PLANT FOOD 47 



The elements which become deficient in the soil through long cultivation 

 and the removal of crops are nitrogen, potassium and phosphorus. Any 

 one familiar with the composition of commercial fertilizers knows that it is 

 these elements in some combination which give them value. 



Nitrogen must be changed into the form of nitrate ; that is, some combi- 

 nation of nitric acid with lime or potash, making a neutral salt, before green- 

 leaved plants can take it as food. Potassium must be changed by oxidation 

 into potash in order that it may be dissolved in the soil-water. Phosphorus 

 must be in the form of phosphoric acid, for the element phosphorus burns 

 up at once on exposure to the air. It is generally combined with calcium, 

 making the phosphate of lime, an insoluble compound, which is rendered 

 soluble by sulphuric acid ; and thus gives us a superphosphate of lime, which 

 is available to plant life. 



Phosphoric acid is a compound of phosphorus, oxygen and hydrogen, 

 but in phosphates the metallic bases replace the hydrogen. Nitrification, or 

 the transformation of organic matter into nitrates so that green plants can 

 get nitrogen, is carried on by minute organisms in the soil, and the life of 

 these organisms depends on the presence of the organic matter in the soil; 

 making it, as we have seen, a living, rather than a dead soil. Of this process 

 of nitrification we will treat more fully further on. 



