356 MICROBIOLOGY OF SOIL 



ot peat and muck soils that are particularly rich in organic acids. In 

 soils left to themselves the formation of basic substances in the break- 

 ing down of silicates and other compounds keeps pace with their 

 neutralization by acid and their removal in the drainage water. When 

 soils are placed under cultivation, lime and other bases are removed 

 more rapidly and the inert humic acids are left behind. The loss of 

 bases is intensified by application of acid phosphate, potash salts and 

 ammonium sulphate, commonly used as fertilizers. This accounts 

 for the less extensive acidity in and among virgin soils as compared 

 with cultivated soils. Arid soils lose scarcely any of their basic sub- 

 stances by leaching and are seldom acid. Residual limestone soils 

 may be alkaline, neutral or acid, according to the loss of bases they 

 have suffered by leaching. Low-lying soils, including meadows 

 and swamps may accumulate large amounts of organic acids because 

 of their imperfect aeration. 



The more recent investigations of the nature of soil acidity have 

 suggested a physical explanation, namely, that the acidity of the 

 soil is due not to the existence of definite humic and other complex 

 organic acids, but rather to selective adsorption. According to some 

 investigators there is a direct adsorption of the base when a soil is 

 treated with a salt solution. Hence, the behavior of the soil towards 

 neutral salts is not due to the presence of organic matter, but to inor- 

 ganic compounds, probably hydrated silicates. According to others the 

 development of acidity in the salt solution is due rather to an exchange 

 of bases : aluminum is given up from the soil in amounts approximately 

 equivalent to the base adsorbed. 



Through the action of microorganisms in the soil, the organic matter 

 is decomposed with the liberation of weak organic acids (oxalic, citric, 

 CO 2 , etc.). By the interaction of these acids in the soil solution and 

 the basic material held adsorbed by the soil, soluble salts are formed 

 which are subsequently removed by leaching: the soil can then adsorb 

 more basic material, giving rise to soil acidity. 



SOIL REACTION AND HYDROGEN-ION CONCENTRATION .--The dif- 

 ferent methods for measuring the lime requirements of soils are merely 

 attempts to measure the total soil acidity, but not the intensity of the 

 acidity or the active acidity. The latter can only be determined by 

 measuring the hydrogen-ion concentration of the soil. Pure water 

 dissociates, producing equal concentrations of H ions and OH ions 



