root growth may be adversely affected when continuously cropping to any single crop (see Figure 11.1). This means that the crop may be less efficient in using soil nutrients and added fertilizers. In addition, rotations that include legumes may supply varying amounts of nitrogen to succeeding crops. An annual legume harvested for seed, such as soybeans, provides little nitrogen for the following crop. On the other hand, a multiyear legume sod such as alfalfa may well supply all the nitrogen needed by the following crop. Growing sod-type forage grasses, legumes and grass-legume mixes as part of the rotation also increases soil organic matter. When you alternate two warm season crops, such as corn and soybeans, you have a very simple rotation that, unless cover crops are used as well, leaves the soil bare for long periods of time. More complex rotations with both warm- and cool-season crops 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 typically 10% higher than those of crops grown in monoculture in normal growing seasons and as much as 25% higher in droughty growing seasons. Rotations involving three or more crops with different characteristics generally lead to positive changes in soil health, thus enhancing crop growth. And when you grow a grain or vegetable crop following a forage legume, the extra supply of nitrogen certainly helps. In fact, yields of crops grown in rotation are often still higher than those of crops grown in monoculture, even when both are supplied with plentiful amounts of nitrogen. Research in Iowa found that even using 240 pounds of nitrogen per acre when growing corn after corn, yields were not as good as corn following alfalfa with little or no nitrogen applied. In addition, following a nonlegume crop with another nonlegume produces higher yields than a monoculture, when using recommended fertilizer rates. For example, when you grow corn following grass hay, or cotton following corn, you get higher yields than when corn or cotton is 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 unexpected climate or market conditions. Other benefits may occur when perennial forages (hay-type crops) are included in the rotation, including decreased soil erosion and nutrient loss. Yields of corn in complex rotations are greater compared to a monoculture or simple rotation both in years of favorable conditions as well as in years when conditions are unfavorable, such as droughty or excessively wet years.
Crop and Varietal Mixtures
Not only do rotations help in many ways, but growing mixtures of different crops and even different varieties (cultivars) of a given crop sometimes offers real advantages. For example, faba (fava) beans help corn to get phosphorus on low phosphorus soils by acidifying the area around its roots. Also, when some varieties of a species are prized for a certain quality, such as taste, but are susceptible to a particular pest, growing a number of rows of the susceptible variety alternating with rows of resistant varieties tends to lessen the severity of the pest damage.
Rotations and 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 inadequate 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 harvested and how you manage 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 aboveground and belowground. Therefore, when soils have become depleted of organic matter due to simple rotations and nutrients being supplied only through inorganic fertilizers, stopping the application of those fertilizers is not the solution. Fertility levels still need to be maintained while also changing the rotation to improve soil health.
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 low level 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 11.2. 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 because rotations that include perennials reduce the total amount of tillage and the associated soil organic matter losses.
A long-term cropping experiment in Missouri compared continuous corn to continuous sod and various rotations. More than 9 inches of topsoil were 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% 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% as much topsoil as was 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% for the unfertilized timothy and 1.2% for the continuous corn plots.
In an experiment in eastern Canada, continuous corn led to annual increases in organic matter of about 100 pounds per acre, while two years of corn followed by two years of alfalfa increased organic matter by about 500 pounds per acre per year, and four years of alfalfa increased organic matter by 800 pounds per acre per year. Keep in mind that these amounts are small compared to the amounts of organic matter in most soils: 3% organic matter represents about 60,000 pounds per acre to a depth of 6 inches. Also, as soil organic matter increases to such an extent that mineral surfaces are fully saturated with organic matter and the soil is already highly aggregated, organic matter content increases plateau no matter how much is added in crop residue, manures and composts.
Two things happen when perennial forages 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 crops such as corn.) Second, grass and legume sods develop extensive root systems, part of which will naturally die each year and add new organic matter to the soil. Crops with extensive root systems stimulate high levels of soil biological activity and soil aggregation. The roots of a healthy grass or legume-grass sod return more organic matter to the soil than do roots of most other crops. Older roots of grasses die, even during the growing season, and provide sources of fresh, active organic matter. Rotations that included three years of perennial forage crops have been found to produce a very high-quality soil in the corn and soybean belt of the Midwest.
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 (to provide food for soil life), high levels of organic matter inside aggregates (to help form and stabilize them), and well-decomposed soil organic matter, or humus (to provide more cation exchange capacity). 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% of the amount found under pasture. Conventional moldboard plow tillage systems are known to decrease the populations of earthworms and other soil organisms. More complex rotations increase soil biological diversity. Including perennial forages in the rotation enhances this effect.
Rotations and energy use, climate change impacts and potential human health impacts
An experiment comparing a typical corn-soybean crop alternation with a rotation that adds a year of oats and red clover (harvesting oats and straw) or a two-year alfalfa crop found many improvements with the more complex rotations: they resulted in less energy use, lower greenhouse gas emissions and better air quality without “compromising economic or agronomic performance.” We know from many other experiments that complex rotations improved soil health in many ways: biologically, physically and chemically.
—Hunt et al. (2020)
More residues are left in the field after some crops than others, as pointed out in chapters 3 and 9. High-residue-producing crops, especially those with extensive root systems, should be incorporated into rotations whenever possible. There is considerable interest in the possible future use of crop residue for a variety of purposes, such as small grain straw for bedding and mulching, or corn stover for producing biofuel. However, farmers should keep in mind that frequent removal of significant quantities of residue from their fields—and there may be more pressure to remove them if production of biofuels from crop residue becomes economically viable—can have a very negative effect on the soil’s health.
