218 GEOLOGY AND CHEMISTRY OF SOILS. 



etc. consist of several substances aJready referred to. Dr. 

 Dana calls the whole geine, and as the fertility of a soil de- 

 pends upon the soluble geine and salts, his method of analysis 

 is well adapted to determine their amount, and the conse- 

 quent fertility of the soil. 



But whatever views are adopted, relative to the nature of 

 the organic constituents of soils, the fact is fully established 

 by experience, that a due mixture of organic, with the mine- 

 ral ingredients, is essential to fertility, and that the power of 

 the soil to bear successive crops for a series of years, depends 

 upon keeping up the supply of humus and salts, which a con- 

 tinued course of cropping takes away. Other substances 

 must exist also in the soil, in a state of partial decomposition, 

 such as the various vegetable products, but they all finally 

 pass into those above described, or pass off in gases, such as 

 ammonia, sulphureted hydrogen, and carbonic acid. 



Sect. 3. Theory of the mutual action of the inorganic and 

 organic constituents of Soil, and of groicing Vegetables. 



The different earths, acids, salts and organic matters, de- 

 scribed in this section, are combined in the soil with each 

 other in definite proportions. They are constantly subjected 

 to the laws of affinity, and as this power exists in different 

 degrees of force in the different compounds, there are frequent 

 and almost constant changes going forward. These changes are 

 aided by the influence of the atmosphere, water, temperature, 

 etc., and tend to alter the relative proportion of the different 

 compounds. The agents concerned in converting the rocks 

 into soils, continue to act, and the same changes continue. 

 These changes are those produced by the mutual action of 

 the organic and inorganic constituents, and those which are 

 produced by the agency of the living vegetable. 



I. Action of the organic and inorganic elements of soil 

 upon each other. The elements of soil, as we have seen, are 



