Clarification of soil degradation mechanism in dryland and environmental restoration for sustainable agriculture development

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To improve the sustainability of the dry land agriculture, appropriate management and rehabilitation methods of salt affected soils were developed based on the analysis of status and causes of soil salinization,1) In Baja California Sur, Mexico, most of irrigation water (underground water) and top soil were salinzed, and have been aggravated in some area. Soil salinization process was mainly explained by two factors. One was irrigation water quality and quantity that influenced salt addition and another was soil texture of subsoils that influenced salt leeching. Proper irrigation management, that is "water-saving", is the key method to solve soil salinization and to recover water resources.2) A convenient procedure to assess soil salinization risk (SSR) was developed. SSR was determined by the sum of the points of various factors that relate to soil salinization. By using the procedure, not only the state but also the cause of salinization was assessable.3) High sustainability of the flood use farming system was verified. Agricultural utilization of surface runoff including suspended sediment in dryland was an extremely useful method, because this method had the following excellent effect ; storage of water, soil fertility improvement by deposition of suspended sediment and salt leeching in saline soil.4) It was clarified that Kochia scoparia, which had strong tolerance for salinity and water shortage, was able to maintain good growth under highly saline condition and would be a promising forage crop in dry region where the salt damage progressed. The mechanism of elongation growth of seedling of Salicornia herbacea that was a promising plant for phytoremediation of salinaized land was clarified under high concentration NaC1 solution.5) Combination use of a plant residue and sulfur might effectively improve sodicity level of soils by enhancing solubility of calcium.