Soil can be enriched and conditioned in a number of ways. In the garden, doing it organically, with the use of live material (green manuring) and dead material (compost, manure), is safer to human health and in many cases less expensive than using inorganic, or chemical, fertilizers.
Using certain crops as green manure has been practiced for generations. It involves growing a crop on idle land and then digging it into the ground rather than harvesting it. Tilling or digging in the plant material adds both organic material (for better texture) and nutrients (for improved fertility) to the soil, thereby cutting down or eliminating entirely the need for chemical fertilizers.
When the green manure is dug into the ground it rots, as it would in a compost heap, leaving a certain amount of humus in the soil which is vital for the soil’s texture, or structure. It also leaves behind a residue of nutrients that it produced while growing. In the case of deep- rooted plants, important elements in the soil are brought up from deep down in the soil and made available to shallow-rooted crops that are planted to follow the green manure. Leguminous plants (peas and lupines, for example) can do some of the work of nitrogen fertilizers. They will actually take nitrogen, an element essential for plant growth, from the air and “fix” it in the soil, making it then available to plants.
Another advantage of green manures is that they act as cover crops. In many areas soil that is left bare between the harvesting of one crop and the sowing of the next is prone to erosion by wind or water run-off. The planting of a fast-growing, intermediate crop helps prevent this. It also helps keep the plot free from weeds.
To be most effective, the seed of the cover crop is sown as soon as possible after the main crop has been harvested. The soil may need digging or it may be sufficient just to rake it over, depending on its condition. The seed can be scattered or sown in drills. Shortly before the next crop is clue to be sown, the green manure is chopped off, left to wilt, and then dug into the soil. The cover crop should not be allowed to get to the stage where it is likely to flower. If it does flower and then go to seed there will be a repeat crop, setting up competition to the main crop planted next.
Some green manures, such as mustard, are very fast growing and can be used on pieces of ground that are empty for only a few weeks. The deeper-rooted crops, legumes for instance, are best left in the ground for much longer periods of time, up to a year, to get their full benefit.