Phytoremediation of Sodic and Saline‐Sodic Soils

Abstract

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+) 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 neglected, especially in areas where significant investments have already been made in irrigation infrastructure. It is imperative to find ways to improve sodic and saline‐sodic soils to ensure that they are able to support highly productive land‐use systems to meet the challenges of global food security. Nearly a century‐old record reveals amelioration of sodic soils through the provision of a readily available source of calcium (Ca2+) to replace excess Na+ on the cation exchange complex; the displaced Na+ subject to leaching from the root zone through the application of excess irrigation water in the presence of a drainage system. Many sodic soils do contain inherent or precipitated sources of Ca2+, that is calcite (CaCO3), at varying depths within the soil profile. However, due to its negligible solubility, natural dissolution of calcite does not provide sufficient quantities of Ca2+ to affect soil amelioration with routine management practices. Consequently, amelioration of these soils has been predominantly achieved through the application of chemical amendments. However, amendment costs have increased prohibitively over the past two decades due to competing demands from industry and reductions in government subsidies for their agricultural use in several developing countries. In parallel, scientific research and farmers’ feedback have demonstrated that sodic soils can be brought back to a highly productive state through a plant‐assisted approach generically termed “phytoremediation.” Typical plant‐based strategies for contaminated soils, such as those containing elevated levels of metals and metalloids, work through the cultivation of specific plant species capable of hyperaccumulating target ionic species in their shoots, thereby removing them from the soil. In contrast, phytoremediation of sodic soils is achieved by the ability of plant roots to increase the dissolution rate of calcite, thereby resulting in enhanced levels of Ca2+ in soil solution to effectively replace Na+ from the cation exchange complex. Phytoremediation has shown to be advantageous in several aspects: (1) no financial outlay to purchase chemical amendments, (2) accrued financial or other benefits from crops grown during amelioration, (3) promotion of soil‐aggregate stability and creation of macropores that improve soil hydraulic properties and root proliferation, (4) greater plant‐nutrient availability in soil after phytoremediation, (5) more uniform and greater zone of amelioration in terms of soil depth, and (6) environmental considerations in terms of carbon sequestration in the postamelioration soil. Phytoremediation is particularly effective when used on moderately saline‐sodic and sodic soils. It is a viable solution for resource‐poor farmers through community‐based management, which would help in strengthening the linkages among researchers, farm advisors, and farmers. These linkages will continue to be fostered as the use of sodic soils becomes more prevalent. The success of phytoremediation of sodic soils requires a greater understanding of the processes fostering phytoremediation, the potential of plant species to withstand ambient salinity and sodicity levels in soil and water, and also of the uses and markets for the agricultural products produced. Strategic research on such aspects would further elucidate the role of phytoremediation in the restoration of sodic soils for sustainable agriculture and conservation of environmental quality.