S Matsumoto Introduction

Alkalinization of soils has expanded rapidly in many irrigated areas of the world due to man's misuse of the land. Alkalinization changes soils to a much poorer support for vegetation than is the case with salinization. Their adverse effects on plant growth can be attributed to impaired aeration, restricted rooting depth, interference in nutrient uptake and plant metabolism, corrosion of root surface, and sodium toxicity.1 In the same way as soil salinization, long periods of dry and warm or hot weather are important factors in the formation of alkali soils. Another important factor is hydrology: high groundwater level, intensive irrigation, and inundation by the sea or brackish water. Relief is also often important, because alkalization is most severe in depressions or at the edge of depressions. Alkalinization of soils is characterized by the formation of soils with a high percentage of exchangeable sodium; often sodium carbonate and sodium bicarbonate also dominate, increasing the pH to beyond 8.5, often to 9 or 10. Clay particles and humus are easily dispersed and eluviated from the A to B horizon under alkali reaction, and accumulate as a natric subsurface horizon. A natric horizon is characterized by a very hard pan with prismatic structure formed by clay and humus coatings on the ped surfaces.2

The total area of the alkali soils in the arid and semi-arid regions of China reaches to 320,000 km2. The very important fact that more than 50,000 km2 of Mollisol distributed in semi-arid regions of China suffer from alkalinization has been revealed by the recent soil surveys carried out by Japanese and Chinese soil scientists.3 Therefore, it would contribute to the more desirable condition of self sufficiency for food in China to rehabilitate these Mollisols suffering from alkaliniza-tion, because normal Mollisol generally shows high productivity.

To reclaim alkali soil, a two step process has been used. The first step involves the replacement of exchangeable sodium with calcium; the second step is to leach the resulting sodium from the soil. Leaching alone of a calcareous soil on which a crop is growing can reclaim alkali soils, but the process is slow. Of the amendments used to bring about exchangeable sodium replacement with calcium, gypsum (hydrated calcium sulfate) is far and away the most used material, because it has the advantages of being nontoxic to plants, easy to handle, and moderately soluble. However, using gypsum as an amendment material for alkali soils has been barely put into practice in China. The main reason is that demands for gypsum in China are pressing, and its agricultural utilization as a soil amendment is not common due to the market price.

Incidentally, the acid rain and air pollution originating from sulfur dioxide released from coal combustion furnaces is one of the serious environmental problems in China. Hence, it would be considered truly a case of killing two birds with one stone if we can produce gypsum from the desulfurization process of each coal combustion facility at power stations and factories, and use it as an improvement reagent for sodic soil. In this report, the possibility of using the byproduct from the desulfurization process for amelioration of alkali soil will be examined for its possible incentive to the construction of desulfuriza-tion processing in China.

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