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Marginal land and problem soil of Thailand


Skeletal Soils
               Skeletal soils refer to soils that contain 35 percent or more (by volume) of rock fragments, cobbles, gravel and laterite concretions or ironstones having diameters greater than 2 mm, within shallow depths (less than 50 cm). These soils occur in a number of landscapes ranging from alluvial terraces, fans, erosional surfaces, peneplains, hill slopes, and mountainous areas. Skeletal soils contain laterite at some period of time on erosional surfaces or as remnants of a peneplain surface. Soils with cobblestones often develop on alluvial terraces. On the other hand, soils shallow to bed rock or soils containing rock fragments are common on areas where the parent rocks are subject to continuous erosion or weathering. They occupy mostly foothill slopes, mountains, and partial peneplains or erosional surfaces. Due to a much different mode of soil formation, the skeletal soils are relatively variable in physical and chemical properties. However, these soils are usually shallow, prone to erosion, and low in natural fertility status.
               Most of these soils have not been used for agriculture. However, due to the great demand for land, parts have been opened for cultivation. The land uses vary depending on the climate and physiographic position. Skeletal soils in lowlands are used for paddy rice. Well-drained skeletal soils in high rainfall areas are mainly used for para rubber growing whereas the drier areas are used for cassava, sugarcane, beans, maize and sorghum. The yields of such crops are generally low, which can be attributed to various causes:
(i)
   the effect of coarse fragments occurring at shallow depths,
(ii)
   the surface layers are always disturbed by cultivation practices, especially the use of machinery,
(iii)
   root penetration and soil volume constraints, and
(iv)
   low plant nutrients and low water holding capacity.
               After a few years of cultivation many areas are abandoned and are covered by low secondary shrubs and weeds. On areas where slopes are not too steep or with slopes less than 20 percent, pasture development should be the most appropriate land use for skeletal soils. Otherwise, these soils should be restricted to woodlands or for commercial fast-growing tress.