On the other hand, it has been shown by Maze and others that under proper cultural condi tions the tuber cle bacteria on artificial media fix (by assimila tion) free nitro gen from the air.
That they do fix free nitrogen from the air, when in the tubercles of t h e legumes under normal condi tions, is abun dantly proved, thus confirming the results of empirical observa tions, that leguminous plants grown in soils poor in nitrogen flourish and sometimes have a larger content of nitrogenous substance at maturity than they could have obtained from the poor soil; that soils poor in combined nitrogen are enriched in this substance when crops of legumes are grown on them, even though the crop of vines and seed is removed, because of the large amount of fixed ni trogen in the bacteroids still within the tubercles in the soil; while with the cereals and grasses the nitrogen content of the soil is decreased. This explains why it is that leguminous crops are more important for green-manuring than the cereals and grasses when there is need of an increase of com bined nitrogen.
Races of nodule bacteria.
While the nodule bacteria are widely distributed in the soil, the fact that there are several different races which dwell in the roots of certain genera of hosts, which cannot attack the roots of others, explains why it is that the bacterial races to which certain genera of legumes are susceptible, are not present in all soils, especially in soils where these hosts do not grow, while other races are present in those soils. This is shown in the case of the pea and lupine organism, which will not attack the roots of cytisus, robinia, trifolium, serradella and others, as shown above. It is also shown by ex periences with the soybean from Japan. When the seed of this bean was planted in America and Europe, no nodules were developed on the roots. It was only when soil from Japan, in which the soybean had grown, was imported and mixed with soil in which the soybean was planted, that the nodules were developed. This organism of the soy bean nodules was thus considered by Kirchner to be a different species and was named Rhizobac terium Japonicum.
Besides the distinct races which cannot infect certain genera of hosts, there are probably sub races or initial races which can infect a wide range of genera, but, by being confined to a limited num ber or to single genera for several years, infect certain genera much more readily than others.
Soil inoculation.
This leads to an important method in practice, i. e.. the inoculation of soils with the specific organism to which the legume which it is desired to grow on the particular plot of ground is susceptible. This method has been developed by the United States Department of Agriculture, especially through the work of Moore and Keller man, and by some of the experiment stations. It consists in obtaining pure cultures of the needed different races on a medium poor in nitrogen com pounds so as to create a state of nitrogen hunger in the organism, which makes it more likely to attack the roots of the legumes than organisms which have a nitrogen surfeit of food. Pure cul tures were distributed, after being dried on cotton, or other suitable material, to the planters, who place them in a quantity of liquid nutrient media for a day or so in order to multiply the germs. This liquid is then scattered on the soil, or, better, the seed is sprinkled with the infusion before being planted. Under certain conditions this prac tice, or some modification of it, promises good returns, especially in soils poor in nitrogen, where the crop in question has not grown for several years or where for any reason the specific organism for the specific crop is absent, or present in small numbers. When the specific organism is present in quantity or in soils already rich in nitrogenous plant-food, the increase in the crop is slight or nil as a result of inoculation of the soil.
A method has not yet been perfected for sup plying and applying cultures of the germ which reliable under all circumstances, due to deteriora tion or contamination of the organisms in cultures, either because of fault, careless or unscrupulous methods on the part of manufacturers, or to imper fect methods of multiplying the organism at the farm and of inoculation of the seed and soil. With some crops it is now a practice to transport the organisms with the soil in which the specific crops have been grown, for inoculation of soils. In this method, however, there is danger of the transpor tation of the germs of fungus and bacterial diseases, which may be present in the soil. [Soil inoculation is fully discussed by Lipman, Vol. I, pages 4-17 450.] Relation between nodule bacteria and their host.