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 
