Identification of dispersive behaviour and the management of red-brown earths

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Abstract
A scheme is proposed, together with a procedure suitable for routine laboratory use, for the prediction of dispersive behaviour of surface layers of red-brown earths and their classification into one of six classes. Each class is defined on the basis of predictive relationships established between dispersion (spontaneous and mechanical), sodium adsorption ratio (SAR) and total cation concentration (TCC). These relationships were established experimentally using 138 samples representing both surface and subsurface layers from 69 red-brown earth profiles. Preliminary studies including samples from red clay and black earth profiles indicated that the proposed scheme is not suitable for these soils. Neither can it be used for soils containing free lime. The procedure proposed enables the prediction of the probable dispersive behaviour of the surface layer of red-brown earths, including exposed subsoils. It provides a rational basis for the formulation of appropriate management strategies for the manipulation of the surface structure of individual red-brown earths used for dryland or irrigated agriculture. Application of the proposed scheme to the estimation of the minimum level of residual gypsum required to maintain aggregate stability via the electrolyte effect is discussed, with special reference to low-sodic soils (i.e. with a SAR below 3, e.g. Classes 2a and 3c).