Physical qualities of acid sulfate soil: its limitations and implications for oil palm production

Global demand has pressured the extension of oil palm production areas in Thailand to acid sulfate soils. Besides their severe acidity and nutrient imbalance, the soil physical quality is expected to restrict production. This research defined the critical physical limitations of acid sulfate soils for oil palm production. The soil parameters of physical quality were assessed, covering 32 points in both the topsoil (Ap) and subsoil horizons. These parameters consisted of penetration resistance in the wet and dry seasons, saturated hydraulic conductivity (Ksat), bulk density (BD), the water characteristic curve, available water capacity (AWCA), available water content (AWCO), infiltration, and the S-index (a single indicator of soil physical quality derived from the parameters of the van Genuchten equation). The results showed critical physical quality constraints, particularly concerning the S-index and water shortage. The poor physical quality was more distributed in the Ap horizon (S-index = 0.024) than in the subsoil (S-index = 0.033). The penetration resistance was moderately restricted (3.26 Mpa) to the plant roots only in the dry season. Corresponding to the S-index data, the Ap horizon had a lower AWCA (0.21) than the subsoil (0.30). Most negative AWCO values in the subsoils (78%) during the wet season suggested a severe water shortage for oil palm production. Further research is needed to refine a specific partial drainage technique under oil palm canopies, focusing on the air-water balance system.