Good soil structure and organic matter are essential to producing good yields, but the key to high yields is the presence of nitrogen in the soil. Organic nitrogen can be boosted by encouraging the growth of certain microorganisms or, more directly, by applying plant and animal manures. Several kinds of bacteria, and other microorganisms such as blue-green algae, take up nitrogen from the atmosphere and convert it to ammonia, which can be used by plants. Some of these microorganisms are free- living in the soil, although they are often associated with the root zones of plants and their growth can be stimulated by certain crops. For example, in the presence of the rice variety IR42 they will produce up to 40 kg of N/ha per year. However, the best practical results have come from exploiting nitrogen-fixing microorganisms that live symbiotically in plants.55
There is a blue-green alga, Anabaena azollae, living in cavities in the leaves of a small fern, Azolla, that is a potentially phenomenal fixer of nitrogen—up to 400 kg N/ha per year, under experimental conditions. The fern will grow naturally in the water of rice fields without interfering with the growth of the rice plants. It quickly covers the surface, and after 100 days some 60 tons can be harvested per ha, containing
120 kg of nitrogen. However, the nitrogen is not directly available to the rice crop; the ferns have to be incorporated in the soil. Rice yields can be increased by a ton per ha or more, and the effect will carry over to a following crop; for example, if wheat is grown after Azolla-treated rice.56
Best known of the symbiotic, nitrogen-fixing microorganisms are the bacteria living in the root nodules of legumes, which can fix 100 to 200 kg N/ha per year. The fertilizing properties of legumes have been recognized for thousands of years. One of the earliest of the world’s cropping systems—dating to soon after agriculture began in the valleys of Central America—was the interplanting of maize and beans; the seed of both crops was often placed in the same planting hole. It is a practice that, in various forms, continues today. For example, when cowpeas are cropped together with maize, the bacteria in cowpea root nodules can provide 30 percent of the nitrogen taken up by the maize.57 Cowpeas and another legume, lablab, are particularly useful in lower-potential lands. Cowpeas are adapted to acid, infertile soils, while lablab is drought tolerant, produces good fodder, and can regrow well after clipping.