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 goals should be to 1) rotate annuals and perennials, and 2) include different species in a rotation, three or more if possible. When compared with row crop monocultures, rotations tend to increase soil organic matter, nitrogen and the mass of microorganisms. Cover crops can help achieve the same goals but may not reach the full benefits of a perennial or biennial crop.
The percent of the time that living roots are present during a rotation is important. The period that active roots are present varies considerably, ranging from 32% of the time for a corn-soybean rotation to 57% for a soybean-wheat rotation to 76% for a three-year, soybean-wheat-corn rotation (Table 11.1). Just adding winter wheat to a corn-soybean alternation can greatly increase the time that active roots are present. (Doing so also assists in controlling weeds, increases corn yields and provides another crop to sell.) This is primarily the result of the fact that winter annuals, perennials and cover crops extend the growing period compared to summer annuals. As mentioned above, when soils are covered with living vegetation for a longer period of time, there tends to be decreased erosion, decreased loss of nitrate and less groundwater contamination.
|Table 11.1 Comparison of Rotations: Percent of Time Active Roots Are Present and Number of Species|
|Rotation||Years||Active Rooting Period (%)||Number of Species|
|Corn–dry beans–winter wheat/cover crop||3||76||4|
|Source: Cavigelli et al. (1998)|
Rotations, Water Quality and Gaseous Losses of N
Diversified rotations offer many benefits when compared to very simple ones. For example, an experiment in South Dakota compared the simple corn-soybean crop alternation with a four-year corn-field pea-winter wheat-soybean rotation. Researchers found that, compared to corn-soybean alternation, soybean yields were higher in the four-year rotation, more organic matter accumulated in the soil, and less nitrous oxide gas (N2O), a greenhouse gas, was lost to the atmosphere. When annual crops are grown and planted in the spring, such as with corn and soybeans, there is a considerable amount of time when the soil is not occupied by living plant roots. This means that for a large portion of the year there are no living plants to take up nutrients, especially nitrate, that can leach out of the soil. This is especially a problem in the midwestern and northeastern United States, where many soils have tile drainage, which accentuates the discharge of high-nitrate water into streams and rivers. In addition to not taking up nutrients, the lack of growing plants means that the soils are wetter and more apt to produce runoff, erosion and leaching. Thus, rotations that include perennial forages and winter grains help maintain or enhance the quality of both ground and surface waters. And, while intensive use of cover crops helps water quality in a similar way, cover crops should not be viewed as a substitute for a good rotation of economic crops. It’s the combination of the positive effects of both good rotations and routine cover crop use that provides the greatest improvements in soil physical, chemical and biological characteristics.
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, and not as much at other times. Including forage hay crops and early harvested crops along with those that are traditionally harvested in the fall would allow you to spread your labor over the growing season, which would make the farm more easy to manage 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-round income and year-to-year financial stability. On the other hand, you can add diversity to the farm even without changing your rotation by growing cover crops that don't need to be harvested or sold (see Chapter 10).
Crop Rotations and Plant Diseases
Carefully selected rotations, especially when alternating between grains and broadleaf plants, can greatly assist control of plant diseases and nematodes.
Sometimes a one-year break is sufficient for disease control, while for other diseases a number of years of growing a nonhost crop is needed to sufficiently reduce inoculum levels. Inclusion of pulse crops in a rotation seems to stimulate beneficial organisms and reduce the severity of cereal root diseases. Severity of common root rot of wheat and barley is reduced by taking a multiyear break to grow broadleaf plants. Rotations can be relatively easy to develop for control of diseases and nematodes that have a fairly narrow host range. However, some diseases or nematodes have a wider host range, and more care is needed in developing or changing rotations if these are present. In addition, some diseases enter the field on contaminated seed, while others, like wheat leaf rust, can travel with the wind for long distances. Other tactics, aside from rotations, are needed to deal with such diseases.
—Krupinsky et al. (2002)
While there are many possible benefits of rotations, there are also some costs or complicating factors. It is critically important to carefully consider the farm’s labor, management capacity and markets 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, like weed cultivation and side-dressing nitrogen fertilizer for corn at the same time as harvesting hay. In addition, some tasks, such as harvesting dry hay (mowing, tedding when needed, baling and storing) can require quite a bit of labor that may not always be available. And the more diversified the farm, the less chance for time to relax.
For many farmers the solution is to diversify even further and bring livestock onto the farm. Well-integrated livestock-crop operations with multi-species grazing have less need for specialized equipment, and the animals can do much of their own harvesting and manure spreading during the grazing season, saving human labor. It also diversifies farm income and overall helps cycle nutrients and carbon on the farm.
Try to consider the following principles when you’re thinking about a new rotation:
- 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.
- Grow less of nitrogen-demanding crops, such as oats, barley and wheat, in the second or third year after a legume sod.
- If possible, grow the same annual crop for only one year to decrease the likelihood of insects, diseases and nematodes becoming a problem. (Note: 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 in isolated regions, and they are able to survive this simple two-year rotation.)
- Don’t follow a crop with a closely related species, since insect, disease and nematode problems are frequently shared by members of closely related crops.
- If specific nematodes are known problems, consider planting nonhost plants, such as grain crops for root-knot nematodes, for a few years to decrease populations before planting a very susceptible crop such as carrots or lettuce. High populations of plant parasitic nematodes will also affect the choice of cover crops (see Chapter 10 for a discussion of cover crops).
- 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.
- Consider livestock as part of a rotational cropping system. Perennial fodder crops have many benefits, and these benefits are enhanced when livestock are grazing them in pastures. In fact, a rotational grazing system can be incorporated as a rotation of animals within a rotation of crops.
