28 FERTILITY OF THE SOIL 



and changing it into nitrogen compounds which can 

 be used by the legumes themselves and the succeeding 

 crops. 



Phosphorus and potassium. Crops remove such 

 large amounts of phosphorus and potassium from 

 soils that it is necessary to replace these elements by 

 the use of fertilizers. Barnyard manure, bone meal, 

 and phosphate rock are used to enrich the soil with 

 phosphorus. Potassium is added to the soil in manure, 

 potash, and wood ashes. 



If the soil is poor in calcium, lime, a compound of 

 calcium, is added. Lime is more often added for other 

 reasons, however, than that of supplying calcium. 



The other elements of plant foods are used in such 

 small amounts that usually the supply in the soil is 

 ample ; and it is not often necessary to add them by 

 use of fertilizers. 



Condition of the soil. The proper physical condi- 

 tion of the soil enables the crops to obtain the food 

 supply present in the soil. When we use such words 

 as mellow, friable, fine, loose, porous, hard, cloddy, 

 compact, sour, and sweet, we are speaking of the 

 physical condition of the soil. Fineness or good tex- 

 ture is the most important physical property of a soil, 

 and it is one of the main objects of tillage. If the 

 soil is cloddy, the plant food is locked up inside the 

 clods, and is beyond the reach of the roots. More 

 soil water and soil air are held in a soil of fine texture 

 than in one of coarse texture. 



