SOIL ACIDITY AND ALKALINITY 



151 



droxyl ions (OH). If H ions only were present the water wonld have 

 the characteristic sour taste of an acid and would turn litnuis paper 

 hlue while if only Oil ions were present the water would have an 

 alkaline reaction, turning litmus paper red, but since the H and OH 

 ions are present in equal numbers in pure water the reaction is 

 neutral. Thus the acidity or alkalinity of a soil depends upon the 

 hydrogen ion, or the hydroxyl ion, concentration in the soil solu- 

 tion. Either inorganic or organic acids may be produced in soils, 

 usually through chemical decomposition of some of the basic rock 

 materials or through the decay of organic materials caused by the 



Fig. 65. — Kalmia latifolia. 



A plant of acid soils. (Photograph by A. G. 

 Eldredge.) 



activities of the living soil organisms. In humid regions the basic 

 salts, with which hydrogen ions might combine, are frequently 

 leached out of the soil to such an extent that hydrogen ions accumu- 

 late in excess and cause the soil to have an acid reaction. Hydrogen 

 ion concentration is determined by colorimetric methods and is 

 expressed by a number indicating the logarithm of the reciprocal of 

 the hydrogen ion concentration which is termed the pH value. The 

 pH value for water, which is neutral, is 7. Any value below 7 indi- 

 cates acidity while any value above 7 indicates alkalinity. 



The great majority of plants, especially economic plants, grow 

 best in a neutral or only slightly acid soil. The harmful effect of 