- Use crop sequences that aid in controlling weeds. Small grains compete strongly against weeds and may inhibit germination of weed seeds; row crops permit midseason cultivation; and sod crops that are mowed regularly or grazed intensively help control annual weeds. Also, rotations including both cool season crops and warm season crops may aid in lowering weed populations. And as weeds develop resistance to more pesticides, it is increasingly important to explore crop sequences that give more opportunities to suppress them.
- 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.
- Try to grow a deep-rooted crop, such as alfalfa, safflowers or sunflowers, as part of the rotation. These crops scavenge the subsoil for nutrients and water, and channels left from decayed roots can promote water infiltration.
- Grow some crops that leave a significant amount of residue, provide a surface mulch for reduced tillage systems, and, together with their roots, maintain or increase organic matter levels. Examples include sorghum or corn harvested for grain.
- 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 (group all early season crops together, for example), type of crop (root versus fruit versus leaf) or cultural practices (for example, if irrigation or plastic mulch are used).
- In regions with limited rainfall, the amount of water used by a crop may be a critically important issue, usually one of the most important issues. The amount of soil water at the time of planting may determine whether to grow a particular crop. Without plentiful irrigation, growing high-water-use crops such as hay, as well as sunflowers and safflowers, may not leave sufficient moisture in the soil for the next crop in the rotation.
- Be flexible enough to adapt to annual climate and crop price variations, as well as to development of soil pathogens and plant parasitic nematodes. For example, dryland rotations have been introduced in the Great Plains to replace the wheat-fallow system, resulting in better water use and less soil erosion. (It is estimated that less than 25% of the rainfall that falls during the 14-month fallow period in the Central High Plains is made available to a following crop of winter wheat.) (See the box “Flexible Cropping Systems” and Table 11.2 for discussion and information on flexible, or dynamic, cropping systems.) Growing winter small grains in a rotation offers a number of possibilities depending on weather and the farm’s needs. Winter grains can serve as a cover crop (killed in the spring while still in the vegetative state), be grazed in the spring if feed is needed, or, if it’s very wet in the spring, be allowed to mature and the grain harvested.
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 in each region and 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 the Northeast during the first half of the 20th century. An example of this rotation:
Year 1. Corn
Year 2. Oats (mixed legume–grass hay seeded)
Years 3, 4 and 5. Mixed grass–legume hay
Years 6 and 7. 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.
In the Corn Belt region of the Midwest, a change in rotations occurred as pesticides and fertilizers became readily available, animals were fed in large feedlots instead of on integrated crop-livestock farms, and grain export markets were developed. 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 nitrates and herbicides, soil organic matter depletion, and in some situations, increased insect problems. Soybeans leave minimal amounts of residues. But research indicates that with high yields of corn grain in a soybean-corn rotation there may be sufficient residues to maintain organic matter. For many years, the Thompson mixed crop-livestock (hogs and beef) farm in Iowa practiced an alternative five-year Corn Belt rotation similar to the first rotation we described: corn-soybeans-corn-oats (mixed/grass hay seeded)-hay. For fields that are convenient for pasturing beef cows, the Thompson eight-year rotation is as follows:
Year 1. Corn (cereal rye/hairy vetch cover crop)
Year 2. Soybeans
Year 3. Oats (mixed/grass hay seeded)
Years 4 to 8. Pasture
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 researched in Virginia used mainly no-till practices as follows:
Year 1. Corn, with winter wheat no-till planted into corn stubble
Year 2. Winter wheat grazed by cattle after harvest; 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 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 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 cereal rye and hairy vetch. Some success was achieved suppressing the cover crop in the spring by just rolling over it with a harrow (with similar effects as a roller/crimper) 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 semiarid regions of the Great Plains and the Northwest commonly include a fallow year to allow storage of water and more nitrogen mineralization from organic matter for the next wheat crop to use. However, the two-year 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 a rate of about 2% per year, in one experiment). In this wheat monoculture system, the buildup of grassy weed populations, such as jointed goat grass and downy brome, also indicates that crop diversification is essential.
Farmers in the dryland regions trying to develop more sustainable cropping systems are considering 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. (See “Flexible Cropping Systems” box.) In the 1970s some farmers began switching from the two-year wheat-fallow system to a three year rotation, commonly winter wheat-grain sorghum (or corn)-fallow. When this rotation is combined with no-till, accumulated surface residues help maintain higher soil moisture levels. 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 are also helping to increase soil organic matter.
Many producers are including sunflowers, a deep-rooting crop, in a wheat-corn-sunflower-fallow rotation. Sunflowers are also being evaluated in Oregon as part of a wheat cropping sequence.
Another approach to rotations in the semi-arid Great Plains of North Dakota combines crop and livestock farming; it uses a multi-species rotation in place of continuous hard red spring wheat. This five-year rotation includes only two cash crops (wheat and sunflowers) with grazing crops grown for three years:
Year 1. Hard red spring wheat (cash crop) with winter triticale and hairy vetch planted after wheat harvest in September
Year 2. Triticale-vetch hay harvested in June. A cover crop consisting of a seven- to 13-species mix is seeded as soon as possible after the hay harvest and then grazed by either cows or yearling steers
Year 3. A silage-type corn variety is planted and grazed first by yearling steers and then by cows in a “leader-follower” grazing plan
Year 4. A field pea-forage barley mix is grazed by yearling steers
Year 5. Sunflowers (cash crop)
Sodic seeps and subsurface sodic clay layers are sometimes found in semi-arid regions and may limit crop growth. (See Chapter 6 for discussion of saline and sodic soils, and for their reclamation see Chapter 20). During the cover crop year of a multi-crop rotation such as the one discussed just above, including adapted crop-types with taproots such as tillage radishes, sunflowers, safflowers, mustard, and canola, as well as sodium-tolerant crops like barley, aids in remediating problem soils when coupled with a diverse crop rotation on all farm acres.
