CHAPTER XVI 

 THE NEED OF COMMERCIAL FERTILIZERS 



Loss ^of Plant-food. The soil is composed 

 chiefly of material that never will enter into the 

 structure of plants, but that serves us by afford- 

 ing a congenial place for plant-roots. It anchors 

 the plants, holds moisture for them, and offers 

 opportunity for all the processes necessary to the 

 preparation of plant-food and to its use. In this 

 material are the abundant supplies of such plant- 

 food as silica, but, as has been previously stated, 

 their very abundance leads us rightly to disre- 

 gard them in our thinking. Our interest is only 

 in the very small percentage of material that is 

 composed of the four constituents which may be 

 lacking in available form in the soil : nitrogen, 

 phosphoric acid, potash, and lime. We believe 

 that the only consideration that now need be 

 given lime is as a soil-corrective and, when there 

 is no acidity, we may assume that there is plenty 

 of lime present. When yields of crops tend to 



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