CHAPTER XI 



FOOD REQUIREMENTS OF CROPS 



307. Amount of Fertility removed by Crops. The 

 amount of fertility removed from an acre of soil pro- 

 ducing average crops varies between wide limits. For 

 example, an acre of mangels removes 150 pounds of 

 potash, while an acre of flax removes 27 pounds ; an 

 acre of corn removes 75 pounds of nitrogen, while 

 an acre of wheat removes 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 is not a measure of 

 the power of crops to obtain food. An acre of corn 

 requires over twice as much nitrogen as an acre of 

 wheat, but wheat often leaves the soil in a more im- 

 poverished condition than corn, because corn has greater 

 power to procure nitrogen and utilize 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 twice as 

 much phosphoric acid as flax, but are a strong feeding 

 crop and need less help in obtaining this element. It 

 was formerly believed the plant food in the matured 

 crop indicated the kind and amount of fertilizing ingredi- 

 ents to apply, and that a correct system of manuring 



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