LAND AND OTHER AGENTS OF PRODUCTION 169 



the bacteria cause changes in the compounds, making them soluble 

 and hence available for the growth of plants. Most soil bacteria live 

 principally on organic matter, or humus, and the products of their 

 own activity. Up to a certain limit, increasing the humus content 

 may, therefore, be expected to increase the bacterial life. The bac- 

 teria, furthermore, not only act on the humus or organic matter in 

 the soil and bring about its solution in the process of obtaining their 

 food, but they also attack the mineral portion of the soil and change 

 insoluble portions of that into soluble. 



The nitrogen problem and Us solution. Soils are very apt to be 

 deficient in nitrogen. This element, then, is generally the limiting 

 factor in the growth of crops. Formerly the lack of nitrogen in a 

 soil was supplied by application of nitrate of soda, which was obtained 

 from the nitrate beds in Chile. With the increasing demands for 

 nitrates, the amounts taken yearly from the nitrate beds were enor- 

 mous, and it was estimated that in a very short time the deposits would 

 be exhausted and the world would face a nitrogen famine. Of course, 

 other nitrogenous materials were available for manure, but in such 

 small amounts that they would be merely a drop in the bucket in sup- 

 plying the demands of the world. 



It was just at this crucial time that soil bacteriologists came to 

 the rescue and quieted the general fears by showing that certain 

 species of bacteria living in soils have the ability to draw upon the 

 inexhaustible supply of nitrogen in the air (which contains 79 per cent 

 nitrogen) and fix it in the soil in a form available for plants. Thus 

 the nitrogen problem was solved and there need be no fear of a nitro- 

 gen famine. 



There are two classes of bacteria which are thus able to utilize 

 the nitrogen of the air in their growth: first, those which live entirely 

 dependent on their own resources, and second, those which grow on 

 the roots of legumes, such as clover, alfalfa, etc. The first are said 

 to live non-symbiotically, or independently, and are known as non- 

 symbiotic; the second are said to live in symbiosis with the legumes, 

 or hi a state of mutual helpfulness, and are called symbiotic. 



The first group of nitrogen fixers, or azotobacter, as they are 

 called, are present in most soils. These bacteria fix nitrogen in 

 amounts which have been estimated at from 15 to 40 pounds per 

 acre per year under ordinary conditions. Proper farm management 

 includes many practices which encourage the fixation of nitrogen from 

 the atmosphere. Thus the ordinary operations of tillage, which open 



