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By comparing undisturbed pristine climax Dipterocarp forest in close proximity to long-term cultivated production systems, the greatest impact of changed land use was observed in the surface charge characteristics of the soil. This was a consequence of a decline in the organic matter content of soils. Using the surface charge characteristics of these two systems a degradation index termed the ‘Saturation Index’ was developed, tested and proposed as a measure for assessing chemical degradation.ഀ
Changes is soil water retention characteristics were quantified using the Freezing Point Depression method on field soils treated with 2:1 smectitic clays showing that the water holding capacity of these soils is increased with the application of clays. This inexpensive approach to measuring the soil water retention characteristics of soils has been introduced into the soil physics research laboratories of LDD Bangkok. ഀ
The high buffering capacity of the un-limed soils as pH declines towards 4 appears to be related to kaolinite dissolution. Indeed kaolinite being the dominant clay mineral showed greater evidence of disorder in the surface horizons than at depth when assessed using XRD. This buffering process will maintain a stable pH for centuries before the effects of continuous acidification will become evident in a pH decrease. Associated with the dissolution of kaolinite, it is expected that the amount of soluble aluminium in the topsoil would increase and the release silica would accelerate cementation processes. A highly significant pedotransfer function was established to estimate pH buffering capacity (pHBC) using soil organic carbon and clay content. Using the pedotransfer function and the net acid addition rates (NAAR) of common cropping systems, acidity risk maps were produced for Northeast Thailand for each of the crop production systems. These maps can be used to assess the potential for accelerated soil acidification and allow informed decisions to be made in order to address or reduce the risk of soil degradation associated with acid generation. ഀ
The application of clay based materials to degraded tropical light textured soils increased the surface charge characteristics of soil and available water content that in turn resulted in 6 fold increases in biomass production. The research has clearly demonstrated the role of cat ion exchange capacity in the soil degradation and its rehabilitation. Increases in surface charge characteristics associated with the application of clay based materials are not influenced by organic matter (variable charge component of soils) and hence these changes are permanent and are not dependent on pH. The responses associated with this one-off application of clay persisted for over 3 years and resulted in significant increases in the water productivity of these rainfed production systems. Farmer field assessments were undertaken by a large number of farmer networks in Northeast Thailand with significant responses in productivity, particularly under rainfed rice production. Farmer networks are promoting the concepts of land degradation and rehabilitation using clay based materials along with organic matter.ഀ
Acid waste bentonite a byproduct from vegetable oil bleaching that is acid (pH < 3.00) and hydrophobic was converted into a high quality soil conditioner using readily agricultural byproducts through co-composting. Co-composted acid waste bentonite with rice husk, rice husk ask, and chicken litter eliminated their acidic reactivity and hydrophobic nature. In addition, these composted materials showed positive impacts on soil physical and chemical attributes resulting in highly significant increases in crop biomass. ഀ
A three years study was conducted in order to evaluate the effect of continuous stylo on the porosity of a compacted layer and its possible influence on root elongation and yield of a subsequent maize crop. Continuous stylo was grown for two years in experimental plots and compared to a currently used stylo-maize rotation. Root distribution and macropore density was measured on the two cropping systems. After 24 months of continuous stylo roots were able to penetrate the compacted layer resulting in a significant improvement in the macroporosity of this layer. The subsequent maize crop developed a deeper root system using these macropores than in the stylo-maize rotation treatment. The yield of maize was not significantly influenced by these physical changes due to the unusually wet growing conditions. The results from this study clearly demonstrate the role of Stylosanthes in bioremediating sandy compacted layers. ഀ
A four year study evaluating the influence of gamba grass (Andropogen gayanua) in remediating sub-soil acidity through the manipulation of fertilizer sources is currently being written up. The results clearly demonstrate that nitrate based fertilizers will result in a significant shifts in soil pH to 50 cm. In addition, roots of gamba were able to penetrate to a depth of 1.2 m that allowed the crop to access stored soil moisture and to produce biomass throughout the year under rainfed conditions. Significant increases in soil organic carbon were measured to a depth of 40 cm. ഀ
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