BOG XEROPHYTES AND ACID SOILS. 133 



the soil, and their subsequent removal by leaching, thus leaving the 

 soil free to absorb more basic material from any source with which 

 it may come in contact. 



Truog (1914 : 505) states that he is even more adverse to accept- 

 ing the colloid-absorption theory in explanation of soil acidity than 

 the so-called humic-acid theory. While the acidity of peat and 

 muck soils is undoubtedly due in part to organic acids, there are up- 

 land soils practically free from organic matter that react strongly 

 acid. It is assumed that this inorganic acidity is due to the reaction 

 of the water solution with the siHcates, which forms a soluble hydroxid 

 or salt taken up by plants or removed by drainage, and a compara- 

 tively insoluble acid silicate that accumulates in the soil to produce 

 acidity. By means of a special method it has been found that it 

 makes little difference whether calcium, barium or sodium hydroxid 

 is used to neutralize acidity, thus indicating strongly that this is due 

 to true acids and not colloids, since the latter should demand different 

 amounts. While there is no question that colloids exist in soils, 

 some absorbing bases and others acid-ions, the amount of absorption 

 in the case of the laboratory study of pure colloids is comparatively 

 very small, and the absorption capacity of these colloids is practi- 

 cally a negligible factor in soil acidity. 



In a comprehensive discussion of sour soils and liming, Frear 

 (1915 : 81) has considered the various causes of soil acidity, appar- 

 ently assigning some value to all of them, but giving the primary role 

 to humous compounds. The other causes discussed are: (1) leaching 

 of basic material by waters containing carbonic acid; (2) acid sili- 

 cates; (3) iron pyrites; (4) furnace and coke-oven fumes; (5) alumi- 

 num salts; (6) decay of plant residues; (7) fermentation in the soil; 

 (8) fertilizers; (9) physiological acidity; (10) sulphur fungicides. 



Truog and Sykora (1917 : 348) conclude that chemical reactions 

 probably play as important a role as such physical phenomena as 

 adsorption and possibly have even a greater effect. Calcium car- 

 bonate, acid kaolin, and other silicates are especially important in 

 this connection. The chemical and physical constitution of most 

 agricultural soils seems to be such that the injurious action of toxins 

 present or arising in various ways is entirely or largely prevented by 

 proper tillage and the use of Hme when needed. 



Conner (1918 : 328) has shown that the acidity of soils varies 

 with the water-content, the acidity being greater in soils kept at 

 half than at fourth saturation. Soils high in organic matter showed 

 the greatest acidity at full saturation. The measurable acidity of 

 acid soils varied much under different conditions of moisture and 

 aeration, owing to chemical rather than physical changes. The water- 

 content of acid soils is acid in reaction as shown by determination 

 of the hydrogen-ions. Acidity in wet soils may be caused by the 



