ABSORPTION OF MINERAL ELEMENTS BY PLANTS 261 



binations of the various fertilizers, e.g., K and i\ K and N, 

 P and N. From the combinations that give the best results, 

 one can judge in which of the elements the soil is most deficient. 

 By means of such experiments, Wagner succeeded in demon- 

 strating very clearly the capacity of leguminous plants to thrive 

 on the nitrogen of the atmosphere. He discovered that legumes 

 do not respond to the appHcation of nitrates (Figs. 78, and 79). 



Fig. 79. — 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 potassium and phosphorus salts. The pot on extreme right has received 

 complete fertilization {after Wagner). 



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

 cating 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. This is the reason Avhy the requirements for 

 fertilizers of plants in pots are always higher than those of plants 

 in the field. For exact determination of the amounts of fertilizer 

 to be added, it is expedient, therefore, to use the field method. 



