THE SOIL SOLUTION 281 



is generally dilute except in arid regions under conditions of 

 alkali. The concentration probably very seldom exceeds 30,- 

 000 parts per million and is normally very much lower. More- 

 over, the greater proportion of the solute is in an ionic state, 

 molecules appearing only when the concentration is relatively 

 high. It is well to note that the plant absorbs most of its 

 nutrients in the ionic condition. 



From the knowledge obtained by the analysis of soil ex- 

 tracts, it is safe to assume that all of the common bases and 

 acid radicals normally occur in the soil solution. Thus, K + , 

 Na + , Mg ++ , Ca ++ , Fe +++ , Ar+ + and NH 4 + ions may be expected 

 as well as such ions as SO;, SiOf, CI - , POJ, NO;, NO; and 

 COg. Since water dissociates slightly, H + and OH - ions will 

 also be present. The reaction of the solution will depend on 

 its hydrogen-ion concentration and may be alkaline, neutral 

 or acid as the case may be. Most soil solutions seem to be 

 slightly acid, 1 possibly due to the action of carbon dioxide. 



Morgan 2 found on an examination of the solutions obtained 

 from soils by the oil pressure method that, as the moisture in- 

 creased, the concentration of the solution decreased. These 

 findings are amply corroborated by the work of Bouyoucos 3 

 with the depression of the freezing point method. The latter 

 presents data regarding the actual concentrations at various 

 moisture contents, which seem to indicate the general differ- 

 ences that may be expected between soils of different types. 



1 Gillespie, L. J., The. Reaction of Soil and Measurements of Hydro- 

 gen-ion Concentration; Jour. Wash. Acad. Sci., Vol. 6, No. 1, pp. 

 7-16, 1916. 



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

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

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



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

 Nutrient Medium to the Growth and Absorption of the Plant; Jour 1 . 

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



'Morgan, J. F., The Soil Solution Obtained by the Oil Pressure 

 Method; Mich. Agr. Exp. Sta., Tech. Bui. 28, 1916. 



'Bouyoucos, G. J., Further Studies on the Freezing Point Lowering 

 of Soils; Mich. Agr. Exp. Sta., Tech. Bui. 31, pp. 14-15, 1916. 