Flexible Cropping Systems
As discussed in point 14 under “General Principles,” it may be best for many farmers to adopt more “dynamic” crop sequences rather than to strictly adhere to a particular sequence. Many things change from year to year, including prices paid for crops, pest pressures and climate. And many farmers do deviate from plans and change what they plant in a particular field; for example, in a wetter-than-normal field a dry spring opens the opportunity for a vegetable farmer to plant an early season crop, thus potentially enhancing the diversity of crops grown in that field. However, this issue is especially important for dryland farmers in water-limiting regions such as the Great Plains. In dryland agriculture, low water availability is usually the greatest limitation to crop growth. In such regions, where much of the water needed for a crop is stored in the soil at planting time, growing two heavy water users in a row may work out well if rainfall was plentiful the first year. However, if rainfall has been low, following a heavy-water-using crop (such as sunflowers or corn) with one that needs less water (such as dry peas or lentils) means that water stored in the soil may be enough, along with rainfall during the growing season, to result in a reasonable yield. Caution is needed when making flexible cropping decisions because carryover of herbicides from the previous crop may interfere with your ability to use a different crop than the one planned. University Extension weed control guides are reliable sources of information relating to herbicide chemical plant-back intervals for various crops (including cover crops). Overall, using an adaptive approach to cropping makes sense for many farm operations but requires a solid understanding of the agronomic principles on the part of the farmer.
|Table 11.2 Comparison of Monoculture, Fixed-Sequence Rotations and Dynamic Cropping Systems|
|Monoculture||Fixed-Sequence Rotations||Dynamic Cropping Systems|
|Numbers and types of crops||Single crop||Multiple crops; number depends on regionally adapted species, economics, farmer knowledge, infrastructure||Multiple crops; number depends on regionally adapted species, economics, farmer knowledge, and infrastructure|
|Crop diversity||None||Diversity depends on the length of the fixed sequence||Diversity high due to annual variation in growing conditions and marketing opportunities, as well as changes in producer goals|
|Crop-sequencing flexibility||None||None, although fixed-sequence cropping systems that incorporate opportunity crops increase flexibility||High; all crops, in essence, are opportunity crops|
|Biological and ecological knowledge||Basic knowledge of agronomy||Some knowledge of crop interactions is necessary||Extended knowledge of complex, multiyear crop and crop-environment interactions|
|Management complexity||Generally low, though variable depending on crop type||Complexity variable depending on the length of the fixed sequence and diversity of crops grown||Complexity inherently high due to annual variation in growing conditions, market and producer goals|
|Source: Modified from Hanson et al. (2007)|
Crop Rotation on Organic Farms
Crop rotation is always a good idea, but a sound crop rotation is essential on organic farms. Supplying nitrogen and controlling weeds is more challenging, and options for rescuing crops from disease are limited, making proactive planning through good crop rotations more important. Disease and weed management require a multiyear approach. Nutrients for organic crop production come largely through release from organic matter in soil. Therefore manure, compost, cover crops, and a crop rotation with regular organic matter inputs and large amounts of nitrogen and active soil organic matter are critical.
Organic farmers usually grow a high diversity of crops to obtain the benefits of a diverse crop rotation and to take advantage of specialty markets. Thus, organic field crop producers commonly grow five to 10 crop species, and fresh market vegetable growers may grow 30 or more. However, because of the large variation in acreage among crops and frequent changes in the crop mix due to weather and shifting market demands, planning crop rotations on highly diversified farms is difficult. Therefore, many organic farmers do not follow any regular rotation plan but instead place crops on individual fields (or parts of fields) based on the cropping history of the location and its physical and biological characteristics (e.g., drainage, recent organic matter inputs, weed pressure). Skilled organic growers usually have next year’s cash crops and any intervening cover crops in mind as they make their placement decisions but find that planning further ahead is usually pointless because longer-term plans are so frequently derailed.
Although precise long-term rotation plans can rarely be followed on farms growing a diverse mix of crops, some experienced organic farmers follow a general repeating scheme in which particular crops are placed by the ad hoc approach described above. For example, some vegetable operations plant cash crops every other year and grow a succession of cover crops in alternate years. Many field crop producers alternate some sequence of corn, soybeans and small grains with several years of hay on a regular basis, and some vegetable growers similarly alternate a few years in vegetables with two to three years in hay. These rest periods in hay or in cover crops build soil structure, allow time for soilborne diseases and weed seeds to die off, and provide nitrogen for subsequent heavy-feeding crops. Some vegetable growers alternate groups of plant families in a relatively regular sequence, but this generally requires growing cover crops on part of the field in years when groups that require less acreage appear in the sequence. Within all of these generalized rotation schemes, the particular crop occupying a specific location is chosen by the ad hoc process described above. Organizing the choices with a general rotation scheme greatly simplifies the decision-making process.
Dividing the farm into many small, permanently located management units also greatly facilitates effective ad hoc placement of crops onto fields each year. By this means, a precise cropping history of every part of each field is easy to maintain. Moreover, problem spots and particularly productive locations can be easily located for planting with appropriate crops.
