346 NATURE AND PROPERTIES OF SOILS 



gen ion concentrations; 1 (2) presence of substances harm- 

 ful to plant growth such as active aluminum, manganese and 

 the like, the presence of which is usually accompanied by a 

 hydrogen ion concentration beyond neutrality; and (3) im- 

 proper nutrition arising from a lack of calcium as a nutrient 

 or as a synergistic agent in facilitating the entrance of other 

 nutrient ions into the plant. 2 



184. Hydrogen ion concentration. — A number of condi- 

 tions are possible if the toxic influence of soil acidity is due to 

 an actual acid. The harmful effect might be due to an ab- 

 normally high hydrogen ion concentration arising from (1) 

 soluble organic or inorganic acids in the soil solution. Again 

 it might be due to (2) insoluble acids or acid salts which, on 

 reaction with water, produce acidity. In this case, the hydro- 

 gen ion concentration of the soil solution at any particular time 

 would not be a measure of the so-called soil acidity. 8 A harm- 

 ful hydrogen ion influence may also be ascribed (3) to soluble 

 acids, either organic or mineral, absorbed by the soil complexes 

 and which would become active only under certain conditions. 

 An additional feature of the actual acidity theory may lie in 

 (4) the selective absorption of bases by the soil, by which acid- 

 ity might be developed from neutral or even alkaline salts. 

 If the actual acidity explanation is entertained, any one or all 

 of these phases might be considered as contributing to the dele- 

 terious effects so noticeable on certain plants. 



185. Active toxic bases. — The explanation of the harm- 

 ful effects of so-called soil acidity as being due to the presence 

 of active toxic bases has of late received much attention. The 



1 Hydrogen is the one essential constituent of all acids. When dis- 

 solved in water, acids dissociate, the hydrogen ion becoming active. The 

 strength of an acid is determined by its hydrogen ion concentration. 



2 True speaks of this cooperative relationship as synergism. By it 

 calcium makes other nutrients physiologically available. True, E. H., 

 The Function of Calcium in the Nutrition of Seedlings; Jour. Amer. 

 Soc. Agron., Vol. 13, No. 3, pp. 91-107, 1921. 



8 Eice, F. E., and Osugi, S., The Inversion of Cane Sugar by Soils 

 and Allied Substances and the Nature of Soil Acidity ; Soil Sci., Vol. V, 

 No. 5, p. 347, 1918. 



