320 Manures, Composts, Green Manures 



in available forms and upon the chemical nature and abundance of 

 the humus produced. The extent of the liberation of the nutrients 

 and the amount of humus produced can thus be controlled by proper 

 selection of plants for green manuring and of the time when these 

 plants are plowed under. 



The humus left from the decomposition of green manures does not 

 completely replace the humus lost from the soil as a result of culti- 

 vation. Mooers has shown that, when cowpeas were grown on a 

 soil and the whole crop was turned under annually, there was a loss 

 of 0.11 per cent of humus, or a total of 2,200 pounds per acre during 

 a 20-year period. When the cowpea crop was removed and only 

 the stubble turned under, the loss of humus from the soil was con- 

 siderably greater: at the end of the 20 years, the total loss was 0.24 

 per cent, or 4,800 pounds per acre. As a result of the turning under 

 of 20 annual crops of cowpea hay containing about 20 tons of dry 

 matter, there was left in the soil 2,600 pounds of humus, that is, only 

 6.5 per cent of the total plant material. Stable manure, on the other 

 hand, not only could fully replace the losses of humus from the soil 

 but actually brought about an increase in humus content. When the 

 soil received stable manure, at the rate of 4 tons per acre annually 

 for 20 years, a gain of 0.11 per cent of humus took place. 



Use of green manures is recommended where an available supply 

 of nitrogen and carbon dioxide is required but where the amount of 

 humus left is not of great importance. When it is essential to in- 

 crease the supply of humus in the soil, stable manures or mature 

 plant residues are to be preferred, either after they have been com- 

 posted or when supplemented with available nitrogen and phos- 

 phorus upon addition to the soil. 