—Charles Mohler, Cornell University
Vegetable farmers who grow a large selection of crops find it best to rotate in large blocks, 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:
Year 1. Sweet corn followed by a hairy vetch/cereal rye cover crop
Year 2. Pumpkins, winter squash or summer squash followed by a rye or oats cover crop
Year 3. Tomatoes, potatoes or peppers followed by a vetch/cereal rye cover crop
Year 4. Crucifers, greens, legumes, carrots, onions and miscellaneous vegetables followed by a cereal rye cover crop
Year 5. (If land is available) oats and red clover or buckwheat followed by a vetch/cereal 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 multiyear 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 and fertility.
Alternating cotton with peanuts is a common, simple rotation in the Southeast coastal region. The soils in this area tend to be sandy, low in both fertility and waterholding capacity, and have a subsoil compact layer. As with the corn-soybean alternation of the Midwest, a more complex system is very desirable from many viewpoints.
A rotation including perennial forage for at least a few years may provide many advantages to the cotton-peanut system. Research with two years of Bahia grass in a cotton-peanut system indicates greater cotton root growth, more soil organic matter and earthworms, and better water infiltration and storage.
The rapid expansion and intensification of agriculture in South America, notably Brazil and Argentina, is strongly driven by the increased global demand for grain crops like corn and soybeans. Many areas in this region also experience extended dry seasons. The system can be made more ecologically sustainable by using no-till and growing soybeans and corn. It is followed into the dry season by a tropical grass like brachiaria that is interseeded into the corn and grazed by beef cattle. While this makes the corn-soybean system less damaging, the participation of these countries in production for global distribution has resulted in the loss of significant portions of important tropical forests and the homelands of the people living in those forests.
Agroforestry is the integration of trees and shrubs into crop and animal farming systems. The idea is that environmental, economic and social benefits are gained by intensively managing an integrated and interactive system. Here, trees do not just exist as an unmanaged plot of woods but rather benefit the crops and animals on the farm either directly or indirectly. In most cases, agroforestry benefits the farm through income diversification, a more favorable microclimate (shade or shelter from strong wind), and by providing wildlife habitat. Also, in many cases it can improve marginal lands that are not suitable for crop production. Agroforestry, however, requires a long-term commitment because the trees often don’t produce income for several years, or even for decades in the case of timber species.
Alley cropping involves planting rows of trees at wide spacings with a companion crop grown in the alleyways between the tree rows. It is often done to diversify farm income, but it can also improve crop production and provide protection and conservation benefits to crops. In the United States, these systems often include cereals, row crops, hay or vegetable crops planted in the alleys between rows of high-value timber, fruit or nut trees (Figure 11.3). High-value hardwoods like walnut and oak trees, or even ornamental trees like woody decorative florals or Christmas trees, are good species and can potentially provide long-term income while short-term proceeds are derived from a companion crop planted in the alleyways. Pecan and chestnut trees are good species for nut production, if that is desired from the tree rows.
Light interception by the trees is a concern when you grow crops in the alleys, especially at higher latitudes. (This is less a concern when the alley crop is shade tolerant, like certain herbs and forages). There are several ways to reduce this effect:
- Space the tree rows more widely.
- Orient tree rows in an east-west direction, which maximizes light interception because the tree obstruction mostly occurs when the sun is at a high angle. This may need to be balanced with other objectives like intercepting wind, which often requires north-south orientation.
- Use trees with fine leaves and less dense canopies that allow for more light penetration for the companion crop.
- Use tree species that leaf-out late or drop leaves early. For example, a late-leafing tree will not intercept light for winter wheat in the early season.
- Thin and prune (coppice) to control large tree canopies and enhance timber quality.
Farmers should tailor the tree layout to the type of species and product. Trees in single rows that are spaced farther apart within the row tend to take longer to close the canopy but also develop more branched crowns, which is desirable for some tree crops, like nut trees. Closely spaced trees in single or double rows encourage more self-pruning and straight trunk development, which is favorable for timber. Sometimes, a taller and shorter tree type can be grown together.
In tropical environments, alley cropping raises fewer concerns related to light interception because the sun is generally more intense and higher in the sky, and there are longer growing seasons. Also, in many tropical countries crop input costs, including fertilizers, are higher while labor costs and mechanization are lower. This creates a greater opportunity to use tropical leguminous trees interspersed with crops to increase the availability of organic nitrogen for the crops, fodder for animals, and firewood for cooking and heating (Figure 11.4).
Although alley cropping can offer advantages, there are some challenges that should be understood. As with other forms of multi-cropping, alley cropping requires more intensive technical management skill and marketing knowledge, and also may demand specialized equipment for tree management. It additionally removes land from annual crop production that may not provide a financial return for several years. Trees may be an obstacle to crop cultivation if their arrangement is not carefully planned and designed. The trees may also result in yield losses for the companion crops grown in the alleys by competing for sun, moisture and nutrients, and in some cases herbicide drift from crops may damage trees.
Other Agroforestry Practices
Forest farming does not separate the land into distinct growing zones like alley cropping but grows understory crops within an established forest, either a natural forest or a timber planting. In this system, the shade from the trees is actually a desired quality because the planted or wild understory crops thrive in such an environment. Typical examples are medicinal herbs like ginseng, certain types of mushrooms, fruits like elderberries, and ornamentals like rhododendrons and moss. Many of these understory crops can be quite profitable.
