l?0 AGRICULTURAL ECONOMICS 



up the soil and admit of a free circulation of air, encourage the growth 

 of the free-living bacteria and bring about greater fixation of nitrogen 

 from the atmosphere. So, also, liming as a remedy for acidity increases 

 the amount of nitrogen in the soil by causing a greater growth of 

 azotobacter. 



The beneficial effect of clover growing on soils was known many 

 years before it was satisfactorily explained. The mystery was not 

 cleared up until the bacteriologists found that certain bacteria were 

 associated with legumes and that these bacteria took nitrogen from 

 the air and fixed it in the soil. 



Legumes will grow and flourish on soils that have absolutely no 

 nitrogen if the proper bacteria are present and the legumes become 

 inoculated with these bacteria. In this process of inoculation the 

 bacteria enter the roots of the legumes. The plants aid the process 

 by a softening of their tissue, and then in so-called " infection-threads" 

 the bacteria pass from cell to cell. They gather at a particular spot 

 and, nourished by the plant, multiply to a large extent and form what 

 are known as nodules, or swellings on the roots. As soon as the 

 organisms begin to multiply they begin to take nitrogen from the at- 

 mosphere and to supply it to the plant. The plant in return sup- 

 plies the bacteria with the necessary carbonaceous food and a close 

 union for mutual benefit is thereby established. So the legumes draw 

 but a small proportion of their nitrogen from the soil, and if the entire 

 crop is turned under for a green manure, which is a common practice, 

 there is a large gain to the soil in nitrogen. 



Legumes will often grow without inoculation and in soils very rich 

 in nitrogen will yield good crops. They then draw their entire nourish- 

 ment from the soil. When that is the case the legumes have no 

 advantage over the non-leguminous crop. But when legumes are 

 inoculated they contain a larger percentage of nitrogen and the soil 

 is not robbed of its stock of nitrogen. 



The activities of soil bacteria with regard to the nitrogen problem 

 are important, not only from the standpoint of increasing the nitrogen 

 content of soils through additions from the atmosphere, but also in 

 the change of organic materials into available forms. Plant and 

 animal remains in the soil, farmyard manure, green manures, or other 

 organic materials, adder! to the soil contain insoluble organic nitroge- 

 nous matter known as protein, and these must be changed into sol- 

 uble nitrates to be of use to plants. This solution is accomplished 

 by the process of decay. Bacteria are the active agents bringing 



