CHAPTER XI 



FOOD REQUIREMENTS OF CROPS 



293. 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 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, for example, takes 

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

 wheat will often leave the soil in a more impover- 

 ished condition than corn, because corn has greater 

 power for procuring nitrogen and for utilizing that 

 formed by nitrification after the wheat crop has com- 

 pleted its growth. The available nitrogen if not 

 utilized by a crop may be lost in various ways. Man- 

 gels require about twice as much phosphoric acid as 

 flax, but are a strong feeding crop and require less 

 help in obtaining this element. 



It was formerly believed that the plant 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 



