350 NATUKE AND PROPERTIES OF SOILS 



concentration, the importance of aluminum in acidity cannot 

 be avoided. 



Table LXXX 



RELATIVE WEIGHTS OF BARLEY AND RYE GROWN IN WATER CUL- 

 TURE. THE HYDROGEN ION CONCENTRATION IS EX- 

 PRESSED IN Ph. 1 



Treatment 



Check 



H 2 S0 4 j 



A1 2 (S0 4 ) 3 



H 2 S0 4 



A1 2 (S0 4 ) 3 



The only conclusion possible at the present time is that 

 there are probably several kinds of acidity and many degrees 

 of the same acidity as far as toxic influences are concerned. 

 Moreover, dissimilar plants seem to be affected differently by 

 the same acidity, while the same plants respond diversely at 

 different times. Hoagland 2 and others 3 have demonstrated 

 that some plants grow better in a slightly acid medium, which 



1 The hydrogen ion concentration of an acid in solution is a measure 

 of the dissociation of that acid and of its strength. The specific acidity 

 of pure water is taken as 1, the number of grams of H+ ions to a liter 

 being .0000001 or 10- T . The exponent of the power is taken as an expres- 

 sion of the acidity. Pure water has a Ph value of 7, which is approxi- 

 mate neutrality. An acid solution containing 4000 times more H+ ions 

 would have a Ph value of 3.4. 



2 Hoagland, D. R., Relation of the Concentration and Reaction of 

 the Nutrient Medium to the Growth and Adsorption of the Plant; 

 Jour. Agr. Res., Vol. XVIII, No. 2, pp. 73-117, 1919. 



3 Gillespie, L. J., The Reaction of the Soil and Measurements of 

 Hydrogen ion Concentration; Jour. Wash. Acad. Sci., Vol. 6, No. 1, 

 pp. 7-16, 1916. 



Sharp, L. T., and Hoagland, D. R., Acidity and Adsorption in Soils 

 as Measured by the Hydrogen Electrode; Jour. Agr. Res., Vol. VII, 

 No. 3, pp. 123-145, 1916. 



Gillespie, L. J., and Hurst, L. A., Hydrogen-ion Concentration — Soil 

 Type— Common Potato Scab; Soil Sci., Vol. VI, No. 3, pp. 219-236, 

 1918. '^ 



