NITROGEN COMPOUNDS OF THE SOIL I 13 



nitrogen. In other soils the nitrogen is present in 

 less complex forms. The organic nitrogen of the soil 

 may vary in complexity from forms like the nitrogen 

 of urea to forms like that of peat. 



124. Amount of Nitrogen in Soils. — The fertility 

 of any soil is dependent, to a great extent, upon the 

 amount and form of its nitrogen. In soils of the 

 highest degree of fertility there is usually present 

 from 0.2 to 0.3 per cent, of total nitrogen, equivalent 

 to from 7,000 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, of nitrogen. 

 There is usually two or three times more nitrogen in 

 the surface soil than in the subsoil. In many sandy 

 soils which have been allowed to decline in fertility 

 the nitrogen 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. 43 A soil with 5,000 pounds of total nitrogen 

 per acre would contain about 100 pounds of available 

 nitrogen of which only a part comes in contact with 

 the roots of crops. Hence it is that a soil may con- 

 tain a large amount of total nitrogen and yet be defi- 

 cient in available nitrogen. 



125. 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 



