THE ORGANIC MATTER OF THE SOIL 151 



capacity of the soil resulting from the presence of organic 

 materials is of some importance in drought resistance, 

 while the black color imparted by the humus tends to 

 raise the absorptive power of the soil for heat. 



The soil organic matter, however, functions in other 

 ways than those strictly physical. The humus or its 

 degradation products may serve as plant-food. Bacteria 

 and other soil organisms are also furnished a source 

 of energy thereby, and the production of carbon dioxide 

 is much increased. This carbon dioxide, as well as the 

 organic acids generated, tends to raise the capacity of 

 the soil water as a solvent agent, and thus the amount of 

 mineral plant food available to the crop is greatly in- 

 creased. The general effect of organic matter, then, is 

 to better the soil as a foothold for plants, and to increase 

 either directly or indirectly the available food supply for 

 the crop. 



102. Maintenance of soil organic matter. — The main- 

 tenance of a proper supply of organic matter in a soil is 

 a question of great practical importance, as productivity 

 is governed very largely by the humus content of the soil. 

 This maintenance of the soil humus depends on two 

 factors — the source of supply and methods of addition, 

 and the promotion of proper soil conditions in order that 

 the organic matter may perform its legitimate functions. 



The organic matter of the soil may be increased in a 

 natural way by. the plowing-under of green crops. This 

 is called green-manuring and is a very satisfactory prac- 

 tice. Such crops as rye, buckwheat, clover, peas, beans, 

 and vetch lend themselves to this method of soil im- 

 provement. Not only do these crops increase the actual 

 carbohydrate content of a soil, but in the case of legumes 

 the nitrogen also is increased in amount, due to the 



