Phosphorus and Potassium Availability in Soil Ameliorated by Different Quality Organic Matters Enriched with Humic Substances

Abstract

The residues of palm oil production and understorey biomass have potential to be used as an ameliorant to release soil accumulated P from prolonged intensive inorganic fertilization and increase potassium (K) availability. The effect of organic matter (OM) is very diverse, low-quality OM that is widely available in the field has high C:N ratio and contains recalcitrant material making it difficult to decompose. So, other materials need to be added to increase the benefit of OM addition to the soil. Humic substances (HS) which are believed to be capable of increasing nutrient availability and plant growth were chosen as additional material. A complete randomize design laboratory incubation with three factors (Soil: P accumulated/ oil palm soil, forest soil; OM: without OM, fern, EFB, frond; and HS: without HS, with HS), and with the addition of inorganic fertilizers as basic fertilizers was done to observe whether this combination can increase the benefit of OM amelioration. There were 16 treatments, and experiments were done in triplicate. Two soils were chosen purposively for this experiment. 1) taken from the oil palm disk zone, which was fertilized intensively at PTPN III, Oil Palm Cikasungka Plantation, Cigudeg, Bogor; and 2) were taken from the forest near the plantation, Cigudeg, Bogor as a comparison. In oil palm soil, P accumulation has occurred as indicated by the very low available P (2.101 mg P2O5 100 g-1) compared to the high potential P (162.297 mg P2O5 100 g-1). OM (ferns/ N. bisserata, oil palm empty fruit bunches/ EFB, and fronds) were also collected from the oil palm plantation, while the HS and inorganic fertilizer used were commercially purchased. The experiment was divided into two sets of laboratory incubations 1) determination of OM decomposition by measuring soil respiration; and 2) determination of soil readily available P and K. Incubation was conducted for 60 days, at room temperature, and 60±10% field capacity. OM decomposition measured on day 1, 10, 20, 30, 40, 50, and 60 after incubation, while readily available P and K were measured on day 0, 3, 7, 14, 28, 45, 60 after incubation, extracted with 1N NH4Cl. The results showed that OM and HS addition have a higher OM decomposition compared to un-ameliorated soil. The general trend of OM decomposition from rapid to slow was in order of ferns, EFB, and fronds. A combination of OM, and HS causes the release of accumulated 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 with single OM addition, however, the values is still much higher than control, means K mineralization from added OM still occur.