The oats germinate first, outcompete weeds for available resources and can be mowed when clover starts to emerge. Other crops are more productive if they have a support to climb. Vetch and field peas, for example, do better when they can twist themselves around a stiff-stalked nurse crop such as oats or rye. Some plants exude chemicals which can control other forms of life. For example, a crop of oilseed radish can greatly reduce populations of harmful nematodes.
Nematodes are microscopic soil-dwelling creatures which bore into the flesh of root and tuber crops.
Scavenger crops Topsoil that has been heavily cropped with shallow-rooted plants such as corn becomes deficient in certain nutrients. In order to restore these removed nutrients the grower must add them or use a plant that is adept at reaching deep into the subsoil to bring minerals to the surface. Scavenger crops such as alfalfa, red clover, sweet clover and bell beans also break up compacted soils. When the deep-rooted plants die, their decaying roots leave holes in the soil. These holes provide a pathway for roots of less aggressive plants to follow and for water to drain from the surface.