SOILS AND FERTILIZERS 103 



plants, through the action of the bacteria on their roots, 

 take the nitrogen of the air and make of it a nitrate 

 for the use of the plant. (See page 113.) 



To gain the nitrogen and not to remove from the 

 soil the other necessary food substances, these crops 

 should be turned under and not harvested. Figure 39 



2TONS CLOVER 



o 



Q. 

 FIG. 39. Showing Plant Food taken from Soil by Clover. 



shows that in a clover crop of two tons taken from 

 an acre of land there is an increase of nitrogen, but 

 there is a great decrease of phosphorus and of potas- 

 sium. 



Note. Under normal conditions of soil fertility, the clover plant 

 will obtain a part of its nitrogen from the soil and a part from the air. 

 It has been estimated that the part of the clover plant removed in a 

 crop as hay will contain just about the amount of nitrogen obtained 

 from the air, the nitrogen in the roots and stubble representing the 

 amount obtained from the soil. If this estimate is correct, it is evident 

 that the soil will neither gain nor lose this element when the hay is 

 removed. 



