The amount of nitrogen these direct-fixing forms are capable of capturing is not definitely known. At Cornell, a series of accumulation tests gave figures of approximately 40 pounds per acre per year. Other authorities have given figures as high as 1,000 pounds per year per acre, but these seem unrealistic.
We might say that direct fixation is important, certainly, to native soils, woodlands, pastures and unplowed fields. It cannot possibly satisfy the needs of flowers and vegetables in home gardens, or the needs of crops like corn and grains which can blot up a couple of hundred pounds of nitrogen per acre per year.
Attempts have been made to isolate and improve superior strains of both Azotobacters and Clostridium so that pure cultures could be added to composts and soils. So far these efforts have been disappointing. Part of the difficulty lies in supplying the cultures with calcium, carbon dioxide and glucose without building up competing populations of more aggressive soil organisms. The use of potassium phosphate, suggested as a source of potash to help preserve nitrogen-fixing bacteria, has not resulted in significant improvement.
NITROGEN RELEASE BY NODULE BACTERIA
Baptisias, lupines, sweet peas, clovers, alfalfas, lespedezas and other legumes-plants belonging to the Leguminosae or pea family -produce nodules or tubercles on their roots when attacked by certain bacteria. These bacteria are able to fix free nitrogen from the air, or at least capture ammonia gas that might otherwise escape into the atmosphere. Originally called Rhizobium radickola, nowadays these are usually classed according to the plant they inoculate, such as R. trifoli for the bacteria that live on clover roots. For simplicity's sake, let's call them nodule bacteria.
The various nodule bacteria are quite specific and will not inoculate all leguminous plants. One that inoculates garden beans will not invade the roots of peas. Lupines have a specific strain peculiar to them alone, and so on. To get around this the Nitragen Company, which probably sells 95 per cent of all the garden inoculants used in the United States, has developed a mixture of all the strains needed by various legume plants.
Some members of the Leguminosae do not harbor nodule bacteria (the redbud or Judas tree, Cercis canadensis, is one), but all lupines, baptisias and other herbaceous species within the family do.
As green manure or sheet composting crops, vetch, clover and others are just as valuable for their nitrogen contribution today as they were centuries ago when farmers grew them and plowed them under without knowing why