538 NATURE AND PROPERTIES OF SOILS 



favorable, an increase in crop production should result. 

 Where there is a shortage of farm manure, the practice be- 

 comes of special importance since roots and crop residues are 

 usually insufficient to maintain the organic content of the 

 soil. Even where manure is available, a green-manuring 

 crop now and then in the rotation does much towards sus- 

 taining normal production. 



The effects of turning under green plants are both direct 

 and indirect — direct as to the influence on the succeeding crop, 

 and indirect as to the soil so treated. In the first place, cer- 

 tain ingredients are actually added to the soil by such a 

 procedure. The carbon, oxygen, and hydrogen of plants come 

 Largely from the air and water, and the plowing-under of a 

 crop, therefore, increases the store of such constituents in the 

 soil. The compounds that result from crop decay increase 

 the absorptive power of the soil, and promote aeration, drain- 

 age, and granulation — conditions that are extremely impor- 

 tant in successful plant growth. If the crop turned under is 

 a legume and the nodule organisms are active, the store of soil 

 nitrogen is markedly augmented, a point of extreme impor- 

 tance in fertilizer practice. 



Green-manures may function also as cover-crops, insofar as 

 they take up the extremely soluble plant nutrients and pre- 

 vent them from being lost in the drainage water. The nitrates 

 of the soil are of particular importance in this regard as they 

 are very soluble and are absorbed only slightly by the soil 

 complexes. Besides this, green-manures, especially those with 

 long roots, tend to carry nutrients upward from the subsoil 

 and when the crop is turned under this material is deposited 

 within the root zone. Again, the added organic material acts 

 as a food for soil organisms, and tends to stimulate biological 

 changes to a marked degree. This biological action is espe- 

 cially important in the production of carbon dioxide, am- 

 monia, nitrates, and organic compounds of various kinds, 

 which are necessary in plant nutrition. 



