62 



ELEMENTARY STUDIES IN BOTANY 



soil in the form of salts called nitrates, which are soluble and 

 therefore " available." It is evident that when crops are 



FIG. 49. Root-tubercles on sweet clover. 



removed from the soil, a large amount of nitrogen compounds 

 is removed also, so that crops drain nitrogen from the soil. 

 This cannot go on indefinitely, without fresh supplies of 



