CHAPTER XXII 

 SOIL FERTILITY AND PLANT GROWTH 



BOTH plants and animals depend on the soil for their ex- 

 istence. Since animals can not draw food immediately 

 from the soil, they are dependent on plants, which have 

 the power to live chiefly from the soil elements. Plants may 

 therefore be looked upon as minute factories, each at work 

 making living tissue out of soil materials. All human food 

 comes either from the plants themselves, or from animals 

 which feed on plants. 



1. Plant Food and Soil Fertility 



The soil is the home of the plant ; there it must find the 

 conditions necessary to its growth and development. The 

 plant must have air and water for its roots, and for its food 

 all the elements that enter into the tissues in its growth. 

 The ability of the soil to supply the elements necessary for 

 plant growth is called its fertility. 



Food required by plants. Agricultural plants re- 

 quire, in all, ten different chemical elements for their 

 growth. These are: 



Carbon "j 



Hydrogen ^Supplied by air and water. 



Oxygen 



Calcium 



Magnesium 'Supplied by soil. 



j ron [Quantity inexhaustible. 



Sulphur 



Nitrogen 1 Supplied by soil. 



Phosphorus 'Quantity limited. 



Potassium 



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