農業開発阻害因子としてのメコンデルタの土壌条件(<小特集>メコンデルタの自然と農業)

Abstract

For the agricultural development of the Mekong delta, two groups of soils pose difficult problems, one being acid sulfate soils that extend over a million hectare and the other being coastal saline soils along the coast line having 1.2 million hectare extension. Acid sulfate soils occur extensively in the Plain of Reeds along the Cambodian border and in the Ha-Tien Plain. A more scattered occurrence is also seen in the Trans Bassac lowlands. They are formed on brackish water sediments in the broad depressions which are located far away from the main river courses. In addition, swampy conditions under which they occur hinder the access of any reclamation means. Strong soil acidity resulting from oxidation of the accumulated sulfides in the sediments can be mollified either by heavy liming or by washing with a large amount of fresh water. Both require quite a heavy investment. In case of the Mekong delta, however, the Ha Tien area produces limestones, which are now being quarried for cement industry. Making the best use of this local resource for the reclamation of acid sulfate soils should merit serious consideration. Coastal saline soils may be classified as saline alkali soils according to Buringh's criteria. They are neutral to slightly acid in reaction, but sufficiently high in free salts and high in exchangeable sodium percentage. The salinity is caused by a salt water intrusion during the low flow period of the Mekong river, i. e., during the dry season. The high tidal range (&acd;3m along the South China Sea coast) is a factor that enhances the salt intrusion in the Mekong delta. Engineering measures such as construction of embankments, dike and channel plugs, and flap-gated culverts, are necessary to prevent the salt intrusion. Where salinity is not very serious, adjusting the rice growing stages to the water salinity condition, such that transplanting of rice seedlings is done during the low salinity period, lessens the adverse effects. As the irrigation water intake in the upstream areas during the low flow period could enhance the salt water intrusion in the lower reaches, care should be taken in planning the irrigation schemes for the dry season crops.