Crop-Based Management Opportunities for Sodium- and Boron-Affected Soils.
Manzoor Qadir, International Center for Agricultural Research in the Dry Areas (ICARDA), Aleppo, P.O.Box 5466, Syria, Andrew Noble, International Water Management Institute (IWMI), South East Asia Office, Kasetsart University, Bangkok, P.O.Box 1025, Thailand, Sui-Kwong Yau, American Univ. of Beirut - FAFS, Bliss St., Beirut, 1107-2303, Lebanon, and Ghulam Murtaza, Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, 38040, Pakistan.
With worldwide occurrence, sodium-affected soils (sodic soils) are characterized by the occurrence of sodium (Na) to levels that result in poor physical properties and fertility problems thereby adversely affecting the growth and yield of most crops. These soils can be ameliorated by providing a soluble source of calcium (Ca) to replace excess Na on the cation exchange complex. Many sodic soils contain inherent or precipitated sources of Ca, i.e. calcite at varying depths within the profile. Unlike other Ca sources used in the amelioration of sodic soils, calcite is not sufficiently soluble to effect the displacement of Na from the exchange complex and hence amelioration. In recent years, phytoremediation — removal of Na by the cultivation of certain plant species tolerant to ambient soil salinity and sodicity levels — has shown promise as a ‘pay-as-you-go' option in the amelioration of calcareous sodic soils. In contrast to phytoremediation of soils contaminated by heavy metals, phytoremediation of sodic soils is achieved by the ability of plant roots to increase the rate of calcite dissolution, thereby resulting in enhanced levels of Ca in soil solution. The process of Na+ removal from calcareous sodic soils through phytoremediation has been found to be driven by the partial pressure of carbon dioxide within the root zone, and the generation of protons by roots of certain plant species at the soil-root interface. Both assist in increasing the dissolution rate of calcite with the added benefit of improved physical properties within the root zone, enhancing the hydraulic conductivity of these soils and allowing the leaching of Na below the effective rooting depth. Although shoot uptake of Na provides a direct source of Na removal from the soil, it is a minor component of sodic soil amelioration. Several plant species of agricultural significance have been found to be effective in the amelioration of calcareous sodic soils. Being common in soils and irrigation waters in dry areas, boron (B) at high levels causes toxicity in plants with subsequent impacts on crop yields. Boron can be leached out of the root zone only in higher rainfall areas or under excessive irrigations. In dry areas or in soils with impermeable sub-soil layers, B concentrations in the root zone can be high, making amelioration extremely difficult. In recent decades, screening of large numbers of accessions or cultivars of different crop species revealed wide variation in B-toxicity tolerance. In few studies, geographical diversity in B-toxicity tolerance among accessions could be attributed to selection over years by the soils of the different regions/countries. Breeding for B-toxicity tolerance has been attempted, and cultivars which give higher yields when grown in boron-affected soils were bred. In dry areas with high soil B, breeding for crop tolerance to B toxicity, in addition to drought tolerance, could make a contribution to improved productivity.