干旱区生态环境知识资源中心

Irrigated agriculture in arid and semiarid regions of the world has resulted in salinity and waterlogging problems that are threatening the sustainability of our lands. Based on pH of the saturated paste, electrolytic conductivity of the saturated paste extract (ECe), and exchangeable sodium percentage (ESP), these soils have been classified as saline, alkali, and saline-alkali per the criteria of the U. S. Salinity Laboratory. Apart from textural B horizon, the Soil Taxonomy considers only ESP for defining a natric and ECe for defining a salic horizon. It does not consider pH and the nature of soluble salts as criteria for classifying these soils. All soils with high pH values (>8.5) as well as high ESP (>15) and high ECe (>4 dS m(-1)) developed in situ, soils with high pH values (>8.5) as well as high sodium adsorption ratio (SAR >13) and high ECe (>4 dS m(-1)) formed due to use of irrigation waters containing high residual sodium carbonate (RSC >2.5 mol m(-3)), and those with moderate pH values (7 to 8.5) but high SAR (>13) and high ECe (>4 dS m(-1)) formed due to shallow saline water table, are classified as saline-alkali soils. These are classified as Salidic Natrustalfs under Alfisols, since such soils contain both salic and natric horizons. Reclamation of such soils requires both leaching to remove soluble salts and application of amendments to lower ESP. This creates confusion in the mind of soil survey officials, planners, and development authorities. An examination of the composition of saturated paste extracts of these soils shows that these are either to be treated as saline or alkali for the put-pose of adopting reclamation techniques. Soils that have the ratio of either (2CO(3)(2) + HCO3-)/(C-l-+2SO(4)(2-)) and/or Na+/(Cl-+2SO(4)(2-)) > 1, expressed in mol m(-3), should be treated as natric and require chemical amendments for reclamation. When soils have both these ratios <1, then, irrespective of their pH and SAR, these should be treated as salic. Under such situations, both SAR and ECe decrease simultaneously on leaching. It is suggested that definitions of salic and natric horizons should be amended to include limits of pH and ionic ratios of the soluble salts to solve the problem of saline-alkali soils. The term "episalic" and "epinatric" should be included to highlight surface salinity or sodicity at the subgroup level similar to the term "external" used in FAO classification.