Manures and Fertilizers 115 



It is thought that to some extent fertihzers act as disin- 

 fectants to reduce damage by unfavorable agents in the soil. 



FERTILIZER PRACTICES 



Elements often present in the soil in too small quantities 

 for the best growth are nitrogen, phosphorus, potassium 

 and calcium. When these are added in the form of fer- 

 tilizers or manure, the yield of most crops is increased. 

 The extent to which farmers may use fertilizers depends 

 on the cost of the fertilizer compared with the net value of 

 the increase in the crop. Another factor which must be 

 considered is the residual effect of fertilizers on the soil. 

 In the early days of the use of fertilizers, small applications 

 were often made without much attempt to maintain the 

 soil supply of organic matter. When the amount supplied 

 was sufficient only to give the young plant a better start, 

 it is probable that the increase in yield removed more of 

 the plant-food elements than was added in the fertilizers. 

 Being usually applied to grain or to tilled crops, there 

 was but little increase in the amount of organic matter 

 returned to the soil in the plant residues, not enough to 

 replace that destroyed by the processes of tillage. This 

 has led to a strong belief among older farmers that " fer- 

 tilizers will run out the soil." The present tendency 

 among potato-growers is to use much larger applications, 

 which supply an excess of soluble plant-food elements 

 over those removed in the crop. 



Practice in the use of fertilizers was for many years 

 based on the theories of the famous German chemist Von 

 Liebig. He taught that plant-food must be returned to 

 any soil to equal exactly that removed in the crop. The 

 practical error in the assumption that the composition 



