Crop Rotation

Because climate, soil type, extent of erosion, and suitable cash crops vary around the globe, rotation schemes vary as well. The principles of crop rotation, however, are universal: to maintain soil health, combat pests and weeds, and slow erosion farmers should alternate crops with different characteristics—sod-base crops with row crops, weed-suppressing crops with those that do not suppress weeds, crops susceptible to specific insects with those that are not, and soil-enhancing crops with those that do not enhance soils.

Farmers use cover crops to stabilize soils during the off-season when a cash crop has been harvested. Cover crops are typically grown during dry or cold seasons when erosion and nutrient depletion threaten exposed soil. Slow-starting legume crops like sweet clover, red clover, crimson clover, and vetch can be planted during the cash crop's growing season. These nitrogen-fixing legumes also restore nitrogen to depleted soils during the off-season, which will benefit the next cash crop. Farmers typically plant fast-growing crops like rye, oats, ryegrass, and Austrian winter peas after harvesting the cash crop. Cover crops are plowed into the soil as "green manure" at the end of the season, a practice that increases soil organic content, improves structure, and increases permeability.

Increasing the number of years of grass, or forage, crops in a rotation schedule usually improves soil stability and permeability to air and water. Sloping land may experience excessive soil loss if row crops like corn, or small-grain crops like wheat, are grown on it for too many years in a row. Rotation with sod-based forage crops keeps soil loss within tolerable limits. Furthemore, forage crops can reverse the depletion of organic nutrients and soil compaction that occur under corn and wheat.

Crop rotation also works to control infestations of crop-damaging insects and weeds. Crop alternation interrupts the reproductive cycles of insects preying on a specific plant. For example, a farmer can help control cyst nematodes, parasites that damage soybeans, by planting soybeans every other year. Crop rotation discourages weeds by supporting healthier crop plants that out compete wild species for nutrients and water, and by disrupting the weed-friendly "ecosystems" that form in long-term monocultures. Rotation schedules that involve small fields, a large variety of rotated crops, and a long repeat interval contain insect infestations and weeds most successfully. Complex rotations keep weeds and insects guessing, and farmers can exert further control by planting certain crops next to each other. For example, a chinch bug infestation in a wheat field can be contained by planting soybeans in the next field instead of a chinch bug host like forage sorghum.

Farmers undertaking crop rotations must plan their planting more carefully than those who plant the same crop year after year. Using a simple principle, that there are the same number of fields or groups of fields as there are years in the rotation, farmers can assure that they produce consistent amounts of each crop each year even though the crops shift to different fields.