MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 299 



content has been restored repeatedly by the application of wood ashes, 

 marl, oyster and dam shells, and various grades of burned or crushed 

 limestone. In the United States acidity is becoming prevalent in many 

 of the cultivated soils, as is shown by the investigations of the Rhode 

 Island, Ohio, Illinois, Oregon and Florida experiment stations.. These 

 investigations, confirmed by experiments in other states, show that 

 there 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 examined were acid, and that 16.25 per cent were strongly 

 acid. 'Similarly, Blair found that of 189 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 or 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 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 appUcation 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 alkaUne, 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. 



Effkct of Reaction on Numbers and ^Species. — Some of the 

 important groups of soil bacteria including nitro, azoto and ammonify- 

 ing 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 , 



