Soil properties of secondary forests under shifting cultivation by the Iban of Sarawak, Malaysia in relation to vegetation condition

Soil properties under secondary forests were studied in relation to vegetation conditions to clarify the site selection method for shifting cultivation of the Iban in Sarawak, Malaysia. Soils and vegetation were surveyed at 25 suitable sites and 17 unsuitable sites for shifting cultivation, classified by the landowners based on their perceptions through knowledge of vegetation condition. Principal component analysis classified soil properties into three principal components. The first was soil texture and organic matter with high factor loadings of clay content, cation exchange capacity, total C, and total N, and, to a lesser extent, exchangeable K. These properties were little affected by the fallow duration and site suitability. Clay content was regarded as one of important factors related to the levels of organic matter and nutrient retention capacity of the soils. The second component reflected soil acidity and exchangeable bases with high factor loadings of pH, exchangeable Ca and Mg, exchangeable Al, and Al saturation. These properties reflected the remnant effects of ash addition and fallow duration. However, no clear differences were apparent in these properties in terms of site suitability. The third component was linked to N availability, including the C/N ratio and exchangeable NH4. The N availability was higher at suitable sites than at unsuitable sites. At suitable sites, N availability tended to be higher with increasing fallow age. The N availability showed a statistically significant correlation with the site score of the first axis obtained from correspondence analysis of plant density, suggesting a strong relationship between N availability and vegetation condition. That relationship was attributable to depletion of the available N of the soils during cropping and at the initial fallow stage, as well as to improvement of N availability of soils with increasing length of the fallow period. This finding related to N availability was consistent with farmers' criteria for fertile land: leaves of trees in fertile land should be dark green. The presence of some specific plant species might be correlated with levels of available P and soil hardness. Results of this study show that the Iban farmers' vegetation based site -selection method might reflect the ability of secondary forest ecosystems to supply nutrients for rice plants through burning practices.