178 THE STUDY OF PLANT COMMUNITIES * Chapter VII 



SOME CHEMICAL FACTORS 



Soil Acidity.— Regardless of the nature of their parent material, 

 soils tend to become acid in reaction if precipitation is sufficient 

 to cause downward percolation of water during much of the year. 

 This is largely the result of leaching of soluble basic salts. To illus- 

 trate, calcium carbonate is relatively insoluble in water but reacts 

 with carbonic acid, ever-present in soil water, to form readily 

 soluble calcium bicarbonate. This, of course, is leached from the 

 surface soil by percolating water. Although the leaching bicar- 

 bonates may be re-precipitated at any time the soil dries out, they, 

 nevertheless, tend always to move downward. Thus the surface 

 horizons tend to be low in basic materials and may have a highly 

 acid reaction because of the acids produced by chemical and 

 biological activity in progress there. The surface strata have the 

 largest accumulation of organic matter, which yields acid prod- 

 ucts upon decomposition, the greatest numbers of soil organisms 

 whose activities may produce acids, and the most active chemical 

 changes in the mineral components, also contributing to acidity. 

 Consequently, acidity is normally greatest at the surface and de- 

 creases in the lower horizons of the soil. 



A solution is acid when the concentration of hydrogen ions 

 (H + ) exceeds that of hydroxyl ions (OH), and it is alkaline if 

 there are more OH~ ions than H + ions. If the two concentrations 

 are equal, as in pure distilled water, the reaction is neutral. Since 

 the concentration at neutrality is known, an expression of the H + 

 ion concentration in a solution indicates its degree of either acidity 

 or alkalinity. 



Because H + ion concentration involves numbers too cumber- 

 some for ordinary use, negative logarithms of the numbers are 

 substituted and preceded by the expression pH. Neutrality is ex- 

 pressed as pH 7.0, indicating a solution that is 0.0000001 (or 10~ 7 ) 

 normal in H ions. A pH value below 7 indicates a greater concen- 

 tration of H ions, or acidity, and a value larger than 7 indicates 

 alkalinity. Since the pH values are logarithmic, the relationships 

 between them are geometric and acidities at pH 5.0, 4.0 and 3.0 

 are respectively 10, 100, and 1000 times as great as at pH 6.0. The 

 pH of most soils will normally fall between 3.0 and 9.0, and, in hu- 

 mid regions, the range to be expected is considerably less, perhaps 

 no greater than pH 4.0 to 7.5. 



