NITRIFICATION EXPLAINED 173 



same time that the nitrogen compounds in the soil organic matter 

 pass through the changes necessary to provide available nitrogen, 

 the mineral elements, which this organic matter contains, hkewise 

 become available. Thus a crop such as corn, for example, secures 

 its supply of nitrogen from one main source — the soil organic mat- 

 ter; and its supply of mineral elements from two sources— from the 

 mineral soil particles and from organic matter. 



Decay of Organic Matter Aids Decay of Mineral Particles. — 

 The organisms which cause the decomposition of the soil organic 

 matter perform a two-fold work. They not only bring about the 

 necessary changes in the organic matter to provide available 

 nitrogen and mineral elements for use by plants, but in an indirect 

 way they aid in the liberation of mineral elements contained in 

 the mineral soil particles. This is explained through the fact 

 that in all organic decay, acids are formed which are effective 

 agents in dissolving mineral matter. We can now understand more 

 clearly why it is important to maintain a good supply of organic 

 matter in soils to enable crops to secure needed and sufficient 

 elements. It is significant that rich, garden soils usually have a 

 high content of organic matter, and that they are much more 

 abounding in life than ordinary field soils. We can explain, too, 

 why some light-colored soils rich in all the important mineral 

 elements and having a low productive power, can be made to pro- 

 duce much larger yields simply by plowing under a good growth 

 of green rye. 



Some Fertilizers Valueless Without Decay. — ^Were it not for 

 the organisms of decomposition, fertilizers such as tankage, blood 

 meal, cottonseed meal, etc., would be of little or no value. More- 

 over, some insoluble mineral fertihzers, such as rock phosphate, 

 would be practically useless, but for the presence of decomposable 

 organic matter in the soil, or because of the organic matter in 

 which the fertilizer may be mixed when appHed. Rock phosphate 

 has been found to give best results in most soils when it is mixed 

 with manure or plowed under with green rye or clover. 



NITRIFICATION" 



Nitrification Explained. — The accompanying diagram (Fig. 

 115) is helpful in gaining a clear idea of the meaning of nitrification. 



This diagram explains that the nitrogen in organic matter is 

 held there in the form of complex, insoluble compounds which 

 must be broken down, through decomposition, into simpler com- 



