622 SOILS: PROPERTIES AND MANAGEMENT 



per ton, alfalfa about 50, cowpeas 43, and soy beans 

 53 pounds. These figures, even though they may 

 be far from correct, at least give some idea as to the 

 possible addition of nitrogen by green-manuring prac- 

 tices, and show how the soil may be enriched by such 

 management. As in the case of farm manures, the in- 

 direct effects of such a procedure may override the 

 direct influences, making the use of legumes as green- 

 manuring crop less necessary than at first thought might 

 be supposed. 



524. Decay of green manure. — As a green crop enters 

 the soil, the process of its decay is the same as that of 

 any plant tissue that becomes a part of the soil body. 

 The organisms that are active are those common to the 

 soil, together with such bacteria as are carried into the 

 soil on the turned-under crop. The decay- should be 

 accomplished under aerobic conditions so that only 

 beneficial products may result. Plenty of water is a 

 necessity, as otherwise the soil would be robbed of a 

 part of its available moisture in facilitating the process of 

 decay. When proper decay has occurred, end products 

 should result which can be utilized as plant-food. The 

 intermediate compounds that are formed should yield a 

 black humus, should readily split up into simple com- 

 pounds, and should be in general beneficial, both directly 

 and indirectly, to crop growth. The decay of green 

 manure under conditions of poor drainage and improper 

 aeration is likely to cause the generation of materials 

 detrimental to the proper development of plants. 



525. Crops suitable for green manures. — The crops 

 that may be utilized as green manures are usually grouped 

 under two heads, legumes and non-legumes. Some of the 

 common green manures are as follows: — 



