90 TEXTBOOK OF PLANT PHYSIOLOGY 



have a higher fertilizer requirement. Legumes, therefore, cannot 

 be regarded as crops that enrich the soil. Though increasing its 

 nitrogen content, they exhaust the mineral reserve. 



In economical 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 

 development of modern marketing, agricultural products are not 

 used locally, but are sold mostly in the city, and the mineral sub- 

 stances 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, 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 paying any attention to the other elements. 



Plants do not absorb the elements at the same rate during the 

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

 the greater part of the salts necessary for their development 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. Not all of the indi- 

 vidual elements found in the ash are absorbed at the same rate 

 during the development of the plants. Potash, for example, 

 accumulates before phosphorus. This may be due to the observed 

 fact that potash takes part in the metabolism of carbohydrates, 

 which proceeds more energetically during the vegetative period, 

 while phosphorus is more important for successful flowering and 

 fruiting. 



The composition of ash of the various organs of the plant 

 changes also with age. Leaves of the beech, picked in May, were 

 found to contain per 100 parts of ash, 42 parts of KoO, 32 parts of 



