54 ORGANIC ELEMENTS OF SOIL. 



ements in organic parts of soil, does not exceed four, 

 oxygen, hydrogen, carbon, and nitrogen. 



90. The great difference between these two di- 

 visions, is this, that while the inorganic are simple 

 combinations of two elementary substances, the or- 

 ganic, are combinations of three or four elements, 

 but never less than three. These are variously com- 

 bined. They have formed the great body of vege- 

 table products, continually changing, the mere ab- 

 straction of a part of one of their elements forms a 

 new product. The four elements, (89) exist gener- 

 ally in such proportion, that the oxygen and hydro- 

 gen would, by their union, produce water, without 

 excess of either element, while the carbon would 

 thus be liberated. It would be found free were it 

 not also acted upon by air and moisture, and changed 

 to carbonic acid. There is not oxygen enough in 

 the organic part, to convert the carbon into carbon- 

 ic acid, and the hydrogen into water. They are con- 

 stantly changing, assuming new forms. This sus- 

 ceptibility of change, is the foundation of tillage. 



91. The relation of agriculture, to silicates and 

 salts, and to the composition of plants alluded to, 

 (89) is of the highest interest. As silicates and 

 salts compose all the earthy ingredients of soil, so are 

 they equally constant in plants. The deduction to 



