ENRICHING SOIL COMMERCIAL RESOURCES 91 



quantities, but only in a fertile soil in sufficient 

 amounts to supply the plant with nitrogen. 



137. Humus is the great storehouse of 

 nitrogen. Humus does not dissolve in water, 

 and so serves as a means of retaining the 

 nitrogen against leaching. But if the nitrogen 

 remained always in the humus, it would not 

 be available to plants, since to be absorbed it 

 must dissolve in the soil-water. Fortunately 

 there is a process whereby the nitrogen in the 

 insoluble humus is made to be available. This 

 process is the work of germs or micro-organ- 

 isms (,35, 35a). These germs are of several 

 kinds. One kind works upon the humus and 

 changes its nitrogen into ammonia, and other 

 kinds change the ammonia into nitric acid. 

 This process of changing nitrogen into the 

 form of nitric acid or nitrate is called nitri- 

 fica*lou. It is probable tiiat nitrogen enters 

 the plant chiefly in form of nitrate, so that 

 all other forms of nitrogen must undergo nitri- 

 fication, or be nitrified, before they are of use. 

 Since tillage promotes the activities of the micro- 

 organisms (35, 52, 89), it thereby increases the 

 supply of available nitrogen. 



138. It has been stated (135) that the great 

 quantity of nitrogen in the atmosphere is not 

 available to most plants, because it is not in 

 a combined state. There are certain plants., 



