54 THE SOII, SOLUTION 



The obvious explanation, at least in the case of the ordinary 

 silicates of the alkalis or alkaline earths, is that by forming bi- 

 carbonates of the hydrolyzed bases, the active mass of the re- 

 action product with water is decreased and hydrolysis thereby 

 increased. But this explanation is apparently insufficient to ac- 

 count for the effects sometimes observed. It has been shown 

 that the passage of carbon dioxide through solutions of the 

 silicates, will produce more or less slowly a precipitation of silica, 

 and there seems little reason to doubt that it does induce to 

 some degree a decomposition and consequent greater solubility 

 of the silicates of the alkalis and alkaline earths. It also in- 

 creases to an appreciable extent the solubility of the phosphates 

 of iron, alumina, and lime. Therefore, the variation in the con- 

 tent of carbon dioxide in different soils, and its continual varia- 

 tion from time to time in any one soil, must be expected to pro- 

 duce corresponding changes in the soil solution with respect to 

 such bases as potassium and lime, and also with respect to phos- 

 phoric acid. This has been verified experimentally with aqueous 

 extracts of soils, the solutions being charged with carbon dioxide 

 while in contact with the soils. 1 It is not conceivable, however, 

 that any great difference can exist in the partial pressures of 

 carbon dioxide in different soils which are in a condition to 

 support crops, and therefore great absolute differences in the 

 mineral content of the soil solution are not to be anticipated, 

 nor are they actually observed. 



It has long been held that the organic substances in the soil 

 have an important solvent effect on the minerals. This assump- 

 tion seems quite unwarranted in the light of our present knowl- 

 edge, although it is not to be denied that occasionally there 

 may be present in the soil some soluble organic substance which 

 influences the mineral content. Generally it has been assumed 

 that the effective organic substances influencing the solubility 

 of the minerals are organic acids, of which a number have found 

 their way into past and even current literature, and which have 

 1 See, for instance, the results obtained by Peter, Proceedings of the 

 I9th Annual Convention of the Association of American Agricultural 

 Colleges and Experiment Stations, Bull. No. 164, Office of Experiment 

 Stations, U. S. Dept. Agriculture, 1906, p. 151 et seq. 



