Soil degradation under contrasting cropping regimes following forest clearance in North East Thailand
Soil degradation of Ultisols after clearance of dry Dipterocarp forest in North East Thailand is quantitatively assessed through measurements of changes in soil properties and soil quality indicators on a time series of study plots under sugarcane and cassava. Research plots with known times since forest clearance on similar Ultisols, were selected by a combination of participatory methods with farmers, collaboration with forestry and agricultural extension officers, information derived from soil maps and field survey, and interpretation of a time series of aerial photographs. Forest control plots were similarly selected for comparison. Results show that soil degradation process are initiated under natural forest and surface effects appear to be related to canopy gaps, but no significant effect of frequency was found at 10-15cm soil depth. Soil degradation increases substantially in the first 20 years after forest clearance for cassava and for sugarcane production and, thereafter, approaches an equilibrium. Significant increases of bulk density under cassava and clay dispersion index under sugarcane indicates oil compaction and soil structure decline. Significant decreases in soil organic carbon, labile carbon and exchangeable cations were observed under both cropping regimes, whilst acidification was significant under sugarcane. Significant negative changes of relative soil quality indices (0 RSQI < 0) over time were detected under cassava but were greatest under sugarcane.