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Black Clayey Soils

          The black clayey soils are characterized by a high content of swelling clays, which crack more than 1 cm wide and more than 50 cm deep in the dry season. Because of swelling and shrinking, which occur alternately, the slickensides are formed at depths between 25-100 cm below the surface. Gilgai relief is observed generally. The texture of these soils is clayey, with a clay content of 50-70 percent. The surface horizons are rather thick and generally black. The granular structure may develop in the top few inches of the soil surface. These soils, in low-lying terrain, are ordinarily mottled with yellowish red, dark brown or strong brown due to periodic flooding by impounded water during the rainy season; otherwise no mottling is visible in their profile.

          The chemical constraints of these soils in Thailand are not very serious.  However, where the soils contain high carbonate content, phosphorus and iron deficiency may occur in some crops. Most of these soils in Thailand have been used for agriculture. However, such land use somewhat depends on whether the topography is flat, especially areas bordering the eastern Central Plain and the lave plain in the Northeast Plateau, where paddy rice is grown exclusively. In other areas where the land is gently sloping, some upland crops such as maize, sorghum, cotton and beans have been grown. Production has been limited due to unfavorable physical soil conditions. Because of poor physical properties the soils become difficult to manage. When they are wet they become sticky, creating poor trafficability. The poor structure of the soils impedes seed germination and root penetration. In addition, due to very low permeability, rain water stagnates easily on the soil surface and thus suppresses the crop yield. These problems force the farmers to abandon the land after a few years of crop failure. It should also be noted that legumes grown on these soils are subject to chlorosis, possibly due to iron deficiency.

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