Salinization, alkalinization and sodification in Punjab (Pakistan): Characterization of the geochemical and physical processes of degradation

Evidence of soil degradation impeding soil tillage and irrigation in cultiv ated soils in Pakistan is identified, described and represented in a genera l process of degradation. Based on a chemical analysis of soil characterist ics, it is shown that a more general geochemical degradation process may oc cur in these soils. Two paths of salinization, i.e, neutral salinization an d alkalinization inducing a process of sodification, are identified. The wi de range of chemical properties of soils and corresponding geochemical proc esses can be explained by the great diversity of quality in irrigation wate r that is taken either from the canal or from the groundwater. The basic mo dule of a geochemical model AQUA (Valles and DeCockborne, 1992) is calibrat ed with the help of a study of the soil geochemical properties (identificat ion of minerals, characterization of exchanges) and then used to assess the effect of four different water qualities on sandy and loamy soils. Based o n these scenarios, the salinity, alkalinity and sodicity hazard of irrigati on water is assessed by taking into account simultaneously the electrical c onductivity and the residual alkalinity (calcite-residual alkalinity, resid ual sodium carbonates) or irrigation water and the soil cation exchange cap acity: these three indicators appear the most relevant in the context of th e study. Copyright (C) 1999 John Wiley & Sons, Ltd.