Crop Rotation and Soil Sustainability
Agriculture is responsible for providing food for an ever-growing population, and as it becomes clear that yields cannot continue to rise without limit, sustainability of agricultural practices becomes an increasingly important question. The soil is a precious resource in which all of agriculture has its base, and careful management of this complex system is essential. Crop rotation is one of the most important management practices in a sustainable agriculture system, both as a means of conserving soil and of maintaining its fertility. "A well-thought-out crop rotation is worth seventy-five percent of everything else that might be done, including fertilization, tillage, and pest control" according to The New Organic Grower (Coleman, 1989, p. 50). Crop rotation is by no means confined exclusively to organic farming, although much of what is considered in planning a rotation sequence encompasses the concerns of the organic farmer. The difference is primarily one of sustainability. "The organic farmer is essentially turning part of his potential income into renewal of the soil (by adding organic matter) in order to assure sustainability of future crop production. The conventional system maximizes present income and is not as concerned about viewing soil as a long-term investment" (Poincelot, 1986, p.23).
A varied sequence of crops provides benefits that a monoculture cannot. A monoculture is an unnatural system; the relationship between soil, plants, and climate is designed to be balanced by diversity. A carefully planned rotation that considers as many aspects of this relationship as possible is a significant step toward establishing sustainability. Although not all the effects of rotation appear to be directly related to the soil, soil is the medium in which plants grow and an investigation of rotational effects necessarily involves it. For instance, a rotational sequence decreases nematode and other insect pest problems by alternating host plants with nonhosts. It decreases disease occurrence due to bacteria, fungi, and viruses by varying the level of susceptibility from year to year. It decreases competition from weeds due to different cultivation and irrigation techniques for different crops that prevent the weeds from establishing a permanent population. And it almost certainly results in an increased yield due to a combination of these and many other factors (Loomis and Connor, 1992). The soil as a medium influences and is influenced by all of these processes.
The term crop rotation includes a variety of practices carried out on a given plot of land over a period of several years. Common rotations make use of a sequence of two or more cash crops with complementary qualities, such as corn and soybeans. Rotations may or may not include a cover crop designed to provide stability and erosion resistance over the winter, a green manure grown for its benefits to the soil and plowed under rather than...