SODIC SOILS - NEW PERSPECTIVES

There are large areas of the world where soils are adversely affected by the presence of sodium (Na) as an exchangeable cation. Unlike their saline counterparts which are more extensive, sodic soils have received less attention in the literature. There has been considerable disagreement concerning the definition of sodicity, owing largely to the fact that many experiments used in the development of definitions did not account for the presence of salts in the water used to measure hydraulic properties. These problems are discussed and the conclusion is reached that a single simple definition is no longer possible. This problem is further exacerbated by the fact that many soils which would never have fallen into a previously defined sodic category, do in fact exhibit sodic properties. The major focus of this account of sodicity will therefore be the soils which contain relatively low levels of exchangeable Na. As such soils are widespread in both humid and subhumid areas of the world and are responsible for the production of a large proportion of the world cereal crop, they deserve special attention. Because swelling and dispersion are the primary processes responsible for the degradation of soil physical properties in the presence of Na, an account of clay behaviour in relation to Na and electrolyte concentration is presented before exploring these new realms of sodicity. Pure clay systems are not always suitable for use as models of soil behaviour in terms of dispersion and flocculation. However, as far as swelling is concerned, the correspondence is much better. Nevertheless, the effects of the exchangeable cations on dispersion are predictable albeit usually only qualitatively. This is partly due to the phenomenon of 'demixing' in which the cations are not distributed over all surfaces in the same proportions. The effects of Na and electrolyte concentration in relation to hydraulic conductivity, infiltration, crusting, runoff, erosion and hardsetting are discussed from which it emerges that the effects of Na are manifested in measurable and often sizeable proportions down to very low levels far below those previously used to define sodic soils. The primary processes responsible for physical degradation are swelling at relatively high levels and clay dispersion throughout the range of exchangeable Na percentage (ESP). Provided that the total electrolyte concentration (TEC) is below the critical flocculation concentration (CFC), clays will disperse spontaneously at high ESP values, whereas at lower ESP levels, inputs of energy are required for dispersion. The TEC of the ambient solution, because of its effects in promoting clay flocculation, is crucial in determining soil physical behaviour. With increasing levels of ESP, correspondingly increased levels of TEC are required to maintain the clay in a flocculated state. Even at very low ESP values, clay can disperse, provided that the TEC is correspondingly low, which means that sodic behaviour can and often is exhibited by soils in humid regions. Because falling raindrops transfer energy to the soil surface, clay dispersion takes place at lower ESP levels than required for dispersion within the soil body. Thus infiltration rate (IR), due to crust formation at the surface, is much more sensitive to Na than is hydraulic conductivity (HC). As a result of crust formation, IR is reduced, resulting in increased runoff and erosion. The effects of weathering of soil minerals resulting in the production of sustained levels of electrolyte in certain soils in relation to clay dispersion and its consequences are also discussed. The role played by organic matter in controlling physical behaviour with respect to sodicity is also explored. Organic matter can both promote and retard clay dispersion. At high levels which are actively being turned over, aggregation is promoted, resulting in decreased clay dispersion. However, as organic matter levels decrease as a result of cultivation, soils often become sensitized to Na due to the charge generating contributions of the broken bonds on the remaining organic fragments., A special case of sodic behaviour is presented by hardsetting soils in which the Ap horizon, having very poor aggregation, slumps on wetting with the dispersible clay becoming reoriented, so that, on drying, very high soil strengths are encountered throughout the entire topsoil. The major difference between hardsetting soils and others exhibiting sodic behaviour stems from their textural composition and very poor degree of aggregation in their natural state. Increased levels of Na promote hardsetting, while increasing TEC which promotes flocculation, reduces soil strength and hardsetting. At relatively low levels of ESP and low ambient soil solution concentrations, physical properties improve in a rather spectacular manner to additions of electrolyte. Thus application of gypsum is a suitable ameliorative strategy. In addition, use of synthetic polymers to stabilize aggregation has also proved useful in improving the physical behaviour of these soils. Lastly, nomenclature for describing sodic soils is discussed and a proposal is made for a classification based on behaviour rather than on arbitrarily set analytical criteria. In addition, fruitful lines for further research are identified.