40 THE COW PEA. 



their nourishment from the roots on which they grow, but 

 the far greater nitrogen supply essential to their life, they 

 gather from the air which circulates through the soil. Each 

 individual of these low forms of vegetable life exists only for 

 a few hours, and at death mingles with the soil and goes 

 through a process of decomposition, similar to that which 

 takes place in other organic matter. The only difference is 

 that when vegetable matter decays, it leaves in the soil 

 only what it has taken from it, but these bacteria add the 

 nitrogen they have absorbed from the air and changed into 

 a form available for succeeding crops. 



When plants, other than legumes, such as rye, mus- 

 tard, oats, etc., are used for green manuring, they really add 

 no plant food to the soil, except what they have absorbed 

 from the same source during their growth. It is true, 

 though, that they change some of the plant food, which the 

 soil already contains, into a more easily available condition, 

 but the cow pea and other legumes do the same and also 

 enrich the soil by their accumulations of nitrogen, which, as 

 stated, is the most expensive element of fertility. 



The value of any material as a soil renovator depends 

 largely on the nitrogen, potash and phosphoric acid it con- 

 tains. The following table, from the Year Book (1895) of 

 the U. S. Department of Agriculture, gives the average 

 percentages of nitrogen, potash and phosphoric acid in 

 common hay and fodder crops. As these percentages are 

 largely influenced by the water content of the material, the 

 table also shows the percentage of moisture: 



