SOIL ACIDITY 351 



seems to indicate that the hydrogen ion concentration less 

 than a Ph value of 7, so often reported in so-called acid soils, 

 is concomitant with a toxic constituent or with malnutrition 

 and is not in itself the harmful agent. 1 This argument, how- 

 ever, does not admit that the hydrogen ion is not in many 

 cases the true explanation of the toxicity of certain acid soils, 

 nor does it suggest that lack of nutrients may not be a serious 

 consideration. 



In light of the explanations offered above, it is evident that 

 the term soil acidity is inadequate to express the inorganic 

 toxicity that accompanies a hydrogen ion concentration below 

 Ph 7, as the condition referred to is, in many cases, not due to 

 the hydrogen ion in detrimental concentration. 2 Since the 

 term is of long standing and since so-called acid soils almost 

 invariably yield an acid reaction with litmus paper, the phrase 

 will continue in use in spite of its misleading inference. 



188. Why soil acidity develops. 3 — No matter what hypoth- 



1 Joffe found that while alfalfa plants experienced difficulty in becom- 

 ing established in soils having high hydrogen ion concentrations due 

 to the addition of sulfuric acid, once the seedlings became established 

 they showed normal color and vigor and made excellent growth on soils 

 having a Ph value as low as 3.8. 



Joffe, J. S., The Influence of Soil Reaction on the Growth of Alfalfa; 

 Soil Sci., Vol. X, No. 4, pp. 301-307, 1920. 



2 Eesearches on Danish soils extending from 1916 to 1920 show that 

 the Ph value on different soils may vary from 3.4 to 8.0. A rather 

 constant relationship was observed between the type of vegetation and 

 the hydrogen ion concentration, many species being found only on 

 soils within a certain range of Ph values. In water culture studies 

 so-called acid-soil plants grew best at a Ph of about 4. Alkaline-soil 

 plants seemed to give the strongest growth at a Ph of 6 to 7. 



Olsen, C, The Concentration of the Hydrogen Ions in the Soil; 

 Science (N. S.), Vol. LIV, No. 1405, pp. 539-541, Dec. 2, 1921. 



'White, J. W., Studies in Acid Soils; Ann. Eep. Penn. State Col., 

 1912-1913, pp. 55-104. 



Skinner, J. J., and Beattie, J. H., Influence of Fertilizers and Soil 

 Amendments on Soil Acidity; Jour. Amer. Soc. Agron., Vol. 9, No. 

 1, pp. 25-35, 1917. 



Conner, S. D., Soil Acidity as Affected by Moisture Conditions of the 

 Soil; Jour. Agr. Ees., Vol. XV, No. 6, pp. 321-329, 1918. 



Martin, W. H., The Relation of Sulfur to Soil Acidity and to the 

 Control of Potato Scab; Soil Sci., Vol. IX, No. 6, pp. 393-408, 1920. 



