SOIL SOLVENTS 163 



change in the composition of the soil gases. In other words 

 the amount of oxygen and carbon dioxide together equal 

 21 per cent, rather constantly. In the air they amount 

 to 21.02 per cent. 



Decomposing organic matter is the principal factor in the 

 variation in composition of soil gases, and to this cause also 

 is due, especially in soils more or less water-logged, the 

 presence of methane and hydrogen sulphide; but these gasc> 

 are rare constituents in any ordinary soil in good condition. 

 Other constituents are not worth considering. 



As to the value of soil gases in decomposing mineral 

 particles, it will be found later that carbon dioxide and 

 oxygen are the most active agents. A solution of carbon 

 dioxide in water is by far the most active solvent for minerals 

 which the soil produces. Of course sulphuric and nitric 

 acids which result from decomposing organic matter are 

 more powerful reagents ordinarily, but their occurrence is 

 very slight compared to that of carbonic acid, which though 

 rated a weak acid is always present in large quantities in 

 practically all soils. Decaying organic matter has been 

 estimated to supply through bacterial action to a depth of 

 eight inches, about 1 ton of carbon dioxide per year. When 

 dissolved in water this makes a very respectable amount of 

 solvent. Organic acids, together with sulphuric and nitric 

 acids, are produced in very much smaller amounts, and 

 being very dilute have not the effect that carbon dioxide 

 has, although these reagents are to be reckoned with in 

 considering mineral decomposition and solubility. 



134. Soil Solvents. — The soil moisture which acts on the 

 mineral particles in the soil consists primarily, of course, of 

 water. Pure water dissolves ordinary minerals but slightly, 

 except gypsum and sodium chloride, of which the latter 

 occurs in normal soils more as a decomposition product 

 than as an original mineral. In the soil, however, water is 

 never pure. Carbon dioxide is always present from the decay 

 of organic matter. Living plant roots excrete carbon dioxide 

 due to respiration, and the soil moisture immediately around 

 such roots is fairlv well concentrated in this constituent. 

 The growing of plants on a polished slab of marble — calcium 

 carbonate — or on one of feldspar leaves a fine tracery of the 



