Decomposition of Humus 



139 



When a virgin soil is brought under cultivation, the humus con- 

 tent is reduced, at first rapidly, and theVi more slowly, depending 

 upon the soil, its climatic conditions, and the manner of cultivation. 

 Although a crop, whether cultivated or unculti\ated, leaves certain 



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 Q0 20 



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Sheep manure + straw 



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Period of decomposition, weeks 



Fig. 61 

 and 



. Influence of straw upon the liberation of nitrogen as nitrate from urine 

 cow and sheep manure, in the process of decomposition ( from Joshi ) . 



residues in the form of roots and stubble, which are gradualh' 

 changed to humus, the amount of humus decomposed as a result 

 of culti\ation is not fully compensated by the humus produced from 

 these residues. To keep up the fertility of the soil in certain types 

 of farming, it is essential to increase the humus content above that 

 made possible by the residues of a cultivated crop grown on the soil. 

 This can be done by growing a special crop to be plowed under for 

 green manuring purposes, by adding stable manures, or by intro- 



