THE ABSORPTION OF MINERAL ELEMENTS 



83 



obtained by adding phosphates, the soil is deficient in phosphorus. 

 If the addition of only one element is not sufficient to secure the 

 highest yield, then experiments are carried out with combinations 

 of the various fertilizers, for instance, K and P, K and N, P and N. 

 From the combinations which give the best results one can judge 

 which of the elements are most deficient in the soil. By means of 

 such experiments, Wagner succeeded in demonstrating very clearly 



Fig. 35. — Influence of various fertilizers on peas in pot cultures. Pot on extreme 

 left without fertilizers; one next to it fertilized with nitrates; the third with potas- 

 sium and phosphorus salts. The pot on extreme right has received complete 



fertilization {after Wagner). 



the capacity of leguminous plants to thrive on the nitrogen of the 

 atmosphere. He discovered that legumes do not respond to the 

 application of nitrates (Figs. 34 and 35). 



Though the pot-experiment method is very valuable in indicat- 

 ing which of the nutrient substances is present in the soil and which 

 fertilizers must be added, it cannot furnish correct quantitative 

 data, for in pots the roots of plants are confined to a volume of soil 

 considerably smaller than when grown under field conditions. 



