PLANT FOOD IN THE SOIL 



77 



a reasonable amount of moisture is necessary, since in very dry 

 soils these processes are retarded. Fourth, basic substances, 

 such as calcium and sodium, are necessary, with which nitric 

 acid may combine to form nitrates. 



A soil which is moist but not too wet, which is well aerated, 

 and which is supplied with lime carbonate is in the ideal con- 

 dition for the processes of ammonification and nitrification to 

 take place. Drainage and thorough tillage both help to bring 

 about such a condition. Obviously, it is necessary to keep 

 within the soil a sufficient amount of organic matter, upon which 

 bacteria may act. 



84. How legumes 

 add nitrogen to the 

 soil. The class of 

 plants known as leg- 

 umes performs a 

 very remarkable and 

 interesting work in 

 supplying nitrogen 

 to the soil. Among 

 the legumes are such 

 plants as clovers, 

 peas, beans, and 

 alfalfa. It has long 

 been known that 

 the growing of these 



plants has a decidedly beneficial effect on the soil. Nearly every- 

 one who has lived in the country has heard men speak of clover 

 as a ''fertilizer." It was not until the latter part of the last 

 century, however, that the cause of the beneficial action of 

 legumes was discovered (Figs. 34 and 35). It was found to be 

 due to their ability to take a part or all of their nitrogen from 

 the soil air. It was also discovered that this property was in 

 some way connected with the nodules, or tubercles, upon the 

 roots of these plants. Careful examination showed these nodules 

 to be literally filled with bacteria. It was finally demonstrated 



FIG. 34. A soy-bean, cowpea, or clover catch crop 

 helps the succeeding corn crop 