Silvopasture systems involve the integration of trees and grazing livestock operations on the same land (Figure 11.5). They provide both harvestable forest products and animal forage, offering both short- and long-term income sources. In temperate climates, cool-season grasses may grow better during the hotter times of the year with partial shade provided by the trees (while critical early growth is not affected until leaf-out). In hotter climates, the trees help keep the grazing animals cool. Silvopasture systems still require the use of agronomic principles, like appropriate selection of forages, fertilization and rotational grazing systems that maximize vegetative plant growth and harvest. As discussed in Chapter 14, silvopasture systems may also be beneficial on landslide-prone slopes by stabilizing the soil (see Figure 14.12). Faidherbia albida is a tropical legume tree that thrives in seasonally dry climates and can be used both in silvopasture systems and in alley cropping systems. Its leaves are feather-like, and its canopy is therefore not overly dense and permits light penetration for crops like corn or pasture grasses. Also, it has a deep taproot and grows foliage in the dry season when other forage sources are limited. Faidherbia blooms at the end of the dry season and thereby provides food for bees. Its seed pods are feed for livestock or wild game, and the woody parts make good fuel.
Riparian buffer systems involve trees or shrubs that are planted along streams, rivers, lakes and estuaries to help filter runoff from upstream agricultural or urban lands. They also stabilize stream banks and provide habitat and shade for aquatic animals. Although mostly used as a conservation practice, interest has recently developed in using buffer zones for income production, including bioenergy crops by planting willows, decorative woody floral crops, and fruit and nut crops. Similarly, windbreaks and shelterbelts are generally planted for conservation purposes like reducing wind erosion, enhancing microclimates and promoting landscape biodiversity (see also Chapter 14), but they are increasingly valued for potential income from the trees themselves.
Transitional systems take advantage of the increased shading and changed microclimate as trees mature. For example, landowners may initially use an alley cropping system where annual crops are grown between young trees, which is then transitioned into silvopasture, forest forming or an orchard. Alternatively they may decide to trim the trees and continue alley cropping.
Chapter 11 Summary
There are literally dozens of ways to increase crop diversity on a particular farm through crop rotations and agroforestry. The specific selection of practices depends on the climate and soils, the expertise of the farmer, whether there are livestock on the farm or nearby, equipment and labor availability, family quality-of-life considerations and financial reality. (While striving for relatively good returns from each crop—potential price minus the cost of production—vegetable farmers will sometimes include low-return crops in their rotations because customers expect to find them in the mix at a farm stand or farmers’ market.) From an ecological view, longer and more complex rotations are preferred over shorter ones, and incorporating trees can provide stable long-term ecological benefits. Livestock can often make a soil-building rotation more attractive. It also makes a lot of sense, once equipment is in place, to stay flexible instead of having a rotation set in stone. If you’re ready to adjust to rapid market changes, shifts in labor availability, crop pest outbreaks or unusual weather patterns, you’ll be in a stronger economic position, while still maintaining a complex and diverse cropping system.
A Case Study, Celia Barss
When Celia Barss became the farm manager of Woodland Gardens Organic Farm, she knew cover crops were going to be a big part of the rotation from the start, a decision she’s grateful she stuck with. “We built up slowly,” she says. “Even the open ground, we just cover cropped it until we had time to start producing cash crops. Some fields got to be cover cropped for three years before we started growing in them.”
The cover crops play a key role in diversifying the 12-acre operation’s rotation, which now includes more than 80 different types of fruits and vegetables, as well as cut flowers, that are sold either to restaurants in Atlanta, to a local farmers’ market or through their CSA. Cover cropping is also done in the greenhouses and hoop houses, which make up 1.5 acres of their eight tillable acres. The remaining acreage is in perennial production, which consists of blueberries, figs, muscadines (a native grapevine) and asparagus. The perennials are grown in their own separate areas on heavy slopes, with grass in between them to protect the soil instead of cover crops.
Barss uses cover crops primarily to build up soil organic matter, which, she says, they are “burning through” due to their climate and tillage practices. She explains that she tills because of their intensive planting schedule and tight crop spacing, but she is trying no-till on two open fields that get planted the earliest. With heavy clay soils and wet springs, Barss felt she was doing too much damage tilling under those conditions, so she decided to create beds, leave them fallow for two months, then cover them with silage tarps for a month prior to production. While it was a compromise for her to leave the soils bare like that, she was impressed with how ready to go the fields were after pulling off the silage tarps. She also makes sure to do a heavy summer cover crop on those fields since they are left bare longer than she prefers. Everything else gets planted with cover crops between cash crops.
The cover crops are also key to dealing with some production challenges, primarily weeds and nematodes. Amaranth has become the farm’s biggest weed challenge in the summer, and Barss is also utilizing landscape fabric to help with suppression. “Weeds are all about prioritizing how you do stuff on the farm,” she says. “Timing of getting [fields] weeded or into cover is everything, and just not letting [weeds] go to seed.”
Nematodes, on the other hand, are a challenge that slowly crept up over the years. Barss started seeing nematode pressure around the tenth year of production in their stationary houses. All of those stationary houses have some level of pressure: the newest ones less, the oldest ones the most. Barss admits this problem occurred from not maintaining a longer period out of a host crop in the houses. But in order to reduce nematodes through rotation, she wouldn’t be able to grow a cash crop for six months, because all of them are hosts for nematodes, she explains.
To help combat the nematodes, they were advised by Elizabeth Little, an Extension plant pathologist at the University of Georgia, to try sunn hemp as a cover crop for its nematicidal traits. But Barss can only have sunn hemp grow and break down before a cash crop in the houses for three months, which she’s realizing is not long enough to break the life cycle. She has been solarizing too, which helps suppress the nematodes long enough for their tomato crops, but after tomatoes and most summer crops, the nematode populations have built back up enough to damage the crops that follow in the fall.
