11.2: Why Rotations?

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.

Figure 11.1. Corn roots: (a) continuous corn with mineral fertilizer, (b) corn following alfalfa with dairy manure compost. Photos by Walter Goldstein (Michael Fields Institute).

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.