The brassicas genus includes such plants as broccoli, cabbage, cauliflower and brussels sprouts, along with kale and mustard greens. Legumes include peanuts, peas, soybeans and lentils, and a variety of beans. Legumes are characterized by their double-seamed seed pods. Green beans are a legume. Each group of plants has specific nutritional needs and growth habits. In gardening or farming, planting brassicas after legumes benefits both the crops and the soil.
Legumes draw nitrogen from the air around them, fixing the nitrogen in the soil. The soil, then, after the legume crop has been harvested, is rich in nitrogen. Brassicas benefit from this surplus as they are heavy nitrogen feeders. The legumes, as a previous crop, amended the soil to the brassicas' liking, resulting in a successful rotation of such plants as broccoli, mustard greens or soybeans. After harvest, the plant debris may be turned into the soil, returning nitrogen to the soil and adding other nutrients as well.
Pest and Disease Management
Rotating crops is a form of pest and disease management. Brassicas release certain biotoxic chemicals that act as pest deterrents. Planting brassicas as a cover crop, that is, one that follows the primary legume crop, brings these chemicals to the soil. The biotoxins aid in eradicating or at least reducing the development of soil-borne diseases and disrupting the life cycles of soil-oriented pests. Legumes do not have these cleansing capabilities. The brassicas then act as cleansing agents for the soil.
Brassicas have strong root systems that reach deep into the soil, breaking into layers of soil untapped by the legumes. The brassicas take up the stores of buried nutrients and in doing so also disperse the untapped nutrients into the upper layers of the soil. This natural tillage results in a revitalization of the soil. Not only are nutrients redistributed after a season of legumes fixing the nitrogen in the upper layers, the soil itself is loosened by the deep rooting systems.
Mustard greens and kale don't have the extensive root systems of those of broccoli or cabbage, but do have fibrous root systems. The decomposition of these roots provides a textural advantage for the soil. The incorporation of the nutrient fortified fibrous roots as decomposing organic material adds humus to the soil, as well as giving it a loamy quality. The soil, then, is loosened after the natural compaction brought about by the shallower roots systems of the legumes.