CHAPTER XI 



FOOD REQUIREMENTS OF CROPS 



290. Amount of Fertility Removed by Crops. — 



From an acre of soil, producing average crops, the 

 amount of fertility removed varies between wide lim- 

 its. For example, an acre of mangels removes 150 

 pounds of potash, while an acre of flax removes 27 

 pounds ; an acre of corn removes about 75 pounds of 

 nitrogen, while an acre of wheat removes about 35 

 pounds. Crops which remove the most fertility do 

 not always require the most help in obtaining their food. 

 This is because the amount of plant food assimilated, 

 and the power of crops to obtain this food, are not 

 the same. An acre of corn, for example, takes over 

 twice as much nitrogen as an acre of wheat, but wheat 

 will often leave the soil in a more impoverished con- 

 dition than corn, because corn has greater power for 

 procuring nitrogen and for utilizing that formed by 

 nitrification after the wheat crop has completed its 

 growth. The available nitrogen if not utilized by a 

 crop may be lost in various ways. Mangels require 

 about twice as much phosphoric acid as flax, but are 

 a strong feeding crop and require less help in obtain- 

 ing this element. 



It was formerly believed that the amount of plant 



