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 receiv ed less attention in the literature. There has been considerable disag reement concerning the definition of sodicity, owing largely to the fa ct that many experiments used in the development of definitions did no t account for the presence of salts in the water used to measure hydra ulic properties. These problems are discussed and the conclusion is re ached that a single simple definition is no longer possible. This prob lem is further exacerbated by the fact that many soils which would nev er have fallen into a previously defined sodic category, do in fact ex hibit sodic properties. The major focus of this account of sodicity wi ll therefore be the soils which contain relatively low levels of excha ngeable Na. As such soils are widespread in both humid and subhumid ar eas of the world and are responsible for the production of a large pro portion of the world cereal crop, they deserve special attention. Beca use 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 concentrat ion is presented before exploring these new realms of sodicity. Pure c lay systems are not always suitable for use as models of soil behaviou r in terms of dispersion and flocculation. However, as far as swelling is concerned, the correspondence is much better. Nevertheless, the ef fects 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 i n the same proportions. The effects of Na and electrolyte concentratio n in relation to hydraulic conductivity, infiltration, crusting, runof f, erosion and hardsetting are discussed from which it emerges that th e effects of Na are manifested in measurable and often sizeable propor tions down to very low levels far below those previously used to defin e sodic soils. The primary processes responsible for physical degradat ion are swelling at relatively high levels and clay dispersion through out the range of exchangeable Na percentage (ESP). Provided that the t otal electrolyte concentration (TEC) is below the critical flocculatio n concentration (CFC), clays will disperse spontaneously at high ESP v alues, 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 le vels 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 tra nsfer energy to the soil surface, clay dispersion takes place at lower ESP levels than required for dispersion within the soil body. Thus in filtration rate (IR), due to crust formation at the surface, is much m ore sensitive to Na than is hydraulic conductivity (HC). As a result o f crust formation, IR is reduced, resulting in increased runoff and er osion. The effects of weathering of soil minerals resulting in the pro duction of sustained levels of electrolyte in certain soils in relatio n to clay dispersion and its consequences are also discussed. The role played by organic matter in controlling physical behaviour with respe ct to sodicity is also explored. Organic matter can both promote and r etard 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 con tributions of the broken bonds on the remaining organic fragments., A special case of sodic behaviour is presented by hardsetting soils in w hich the Ap horizon, having very poor aggregation, slumps on wetting w ith the dispersible clay becoming reoriented, so that, on drying, very high soil strengths are encountered throughout the entire topsoil. Th e major difference between hardsetting soils and others exhibiting sod ic behaviour stems from their textural composition and very poor degre e of aggregation in their natural state. Increased levels of Na promot e hardsetting, while increasing TEC which promotes flocculation, reduc es 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 appli cation 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, nomenclat ure for describing sodic soils is discussed and a proposal is made for a classification based on behaviour rather than on arbitrarily set an alytical criteria. In addition, fruitful lines for further research ar e identified.