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TEXTBOOK OF PLANT PHYSIOLOGY 



unfertilized. These two series serve as controls, the first showing 

 the maximum yield obtainable under the conditions of the experi- 

 ment, the second, the minimum yield. The rest of the containers 

 are fertilized with one of the elements mentioned. For instance, 

 one series receives only nitrogen, another potassium, and a third, 



Fig. 34. — Effects of different fertilizers on wheat in pot cultures. On the extreme 



Left without fertilization; one next to it fertilized with nitrates; the third with 



potassium and phosphorus salts. The pot on the extreme right has received 



complete fertilization (after Wagner). 



phosphorus. The experiment is continued throughout the vege- 

 tative period, and at the end of it the condition of plants in the 

 different series is compared. If the addition of one of the fer- 

 tilizers, for instance, nitrogen, increases the yield to its maximum, 

 this shows that the soil is in need of nitrogen. If the same result is 



