Effect of organic soil amendments on soil quality in oil palm production

Soil quality (SQ) evaluation is an essential tool for the enhancement of agricultural soil management and use. An array of soil attributes, termed soil quality indicators, are used in soil quality estimation due to their sensitivity to disturbance from management practices. Therefore, the objective of this study was to assess the soil quality (using multivariate ordination technique) of oil palm (Elaeis guineensis, Jacq.) plantations amended with on-farm biomass. The experimental sites were three oil palm plantations located in the southernmost Pattani province of Thailand namely, Ban Chamao (BC), Ban Lipasango (BL) and Ban Bothong (BB) fertilised with fish effluent (FE), empty fruit bunch (EFB) compost and NPK (plus EFB mulch), respectively with completely randomised block design. A minimum dataset (MDS) of soil indicators were defined using multivariate principal component analysis (PCA). The selected MDS comprised mainly geometric mean of microbial enzyme activity (GME), available phosphorus (AP) and moisture content (MC) with per cent contribution towards soil quality index (SQI) (assessed based on PCA after scoring by normalised linear function) in the order: GME (77.3%) > AP (13.6%) > MC (9.1%). Essentially, the activity of enzymes is a crucial ecosystem function (nutrient cycling), further indicating that decreases in soil enzyme activities can be signal for a decline in SQ. GME, AP, and MC effectively differentiated the oil palm plantations and could be important not only in monitoring soil quality enhancement in the present study locations but also in a similar tropical environment with similar edaphic conditions. SQI was not significantly different between the BB (0.81) and BL (0.79) plantations (p = 0.46). However, both BB and BL were higher than in BC (0.65). BB and BL showed low geometric mean of microbial biomass GMB, high soil pH, C, N, GME and qCO(2) (metabolic quotient, i.e., the amount of CO2 respired per unit microbial biomass), while the fish effluent amended BC plantation showed the opposite. EFB contains a substantial quantity of nutrients and organic matter (OM) capable of reducing soil acidity, stimulating enzyme activity, and enriching the soil with nutrients with the resultant improvement in SQ. Therefore, a regular practice of returning EFB (either as compost or mulch, with or without mineral NPK) to the soil should be encouraged for smallholder oil palm plantations to improve soil quality and serves as an alternative channel for on-farm biomass disposal.