ENRICHING SOIL— COMMERCIAL RESOURCES 91 



quantities, but only in a fertile soil in suflBcient 

 amDimts to supply the plant with nitrop^ou. 



\'M. Ilunuis is the great storehouse of 

 nitro,ir«Mi. lliimus (Iocs not dissolve in water, 

 and so serves as a means of retaining tln^ 

 nitrogen against K'arliing. But if tlu> nitrogen 

 remained always in the hunms, it would not 

 he availal)le to plants, sinee 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 mi(M"o- organ- 

 isms (.T), 37)(i) . These germs are of several 

 kinds. One kind works u]>on tlu" hunnis and 

 changes its niti-ogen into ammonia, ami other 

 kinds change the annnonia into nitric acid. 

 This process of changing nitrogen into the 

 form of nitric acid or niti-ate is called nitri- 

 ticatiou. It is probable tii.it nitrogen enters 

 the plant chielly in form of nitrate, so that 

 all other forms of nitrogen must undergo nitri- 

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

 Since tilla^'e promotes th»> activities of the micro- 

 organisms (3.'), f)!,*, Hi)), it thereby incjeases tlie 

 supply of available nitrogen. 



l.'.s. It has been stated (135) that the great 

 quantity of nitrogt»n in the atmosphere is not 

 available to most })lants, because it is not in 

 u conjbine<l state. Thei*e uie certain jdaiits, 



