<![CDATA[The residues of palm oil production and understorey biomass have potential to be used as ameliorants to release soil accumulated P from prolonged intensive inorganic fertilization and increase potassium (K) availability. However, low-quality OM that is widely available in the field has high C:N ratio and contains recalcitrant materials, making it difficult to decompose and reduce nutrient availability. Humic substances (HS) which are believed to increase nutrient availability were chosen as additional material to enhance the amelioration benefits of OM. Two complete randomized design laboratory incubation (60 days, at room temperature, and 60±10% field capacity) with three factors: Soil (P accumulated/ oil palm soil, forest soil); OM (without OM, ferns, empty fruit bunches/ EFB, fronds); HS (without HS, with HS), and with the addition of inorganic fertilizer as basic fertilizer was done to observe the effect of those combination on soil respiration and soil readily available P and K. The results showed that soil respiration rate were higher in soil with OM and HS addition compared to un-ameliorated soil. The general trend of soil respiration from rapid to slow was in order of ferns, EFB, and fronds. A combination of OM and HS causes the release of insoluble P in oil palm soil. Immobilization occurs in the initial incubation period, but after day 7, readily available P increased, indicating that OM and HS affect positively on available P. Meanwhile, readily available K increased immediately after OM and HS addition and did not show any immobilization process. OM with HS treatment shows lower available K compared to OM without HS addition, however, the values is still much higher than control, means K mineralization from added OM still occur.]]>
