Some recent researches on soil colloids

Abstract

ALTHOUGH soil colloids, both organic or inorganic are known to occur normally as gels, their properties have generally been investigated in the sol state. This is natural, because in the present stage of development of colloid chemistry the properties of sols can be expressed far more quantitatively than those of gels. The greatest restraint is therefore necessary in using the results of experiments with sols in order to describe the properties of the complex gel system in the soil. In this connection Wiegner(i) points out that the factors influencing the degree of dispersion are qualitatively the same for gels as for sols.Two soil colloids have been studied, clay and humic matter, neither of which is a chemical individual. The clay (Ger. Kolloidton) usually represents all the inorganic particles of equivalent diameter less than 2 fj, (United States Department of Agriculture clay particles < 5 /J., ultraclay particles < 1 fi) and the humic matter, the organic colloid which can be extracted with alkali. As ordinarily prepared, however, each of these is contaminated to some extent with the other.It should be noted also that in certain types of soil the silt and even the fine sand may play a considerable part in the adsorption and base exchange phenomena which are often ascribed wholly to the colloidal clay. CLAY.Composition. The American workers (2, 5, 6,14) employ the high speed centrifuge for the separation of the clay from the coarser particles and obtain a product whose particles have an equivalent diameter of < 1 fi. Bradfield(2) has subjected such clay to a further fractionation using the centrifuge and has obtained three fractions. The coarsest of these contained particles from 1-2 /x to -4 /x. in diameter. The two finer fractions, which Bradfield terms emulsoids A and B contain no particles visible by the ordinary microscope. Analysis showed little difference in composition between the finer emulsoid A and the coarsest fraction, but the alumina extracted by hydrochloric acid was less in the latter than in the former. The titration curves of the clay fractions from various soils