66 ORGANIC CONSTITUENTS OF SOIL. 



product of substances once endowed with life. This power 

 influences the elements, recombines them in forms so essen- 

 tially connected with life, that they are, with few exceptions, 

 produced only by a living process. They are the products 

 of living organs, hence termed organic ; and when formed, 

 are subject to chemical laws. The number of elements in 

 the inorganic parts of soil is twelve. Oxygen, sulphur, 

 phosphorus, carbon, silicon, and the metals, potassium, 

 sodium, calcium, aluminium, magnesium, iron, and manga- 

 nese (56). The number of elements in organic parts of 

 soil, does not exceed four, viz. : oxygen, hydrogen, carbon, 

 and nitrogen. 



90. The great difference between these two divisions is 

 this, that while the inorganic are simple combinations of two 

 elementary substances, the organic are combinations of 

 three or four elements, but never less than three. These 

 are variously combined. They have formed the great body 

 of vegetable products ; continually changing, the mere ab- 

 straction of a part of one of their elements forms a new 

 product. The three elements (89), exist generally in such 

 proportion, that the oxygen and hydrogen 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 carbonic acid, 

 and the hydrogen into water. They are constantly changing, 

 assuming new forms. This susceptibility of change is the 

 foundation of tillage. 



91. The relation of agriculture to silicates and salts, and 

 to the composition of plants, which has been alluded to (89), 

 is of the highest interest. As silicates and salts compose 

 all the earthy ingredients of soil, so are they equally con- 



