THE ORGANIC MATTER OP THE SOIL 125 



conditions not only prevent the proper decay of organic mat- 

 ter, bnt also tend to encourage the production of products in- 

 imical to plant growth. Therefore, in order that organic ma- 

 terials added to any soil may produce the proper decomposi- 

 tion products and perform their normal functions, soil con- 

 ditions in general must be of the best. Tile drainage should 

 be installed, if necessary, in order to promote aeration and 



•AMIMAL 



"^^'^^OREEN MANURES 

 CROP RESIDUES 



501L 

 ORGANIC MATTER 



SOIL 

 MINERALS 



#^^^ 



J^^ 



CO 



Fig. 22. — ^Diagram showing the practical sources of the soil organic 

 matter and the cycle through which its constituents pass. Note 

 that the carbon, oxygen and hydrogen come very largely from air 

 and water and that fixation of nitrogen may occur if the crop is 

 a legume. Only about 25 per cent, of the organic matter fed to 

 animals ever reaches the soil in farm manure under average con- 

 ditions. 



granulation. Lime shonld be added if basic materials are 

 lacking, for it promotes bacterial activity as well as plant 

 growth. The addition of fertilizers will often be a benefit, 

 as will also the establishment of a suitable rotation. The 

 rotation of crops not only prevents the accumulation of toxic 

 materials, but also, by increasing crop growth, makes pos- 

 sible a larger addition of organic matter by green-manuring. 



