A method is described for measuring the relative mobility (RM) of colloidal suspensions. It was used to assess the potential mobility of clay in the horizons of 12 profiles from Belgium, Bolivia, Brazil and India.The colloid from surface horizons of soil was always more mobile than that from deepcr layers. The values of relative mobility differed between profdes and appeared to give a useful indication of potential clay migration under natural conditions.
InmductionUNDER some natural condtions large amounts of materials are moved from the soil surface and are deposited in the subsurface layers, the movement of clay being termed clay migration (Oertel, 1968; Thorp et al., 1959;Brydon, 1965).Information on clay migration under natural conditions may be obtained from colloid stability measurements (Combeer and D'Hoore, 197 1,1973). These authors used the incipient flocculation ratio (IFR), obtained from optical density measurements, as a measure of colloid mobility. (The term colloid is used in this paper for particles <1 pm which include clay and some dispersed organic matter.) However, a more precise way to assess colloid stability is to measure the relative mobility of colloids on chromatography paper. Soil profiles can then be characterized by their relative mobility curves, obtained at various clay/water ratios, in relation to pedon depth (Dixit, 1976). The results reported in this paper indicate that such curves can be used to distinguish distinct soil types, and at the same time may also yield information on the direction of clay migration and accumulation in the profiles.
Experimental SamplesSoil samples were taken for relative mobility measurements from 12 different profiles classified as Hapludalf, from Belgium; Tropudalf, from Brazil; Luvisol, from Bolivia; and from India, Ustochrept with a sandy phase -Ustipsamment, Thapto Ustochreptic -Ustipsamment, Typic -Ustochrept, Natraquic -Camborthid. The physicochemical properties of these samples have been communicated elsewhere (Dixit, 1977~).
Meusuremnt of rehtive mobilityQuantities of soil, containing the required amount of <1 pn clay, were shaken overnight in distilled water on a reciprocating shaker, and suspensions of <1 pn colloids were separated by centrifuging (D'Hoore, 1970). From all the horizons of various profiles the suspensions were obtained at 0.001,0.005,0.01,0.03 and 0.05