226 SOILS AND FERTILIZERS 



food present in the matured crop indicated the kind 

 and amount of fertilizing ingredients to apply, and 

 that a correct system of manuring required a return 

 to the soil of all elements removed in the crop. Ex- 

 periments have shown that both of these views are in- 

 correct. The composition of plants cannot be taken as 

 the basis for their manuring. For example an acre of 

 wheat contains 35 pounds of nitrogen while an acre of 

 clover contains 70 pounds. If 70 pounds of nitrogen 

 were applied to an acre of clover and 35 pounds to an 

 acre of wheat, poor results would follow, because clo- 

 ver can obtain its own nitrogen while wheat is nearly 

 helpless in obtaining it, and the 35 pounds would not 

 necessarily come in contact with the roots so that it 

 could all be assimilated. While the amount of plant 

 food removed in crops cannot serve as the basis for their 

 manuring, valuable results are obtained from the study 

 of the different elements of fertility removed in crops, 

 and in making use of the following figures, other fac- 

 tors, as the influence of the crop upon the soil and the 

 power of the crop to obtain its food, must also be con- 

 sidered. 



