28 NATURE AND PROPERTIES OF SOILS 



atmosphere carries about .03 per cent, of carbon dioxide by 

 volume, while considerable amounts are brought down on 

 rocks and soil by rain and snow. Traces of nitric and sul- 

 furic acid are also found in rain water. The carbon dioxide 

 produced within the soil by decaying organic matter keeps 

 the concentration of this gas high at points where it can act 

 most effectively. 



Solution, 1 accelerated both by mechanical and chemical 

 means, is of particular importance in two directions. In the 

 first place, it allows a continual loss of plant nutrients not 

 only as the soil is being formed but after it becomes a proper 

 medium for plants. This constant drain accounts for the 

 deficiency of certain elements in the soil and the need in cer- 

 tain cases of such additions as lime and fertilizers. On the 

 other hand, this solution, however wasteful, is necessary since 

 plants absorb nutrients from the soil only in soluble form. 

 The concentration and composition of the materials in the 

 soil water is thus a function of solution, which is a culmina- 

 tion of the activities of the soil processes already discussed. 



20. General statement of soil formation. — By a very 

 complicated coordination the mechanical and chemical forces 

 of weathering reduce the solid rock to small fragments and 

 mix therein the necessary organic matter. The process slowly 

 proceeds until a suitable medium for the growth of higher 

 plants is produced. As a rule, the chemical processes are in- 

 complete and all stages of decay are exhibited. This is for- 

 tunate, as solution may thereby continue to renew the nutrients 

 in the soil-water for a long period and thus maintain the 

 continuous productivity of the soil. 



The products of disintegration and decomposition are com- 

 monly classified into two general groups, sedentary and trans- 



1 While the formula for water is generally given as H,0 the molecule 

 is not as simple as this, being at low temperature as high as (H 2 0) 4 . 

 The remarkable power of water as a solvent may be due to extra oxygen 

 valences as well as to the high dielectric constant which favors ioniza- 

 tion, thus hastening chemical reaction. 



