112 WHEAT 



Hopkins is authority for the statement that 

 the fertility of all normal soils in the United 

 States may be permanently maintained and even 

 increased by adopting systems of farming which 

 will supply three constituents regularly and in 

 proper amounts, namely, limestone, phosphorus 

 and organic matter. The limestone is required 

 to correct acidity, and is most cheaply applied 

 in the form of ground limestone rock as a surface 

 dressing. The phosphorus is needed solely for 

 its plant food value, and its most economical 

 source is ground rock phosphate. The supply 

 of organic matter must be renewed to provide 

 nitrogen and to keep the soil in favorable physical 

 condition. It has to do also with making avail- 

 able the potassium and other essential plant food 

 elements contained in the soil in abundance. 



The ideal farm practice is to return to the soil 

 all plant food not sold from the farm. The or- 

 ganic matter may be in part maintained by plow- 

 ing down the stubble and by saving the straw 

 and manure and returning it again to the soil. 



Investigations at several experiment stations 

 have determined that about one-third of the total 

 organic matter and nitrogen in a red clover plant 

 is contained in the roots and stubble. Alfalfa con- 

 tains a larger proportion of nitrogen in its roots, 

 while sweet clover and crimson clover may con- 

 tain a little less than red clover. If a crop of any 

 of these legumes is plowed under for green manure 

 there is added to the soil a definite amount of 

 organic matter and nitrogen which may be readily 

 computed. Thus the plowing under of a crop of 



