Introduction 199 2. Description of Sodic and Saline-Sodic Soils 201 3. Degradation Processes in Sodic and Saline-Sodic Soils 203 4. Phytoremediation of Sodic and Saline-Sodic Soils 206 4.1. Historical perspective 208 4.2. Mechanisms and processes driving phytoremediation 212 4.3. Comparative efficiency of phytoremediation 223 4.4. Plant species for phytoremediation 233 5. Perspectives 236 Acknowledgments 239 References 239 Sodicity-induced soil degradation is a major environmental constraint with severe negative impacts on agricultural productivity and sustainability in arid and semiarid regions. As an important category of salt-affected soils, sodic soils are characterized by excess levels of sodium ions (Na þ) in the soil solution phase as well as on the cation exchange complex, exhibiting unique structural problems as a result of certain physical processes (slaking, swelling, and dispersion of clay) and specific conditions (surface crusting and hardsetting). Saline-sodic soils, another category of salt-affected soils, are generally grouped with sodic soils because of several common properties and management approaches. Sodic and saline-sodic soils occur within the boundaries of at least 75 countries, and their extent has increased steadily in several major irrigation schemes throughout the world. The use of these soils for crop production is on the increase as they are a valuable resource that cannot be

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Water for irrigation is a major limitation to agricultural production in many parts of the world. Use of waters with elevated levels of salinity is one likely option to meet the supply of increased demands. The sources of these waters include drainage water generated by irrigated agriculture, municipal wastewater, and poor quality groundwater. Soil salinity leaching requirements that were established several decades ago were based on steady-state conditions. Recently transient-state models have been developed that potentially can more correctly predict the dynamics of the chemical-physical-biological interactions in an agricultural system. The University of California Center for Water Resources appointed a workgroup to review the development of steady-state analyses and transient-state models, and to determine whether the current recommended guidelines for leaching requirement based on steady-state analyses need to be revised. The workgroup concludes that the present guidelines overestimate the leaching requirement and the negative consequences of irrigating with saline waters. This error is particularly large at low leaching fractions. This is a fortuitous finding because irrigating to achieve low leaching fractions provides a more efficient use of limited water supplies.

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