Two ways of enriching the soil involve growing your own fertilizer. This can be done through a class of plants that are able to extract nitrogen from the air, and through the growing of “green manure.”
Some plants—chiefly those belonging to the group called legumes—associate with nitrogen-fixing bacteria to get their nitrogen from the air. The process involves a symbiotic relationship between the leguminous plants and various strains of Rhizobium bacteria found in the soil. The bacteria “infect” the plant roots, causing small nodules to grow. Within these nodules, the bacteria extract nitrogen from the air and fix it into forms the plants can use to make protein. In return, the bacteria receive sugars from the plant for their own nutrition.
Nitrogen absorbed and fixed into plant-usable forms by bacteria is a very important source of organic nitrogen. For centuries, the bacteria that live on the roots of legumes such as clovers and alfalfa were the principal way (except for manures) to add nitrogen to soil. In parts of New Zealand where clovers grow the year round, bacteria living on the roots will supply all the nitrogen needs of some crops for a full season.
The bacteria on the roots of alfalfa are usually the most nitrogen-productive. Following alfalfa, in descending order of nitrogen production, are ladino clover, sweet clover, red clover, and white clover. Although these are the most productive nitrogen fixers we know, many other bacteria that live in the soil or on plant roots have the ability to capture and use nitrogen from air.
Legumes can be used as a nitrogen source in several ways. Clover, mixed with grass in a lawn, supplies much of the nitrogen needed by the lawn. (However, the clover blossoms attract bees, which pose a stinging hazard.) Legumes can be grown in a rotation with other crops, leaving more nitrogen in the soil than before they were planted. Or they can be grown as a cover crop, to be turned in when a new crop is to be planted.
Cover crops are also known as green manures. In contrast to crops grown for their fruit or flower, cover crops are grown for the express purpose of being tilled back into the soil. Tilling these crops into the soil has the effect of improving the physical structure of the soil, increasing the organic matter content, and increasing the soil’s fertility.
Cover cropping is a common practice on farms and ranches, but the use of cover crops can also benefit the home gardener. In addition to the legumes mentioned above, the following crops make excellent green manures in the geographical areas listed. In the northern states, ryegrass (Lolium multiflorum); in the southern states, crimson clover (Trifolium incarnatum), horse bean (Vicia faba), and rough pea (Lathyrus hirsutus); in the West, bitter vetch (Vicia ervilia) and common vetch (V. sativa).
Cover crops should be tilled into the soil shortly before the crop reaches full maturity. If grown on soils of very low fertility, an application of a commercial fertilizer during the growing season will greatly improve growth of the crop and its eventual benefit to the soil.
In the home garden, cover crops can be used to improve the soil before planting. Kill any weeds, then prepare the soil and sow a cover crop. When it begins flowering, cut it, let it lay in the sun for a day to wilt, then till it into the soil. If you have time, repeat for greater benefit.