Morphology and genesis of Gray Podzolic soils in Thailand Public Deposited

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  • Nine Gray Podzolic soils from different physiographic regions of Thailand were studied to determine their characteristics and genesis in relation to their parent materials, land form, pedogenic environment, classification and management. The clay was characterized by X-ray diffraction and standard chemical and physical analyses were performed. Mineralogy of the light ana heavy minerals from the fine sand to coarse silt fractions was determined by randomized grain counts. Micromorphological study was made of thin sections from selected horizons in each profile. Field and laboratory study showed that Gray Podzolic soils formed under distinctly different climatic zones and vegetation have quite similar characteristics. They occupy similar low terraces with nearly level topography and are associated closely with Low Humic Gley soils. Parent materials and topography are the two most important factors influencing the genesis of these soils. Mineral assemblages in the profiles indicate the parent materials were preweathered sediments from acidic rocks. They are quartz rich, with very few if any weatherable minerals, ubiquitous heavy minerals and with dominantly kaolinitic clay. The few lower stability minerals found in several profiles can be traced to some rocks rich in those minerals in the catchment areas. Climatic variations between the soils are not consistent with the weathering stages found and differences in soil age are not enough to account for mineralogical variation through differential in situ, weathering. Therefore, differences in composition and preweathering of parent materials most logically account for the presence of minor amounts of relatively unweathered feldspar and hornblende in some of these soils. Soil development has resulted in a loosely bound, clay poor, s-matrix with low degree of orientation in the upper horizons, and illuvial B horizons with more oriented plasma, illuviated cutans and vughs in the lower part of the profiles. Pedoplasmation activity by termites and other soil dwellers is partially responsible for the high porosity and mixing of the soil materials and, possibly, for the addition of lime. Plinthite formation in the lower part of some profiles relates directly to iron mobility and accumulation associated with fluctuations of the water table. Cation exchange capacities of the soils were low and the base saturation was generally less than 20% in the lower horizons. These characteristics reflect the low organic matter, sandy texture, kao- linitic clay and high porosity of these soils and strong leaching of bases under the humid climate. Depletion of bases and organic matter has been accelerated by the clearing of forest for shifting cultivation. Eight of the profiles were classified as Ultisols in Paleustult and Paleudult suborders; four of these with plinthite layers were grouped in plinthitic great groups. One profile with low clay content and higher, fluctuating base saturation was placed in the Ustropept great group. The CEC of less than 24 me /100 g clay placed all nine soils in ustoxic, orthoxic or oxic subgroups. Leached Ferruginous Tropical soils of Africa and Australian Lateritic Podzolic soils are similar to Gray Podzolic soils in this study except that they have higher base saturations. In northeastern Thailand, Gray Podzolic soils have been shown to have both high and low base saturations. The higher base saturation may be due to lime accumulation by termites and low buffering capacity of the sandy soils. Characteristics of Gray Podzolic soils indicate serious problems in their management under cultivation. The nutrient level in these soils is very deficient for most crops. In the northeast, where the rainfall is less reliable, development has been most difficult. With the present crops and level of management, response to fertilization and irrigation is very poor. Multiple improvements in other essentials of production is badly needed. Improvements are taking place, but sustained efforts on all aspects of the problem are required for effective and lasting progress.
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