112 THE INVISIBLE WORLD 



the soil is, for the most part, nitrogenous ni- 

 trogen and its compounds. This element is abun- 

 dant in Nature. Four-fifths of all the atmosphere 

 is nitrogen. All animal bodies contain sixteen per 

 cent, of nitrogen. Plants contain still more. 

 Animals derive their nitrogen from the plants on 

 which they subsist. Animals, which subsist on 

 other animals, derive their nitrogen from these 

 other animals which subsist on plants. Plants get 

 their nitrogen food from the soil, and the soil gets 

 it from the air. 



But microbes are the means of filing nitrogen- 

 ous food in the soil. Many species of microbes live 

 in the soil as their only abiding place. The air 

 presses everywhere upon the soil with the weight 

 of about fifteen pounds to the square inch. This 

 sends the air deeply into the soil everywhere. 

 Some species of microbes in the soil come in con- 

 tact with air, and, by their life processes, transfer 

 its nitrogen to the soil in such form that the fine 

 rootlets of plants get hold of it and appropriate it 

 as food. The more nitrogen is thus transferred to 

 the soil, the richer is the soil, and the better the 

 crops of the farmer. 



The soil in some large tracts of land becomes bar- 

 ren. Its crops do not pay for the labor required 

 to cultivate it. It is because of the absence of 

 these microbes. For some reason they have been 

 destroyed, or removed from the soil. 



Now make a pure culture of these required 

 species of soil microbes. Prepare the culture in 



