Nitrogen in Soils. 



83 



below gives the distribution and amount per acre in each 

 of the upper four feet of four Manitoba soils: 



Thus it is seen that in the upper four feet of these rich 

 soils there was found from 6 to 16 tons per acre of nitrogen. 



97. Forms in Which Nitrogen Occurs in the Soil. Nitro- 

 gen occurs in the soil in several distinct forms : 



1. In humus, described in (88), which is by far the most 

 important form and the substance which carries the largest 

 proportion of that which the soil contains. 



2. In organic matter in the form of roots, stubble and 

 farmyard manure, which by slow degrees is converted into 

 humus to make good that which hhs been used. 



3. As free nitrogen in soil-air which is seized upon by 

 some forms of microscopic life described in (101) and con- 

 verted into organic form for their use. 



4. As nitrates of lime, magnesia, potash and soda, and 

 this is the form from which most of the higher plants get 

 their supply. 



5. As ammonia, nitrous acid and nitric acid, which are 

 transition stages to one of the nitrates named above and 

 which are formed either from the humus or organic matter 

 or are brought down with the rain. 



98. Distribution of Nitrogen in the Soil. In humid cli- 

 mates the largest amount of nitrogen is found in the surface 

 6 to 12 inches, but as already shown in (96) large quan- 

 tities are found as deep as four feet below the surface. 



