620 SOILS: PROPERTIES AND MANAGEMENT 



the air, and the plowing-under of a crop therefore in- 

 creases the store of such constituents in the soil. If the 

 plant is a legume and the nodule organisms are active, 

 the nitrogen content of the soil is also augmented. The 

 mineral parts of the turned-under crop, of course, come 

 from the soil originally and they are merely turned back 

 to it again. As they return, however, they are in intimate 

 union with organic materials, and are thus readily avail- 

 able as plant-food as the decay process goes on. Indeed 

 they are much more readily available than they previously 

 were, when the green-manuring crop acquired them. 

 Actual additions are -thus made to the soil, together with 

 a promotion of an increased availability of the constit- 

 uents dealt with. 



Green manures may function also as cover crops, in 

 so far as they take up the extremely soluble plant-food 

 and prevent it 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 adsorbed only 

 slightly by the soil particles. Besides this, green manures, 

 especially those with long roots, tend to carry food up 

 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 bacteria, 

 and tends to stimulate biological changes to a marked 

 degree. This bacterial action is especially prone to in- 

 crease the production of carbon dioxide, ammonia, ni- 

 trates, and organic acids of various kinds, which are very 

 important in plant nutrition. The humus that results 

 from this decay increases the adsorptive power of the soil, 

 and promotes aeration, drainage, and granulation con- 

 ditions that are extremely important in successful crop 

 growth. 



