236 MICROBIOLOGY OF SOIL. 



is a marked removal of lime and of other basic materials from the soil 

 as cultivation and the use of commercial fertilizers become more thorough. 

 Knisley shows, for instance, that 38.75 per cent of the Oregon soils ex- 

 amined were acid, and that 16.25 per cent were strongly acid. Similarly, 

 Blair found that of 189 soil samples of different Florida soils and subsoils, 

 examined, 68.22 per cent of the former and 51.35 per cent of the latter 

 were acid. He also found that virgin soils were less acid than cultivated 

 soils. 



CAUSES OF SOIL ACIDITY. Soil acidity may be due to acids or acid 

 salts, both inorganic and organic. ' Under ordinary conditions the latter 

 are of much greater importance than the former as a cause of soil acidity. 

 This is demonstrated by the extremely acid conditions of peat and muck 

 soils that are particularly rich in organic acids. In soils left to themselves 

 the formation of basic substances in the breaking 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 substances 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. 



EFFECT OF REACTION ON NUMBERS AND SPECIES. Some of the 

 important groups of soil bacteria including nitro, azoto and ammonifying 

 species will develop slowly or not at all, when the amount of acid in the 

 medium is increased beyond a certain point. Hence it is realized by 

 progressive farmers that a proper supply of lime is essential for the satis- 

 factory decomposition of organic matter in the soil, and the abundant 

 supply of available nitrogen compounds, as well as of other constituents 

 of plant food to growing crops. The influence of lime on the multipli- 

 cation of soil bacteria is well illustrated, for instance, by the experiments 

 of Fabricius and von Feilitzen. These investigators found only 138,500 

 bacteria per g. in newly broken and unlimed peat soils; whereas in 

 similar soils that had been limed and cultivated for several years the 



