102 PLANT PHYSIOLOGY 



dissolved in sufficient quantities for normal growth. Where 

 man interferes by defective cultivation or mistaken selection 

 of crops, the soil must be artificially enriched. In nature 

 the constant supply of adequate amounts of all the food- 

 materials is secured by the action of those forces and or- 

 ganisms which break down the inorganic and organic mat- 

 ters on and in the upper layers of rock and soil. 



