TENTH CHAPTER. 



WHAT OUR SOILS ARE MADE OF. 



T^HE SOIL, which we work, is in itself the most 

 important of all of our available sources of 

 plant food. It must be of interest to examine its 

 structure and general constitution, even before en- 

 tering the question of the food elements which are 

 contained in it. 



All soils have more or less organic matter de- 

 rived from the decay of animal, and still more of 

 vegetable, substances. The proportion of such or- 

 ganic matter in naturally productive soils varies 

 between a mere fraction of one, and seventy per cent 

 of its entire weight. In our best soils the organic 

 matter ranges from five to twelve per cent, seldom 

 more. Only in mucky and peaty soils does the 

 amount of organic matter ever exceed that of the 

 Inorganic or earthy matter. 



The inorganic or earthy part of the soil consists 

 principally of silica or silicious sand, alumina (as 



