. . . 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 plant pathogens. Rotations that include non-host plants are effective for controlling insects like corn rootworm, nematodes like soybean cyst nematode, and diseases like root rot of field peas. When specific soil diseases are present, the length of time between growing the same or similar crop may vary from relatively short (one to two years for leaf blight of onions) to fairly long (seven years for clubroot of radish or turnip). Also, the rotation should contain some crops that are non-hosts or actually suppress the disease. Root growth may be adversely affected when continuously cropping to any single crop (see figure 11.1). This means that the crops may be less efficient in using soil nutrients and added fertilizers. In addition, rotations that include legumes may supply significant amounts of nitrogen to succeeding crops. A legume harvested for seed, such as soybeans, provides little N 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 crops, such as corn and soybeans, you have a very simple rotation. More complex rotations require three or more crops and a five to ten-year (or more) cycle to complete.
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) bean helps corn to get phosphorus on low P 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 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. When you grow a grain or vegetable crop following a forage legume, the extra supply of nitrogen certainly helps. However, yields of crops grown in rotation are often 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 N per acre when growing corn after corn, yields were not as good as corn grown following alfalfa with little or no N applied. In addition, following a non-legume crop with another non-legume produces higher yields than a monoculture 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.