AMOUNT OF PLANT FOOD IN THE SOIL 



119 



78. Amount of Plant Food in the Soil. An acre of 

 average productive soil to the depth of 6|^ inches, rep- 

 resenting the amount usually turned over in plowing, 

 weighs about 2,000,000 pounds and will contain nearly 

 8000 pounds of nitrogen, 2000 pounds of phosphorus, and 

 35,000 pounds of potassium. This, however, is a rough 

 estimate only. The nitrogen content may vary from 1000 

 to 35,000 pounds ; the 

 phosphorus from 160 to 

 15,000 pounds ; and the 

 potassium from 3000 to 

 60,000 pounds. In some 

 soils there is scarcely 

 any lime at all; in 

 others the lime runs as 

 high as 20 per* cent. 



While there are enor- 

 mous quantities of po- 

 tential plant food in an 

 average soil, yet only a' 

 small fraction of it is 

 liberated as available 

 plant food during -an av- 

 erage season of average 

 farming. That is, two 

 per cent of the nitrogen (8000 x .02 = 160 pounds of 

 nitrogen) ; one per cent of the phosphorus (2000 x .01 = 20 

 pounds of phosphorus) ; and ^ of one per cent of potassium 

 (35,000 x .0025 = 87.5 pounds potassium) is estimated to 

 be liberated in one season. In other words the surface soil 

 to the depth of 6| inches should have resources enough to 

 supply the nitrogen for 50 years, the phosphorus for 100 

 years, and the potassium for 400 years, even were we to 

 ignore all additional plant food from whatever source. 



SWEET CLOVER ON SHALLOW LIMESTONE 

 SOIL.- 



