266 PLANT PHYSIOLOGY 



The table (p. 265) shows that highly cultivated crops, like 

 potatoes and legumes, exhaust the soil more than do the cereals 

 and thus increase the fertilizer requirements of the soil. Legumes, 

 therefore, cannot be regarded as crops that only enrich the soil. 

 Though increasing its nitrogen content, they exhaust the mineral 



reserve. 



In farm practice, where almost all the mineral elements taken 

 from the soil are returned to it in the form of manure and other 

 refuse, soil exhaustion takes place slowly. But with the develop- 

 ment of modern marketing, agricultural products are not used 

 locally but are sold mostly in the city, and the mineral substances 

 contained in the crop are lost forever. Therefore, the farmer 

 must return to the soil the substances removed. This statement 

 was clearly formulated for the first time by Liebig and termed by 

 him ''the law of return." He took this law literally, stating 

 that without exception all the mineral elements removed from 

 the soil must be returned, preferably in the form of artificial 

 fertilizers, which he was the first to introduce into agricultural 

 practice. In his first artificial fertilizer mixtures, he had all the 

 ash elements, including Mg, Na, S, and Si. 



It was soon found, however, that the law of return must not 

 be taken literally and that it is quite unnecessary to return some 

 of the elements, for inexhaustible reserves of them are contained 

 in the gradually disintegrating rocks. That is why at the 

 present time usually phosphorus, potash, and nitrogen are 

 returned to the soil, without much attention being paid to the 

 other elements. 



Plants do not absorb the elements at the same r?fte during 

 the different stages of growth. As a rule, most of the annuals 

 absorb the greater part of the salts necessary for their develop- 

 ment before the flowering stage. In some plants, as for instance 

 corn, all the soil nutrients required for their further development 

 are already accumulated in the stem at the time of flowering, 

 when their further absorption ceases almost completely. There 

 is also a difference in the rate of absorption of individual elements 

 during the development of the plants. 



Thus, for instance, according to Arendt's data, potassium and 

 calcium are absorbed by oat plants chiefly during the first half 

 of their vegetation, while the absorption of magnesium is very 

 uniform throughout their growth period. Dividing the vege- 



