128 SOILS AND FERTILIZERS 



amount and form of its nitrogen. In soils of the 

 highest fertility there is usually present from 0.2 to 

 0.3 per cent of total nitrogen, equivalent to from 

 7000 to 10,000 pounds per acre to the depth of one 

 foot. Many soils of good crop-producing power contain 

 as low as 0.12 per cent. There is usually two or three 

 times more nitrogen in the surface soil than in the sub- 

 soil. In sandy soils which have been allowed to decline 

 in fertility, there may be less than 0.04 per cent. Of 

 the total nitrogen in soils there is rarely more than 2 

 per cent at any one time in forms available as plant 

 food. 44 A soil with 5000 pounds of total nitrogen per 

 acre may contain less than 100 pounds of available nitro- 

 gen soluble in the soil water, of which only a part is 

 assimilated by the roots of crops. Hence it is that a 

 soil may contain a large amount of total nitrogen, and 

 yet be deficient in available nitrogen. 



139. Amount of Nitrogen removed in Crops. - - The 

 amount of nitrogen removed in crops ranges from 25 to 

 100 pounds per acre, depending upon the nature of the 

 crop. It does not necessarily follow that the crop which 

 removes the largest amount of nitrogen leaves the land 

 in the most impoverished condition. Wheat and other 

 grains, while they do not remove so much in the crop, 

 leave the soil more exhausted than if other crops were' 

 grown. This, as will be explained, is caused by the loss 

 of nitrogen from the soil in other ways than through the 

 crop. 