While Barss would be happy to do more cover cropping in the houses because of the difference they make in soil tilth—“it’s amazing the difference when we go in after a cover crop,” she says—the farm can’t afford to be out of production for longer than three months. Instead, she’s moving from solarizing to soil steaming so she can cover crop and treat for nematodes. This approach will still allow her to do more cover cropping because while solarizing takes six weeks, steaming only takes half an hour. “I can do a quick cover crop and then do the steamer before going into a cash crop, instead of solarizing in addition to the cover crop,” Barss explains. But soil steaming requires a lot of energy and can be a big financial investment for the steamer, so it’s considered an alternative when other options aren’t available.
In addition to sunn hemp, Barss uses a lot of cowpeas and sorghum-sudangrass together in the summer because they do well in the heat. In areas where she has a shorter window, like six weeks, she’ll use millet or buckwheat instead, since it’s not enough time to let the cowpeas and sorghum-sudangrass grow. In the winter and cooler seasons, she’ll use rye, hairy vetch and Austrian winter peas in fields that will be in cover for longer periods. In fields where she’ll be planting early or needs to fill in shorter gaps in the spring and fall, she’ll use oats because they’re easier to terminate.
Oats will also follow in the spring in places where brassicas may have gone too late. Unlike the hoop houses, fields have a good three- or four-year rotation between plant families, Barss says, which is mostly dictated by the brassicas. “The brassicas really push the rotation, and that’s the family I’m finding myself always doing less than I want to because of rotation.” Her rotations vary by field, as she has to stay out of some later than others because they’re too wet in the spring, but a typical rotation might include early spring brassicas, followed by the field peas and watermelons split 50/50 across the field, then two cycles of a cover crop.
Barss tries to get as much out of the cover crops as possible while they’re growing by mowing sorghum-sudangrass, for example, to about a foot and letting them regrow. This extends their life while preventing them from going to seed. “Our goal is to have [the cover crops] go as
long as possible and keep the ground covered because we have such a long summer,” she says. When it is time to terminate them, she’ll flail-mow and incorporate them into the soil.
Her focus on cover cropping has paid off. Barss says initially there were fields she didn’t want to plant certain crops in because she didn’t think the soil quality was good enough. Instead she would plant a crop that didn’t need a lot of nutrients, such as field peas, and then focus on
cover cropping it. Now she can plant any of their crops in those fields, stating: “it’s amazing the difference in a field that I went into 10 years ago that hadn’t been cover cropped.”
“I attribute everything to the cover cropping, honestly, for the quality of our soils,” Barss says. “I could grow a lot more, but I wouldn’t be able to do the cover cropping the way I am. Just forcing yourself to stick to those ideals that you set up and making sure you stick to those rotations and
not just trying to plant more and more. Because it’s easy to start doing, but then you definitely see your soil quality start to go down.”
|Field||Season||2019 Season||2020 Season|
|2||Winter||strawberries, onions, flowers||Covers: rye/peas/vetch|
|Spring||strawberries, onions, flowers||Covers: sorghum-sudangrass/cowpeas|
|Summer||cover crop||Covers: sorghum-sudangrass/cowpeas|
|6||Winter||cover crop||cover crop|
|Spring||cover crop||cover crop|
|Summer||sunchoke, edamame, flowers||tomatoes/flowers|
|Fall||cover crop||cover crop|
|7-B||Winter||cover crop||cover crop|
|Spring||peppers, eggplant||cover crop|
|Summer||peppers, eggplant||cover crop|
|8-A||Winter||cover crop||cover crop|
|Spring||cover crop||peppers, eggplants, herbs|
|Summer||watermelons, flowers, beans, cukes||peppers, eggplants, herbs|
|Fall||cover crop||cover crop, herbs|
|8-B||Winter||cover crop||cover crop|
|Spring||brassicas, scallions, beets||flower/cover crop|
|Fall||cover crop||cover crop|
|Summer||cover crop||cover crop|
|Fall||strawberries, onions, flowers||cover crop|
|9-B||Winter||cover crop||cover crop|
|Spring||cover crop||cover crop|
|Summer||cover crop||cover crop|
|9-C||Winter||cover crop||cover crop|
|Spring||cover crop||cover crop|
|Summer||cover crop||cover crop|
|10||Winter||cover crop||cover crop|
|Spring||flowers, melons, corn, cukes||potatoes|
|Summer||cover crop||cover crop|
|Fall||bedded up for spring||cover crop|
|11||Winter||cover crop||cover crop|
|Summer||field peas, watermelon||corn, beans, squash|
|Fall||cover crop||cover crop|
|12||Winter||cover crop||cover crop|
|Spring||cover crop||cover crop|
|Summer||cover crop||field peas/flowers|
|Fall||brassicas, chicories||cover crop|
|13||Winter||garlic, cover crop||fallow|
|Spring||garlic, cover crop||brassicas, scallions, lettuce|
|Summer||cover crop||squash, corn|
|Fall||prep for spring||cover crop|
|14||Winter||cover crop||garlic, cover crop|
|Spring||cover crop||garlic, cover crop|
|Summer||tuberoses, field peas to cover crop||cover crop|
|Fall||garlic, cover crop||cover crop/prep for spring|
|15||Winter||cover crop||cover crop|
|Spring||cover crop||squash, corn|
|Summer||sweet potatoes, melons||cover crop|
|Fall||cover crop||garlic, cover crop|
|17-A||Winter||cover crop, herbs||cover crop|
|Spring||peppers, herbs||sweet potatoes, tuberoses|
|Summer||peppers, cover crop||sweet potatoes, tuberoses|
|Fall||cover crop, brassicas||cover crop|
|Spring||squash, corn, beans||brassicas|
|Summer||cover crop||field peas, watermelons|
|Fall||prep for early spring||cover crop|
|18||Winter||cover crop||cover crop|
|Spring||cover crop||cukes, melons, winter squash|
|Summer||tomatoes, okra, flowers||cukes, melons, winter squash|
|19||Winter||cover crop||cover crop|
|Spring||cover crop||cover crop|
|Summer||winter squash, corn, beans, summer squash||melons, watermelons, edamame, okra, beans, tuberoses|
Chapter 11 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: 117–121.
