SOILS AND FERTILIZERS 113 



as possible over the surface, then plowed or harrowed 

 in, for if they are not thoroughly mixed with soil, they 

 will do little good. 



If immediate results are desired in the form of a 

 large yield, soluble fertilizers must be applied in the 

 spring, but if the soil is to be gradually enriched by 

 their use for a number of years, a slow, insoluble fer- 

 tilizer should be put on in the fall. 



Nitrogen. Much has already been said about the 

 need of nitrogen. The atmosphere is largely com- 

 posed of this element, about 80 per cent, and yet 

 plants surrounded with air may starve for lack of ni- 

 trogen. The reason is that they are so constituted 

 as to be unable to take in the free nitrogen of the air. 

 The air must first enter the soil, and there give up 

 its nitrogen to make soluble nitrates nitrogen com- 

 pounds before the plant can take it in and use it. 

 A small amount of nitrates nitric acid and ammonia 

 falls with rain and snow and enters the soil, but as 

 this is variable and insufficient for plant needs, we must 

 consider what the conditions are under which soluble 

 nitrates are formed in the soil, in order to understand 

 how plants get their needed food. 



Legumes. It has remained for modern science to 

 explain a fact well known in ancient times, that other 

 crops yield more abundantly when the land has been 

 sown to clover, peas, or beans the preceding year. 

 We now know why this is so. These plants and their 

 kind, known as legumes, or pod plants, have little 

 swellings, or tubercles, on their roots, caused by cer- 

 tain small organisms, called bacteria. Through the 

 agency of these microscopic plants the free nitrogen 

 of the air and that found in organic matter in the 



M. & H. AG. 8 



