124 SOME PRACTICAL LESSONS FROM SOIL BACTERIOLOGY. 



besides all the advantages of a cover crop. It not only prevents 

 the loss of available plant foods by drainage, but it also adds a 

 considerable quantity of food to the soil. It not only serves as a 

 catch crop to hold the nitrates that may form during the season 

 when the main crop is not growing, but it may add to the total nitro- 

 gen of the soil. The general plan of green manuring is to grow upon 

 the soil some leguminous crop that increases the nitrogen content 

 of the soil, and, after proper growth, to plow the whole crop into the 

 soil. The addition of this large amount of organic matter to the soil 

 will stimulate the bacterial activities of decomposition. The roots, 

 stems, leaves, and fruits of the crop undergo a decomposition and 

 subsequent nitrification, resulting finally in the formation of nitrates 

 which can be utilized by the next crop grown on the same soil. 



When adopting the plan of green manuring the first thing is to 

 select the crop that is to be so utilized. Manifestly, from what has 

 been learned, this should be some one of the legumes, since this family 

 of plants alone assimilates nitrogen from the air, at least in any con- 

 siderable quantity. Other plants have been used for the purpose, 

 but while they are valuable in supplying some organic material 

 which helps maintain the store of humus, they are far inferior to 

 legumes which, in addition to all the other advantages, add usable 

 nitrogen in quantity. Attention should be given to the nature of 

 the soil, and to the kind of legume that will best flourish in the soil ; 

 the legume must produce plenty of root tubercles, otherwise the 

 chief value of the green manuring is lost. Green manuring is of 

 particular value in sandy, loose soils, where the humus is scanty, 

 and where the texture of the soil facilitates losses by draining. In 

 such soils so rapid is the draining that it is sometimes difficult to get 

 fertilizers to remain in the soil long enough for their proper assimila- 

 tion by the plant. The use of legumes, plowed under to furnish a 

 mass of decaying vegetation, greatly improves the texture of the soil 

 and will, in time, give them a fair humus content. By this means 

 very unpromising sandy soils can be reclaimed to a fair condition of 

 fertility. The legumes found to be best adapted to such sandy soils 

 are the cow pea, the soy bean, the velvet bean and the crimson clover. 

 With clay soils, on the other hand, green manuring must be 