Baldock, J.O. and R.B. Musgrave. 1980. Manure and mineral fertilizer effects in continuous and rotational crop sequences in central New York. Agronomy Journal 72: 511–518.
Barber, S.A. 1979. Corn residue management and soil organic matter. Agronomy Journal 71: 625–627.
Cavigelli, M.A., S.R. Deming, L.K. Probyn and R.R. Harwood, eds. 1998. Michigan Field Crop Ecology: Managing Biological Processes for Productivity and Environmental Quality. Extension Bulletin E-2646. Michigan State University: 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, ed. C.A. Edwards, R. Lal, P. Madden, R.H. Miller and G. House. Ankeny, IA: Soil and Water Conservation Society.
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: 74–77. This source describes the long-term cropping experiment in Missouri.
Gold, M., H. Hemmelgarn, G. Ormsby-Mori and C. Todds (Eds). 2018. Training Manual for Applied Agroforestry Practices—2018 Edition. University of Missouri Center for Agroforestry.
Grubinger, V.P. 1999. Sustainable Vegetable Production: From Start-Up to Market. Natural Resource and Agricultural Engineering Service: Ithaca, NY.
Hanson, J.D., M.A. Liebig, S.D. Merrill, D.L. Tanaka, J.M. Krupinsky and D.E. Stott. 2007. Dynamic cropping systems: Increasing adaptability amid an uncertain future. Agronomy Journal 99: 939–943.
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: 448–452.
Hunt, N.D., M. Liebman, S.K. Thakrar and J.D. Hill. 2020. Fossil Energy Use, Climate Change Impacts, and Air Quality-Related Human Health Damages of Conventional and Diversified Cropping Systems in Iowa, USA. Environmental Science & Technology. DOI: 10.1021/acs.est.9b06929
Karlen, D.L., E.G. Hurley, S.S. Andrews, C.A. Cambardella, D.W. Meek, M.D. Duffy and A.P. Mallarino. 2006. Crop rotation effects on soil quality at three northern corn/soybean belt locations. Agronomy Journal 98: 484–495.
Katsvairo, T.W., D.L. Wright, J.J. Marois, D.L. Hartzog, K.B. Balkcom, P.P. Wiatrak and J.R. Rich. 2007. Cotton roots, earthworms, and infiltration characteristics in sod–peanut–cotton cropping systems. Agronomy Journal 99: 390–398.
Krupinsky, M.J., K.L. Bailey, M.P. McMullen, B.D. Gossen and T.K. Turkington. 2002. Managing plant disease risk in diversified cropping systems. Agronomy Journal 94: 198–209.
Lehman, R.M., S. L. Osborne and S. Duke. 2017. Diversified No-Till Crop Rotation Reduces Nitrous Oxide Emissions, Increases Soybean Yields, and Promotes Soil Carbon Accrual, Soil Science Society of America Journal 81(1): 76–83.
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. In Sustainable Agriculture Research and Education in the Field, pp. 183–205. Proceedings of a conference, April 3–4, 1990, Board on Agriculture, National Research Council. National Academy Press: Washington, DC. This is the reference for the rotation experiment in Virginia.
MacFarland, K. 2017. Alley Cropping: An Agroforestry Practice. USDA National Agroforestry Center. https://www.fs.usda.gov/nac/assets/documents/agroforestrynotes/an12ac01.pdf
Mallarino, A.P. and E. Ortiz-Torres. 2006. A long-term look at crop rotation effects on corn yield and response to nitrogen fertilization. In 2006 Integrated Crop Management Conference, Iowa State University, pp. 209–217.
McDaniel, M., L. Tiemann and A. Grandy. 2014. Does agricultural crop diversity enhance soil microbial biomass and organic matter dynamics? A meta-analysis. Ecological Applications 24(3): 560–570.
Merrill, S.D., D.L. Tanaka, J.M. Krupinsky, M.A. Liebig and J.D. Hanson. 2007. Soil water depletion and recharge under ten crop species and applications to the principles of dynamic cropping systems. Agronomy Journal 99: 931–938.
Meyer-Aurich, A., A. Weersink, K. Janovicek and B. Deen. 2006. Cost efficient rotation and tillage options to sequester carbon and mitigate GHG emissions from agriculture in eastern Canada. Agriculture, Ecosystems and Environment 117: 119–127.
Mohler, C.L. and S.E. Johnson. 2009. Crop Rotation on Organic Farms: A Planning Manual. No. 177. Natural Resource, Agriculture, and Engineering Service: Ithaca, NY.
National Research Council. 1989. Alternative Agriculture. National Academy Press: Washington, DC. 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 21–23, 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.
Schlegel. A., Y. Assefa, L. Haag, C. Thompson and L. Stone. 2019. Soil Water and Water Use in Long-Term Dryland Crop Rotations. Agronomy Journal 111: 2590–2599.
Tsonkova, P., Böhm, C., Quinkenstein, A. and D. Freese. 2012. Ecological benefits provided by alley cropping systems for production of woody biomass in the temperate region: a review. Agroforestry Systems 85 (1): 133–152.
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): 24–32. Wolz, K.J. and E.H. DeLucia. 2018. Alley cropping: Global patterns of species composition and function. Agriculture, Ecosystems and Environment 252: 61–68.