180 SOIL ORGANISMS IN RELATION TO SOIL FERTILITY 



the bacteria returning to the soil in vast numbers. Here they may 

 remain for a considerable length of time before they have an 

 opportunity to enter other plants and again gather nitrogen 

 and multiply. 



How the Growing of Legumes Improves Soils. The growing 

 of legumes may improve soils directly in two ways: by adding 

 organic matter and by increasing the nitrogen content. The 

 amount of nitrogen that may be actually added to the soil in grow- 

 ing a crop of clover, for example, depends upon the soil and 

 especially upon what disposition is made of the crop. The crop 



FIG. 121. A few of the bacteria which fill the cells, highly magnified. (U. S. D. A.) 



may be plowed under, it may be cut for hay and sold off the farm, 

 or it may be fed on the farm, either as pasture or hay. 



In studying the clover plant, it has been found that practically 

 two-thirds of the total nitrogen contained in it (roots and all) is 

 in the hay, and one-third in the roots. Under average, normal 

 soil conditions, the clover gets about two-thirds of its nitrogen 

 from the nodule bacteria, and one-third from the soil reserve. Thus 

 when the crop is taken off the field, there is left in the roots practi- 

 cally the same amount of nitrogen as was taken from the soil 

 supply. The nitrogen content of the crop harvested, there- 

 fore, represents the amount fixed or taken from the air by the 

 nodule bacteria. 



It follows that when a clover crop is plowed under, the soil is 

 enriched by the amount of nitrogen contained in the clover plowed 

 under. This amounts to about forty pounds for every ton of hay 



