Assessment of soil health and remediation in coastal palm oil plantations of Sabah, Malaysia

Thesis Abstract

Coastal palm oil plantations in Sabah, Malaysia, face progressive soil degradation driven by nutrient imbalances, salinization, acidification, and compaction, threatening long-term productivity and coastal ecosystem services. This study addresses the need to assess soil health status and identify effective remediation strategies tailored to tropical oil palm systems in littoral environments. The aim is to evaluate spatial and temporal patterns of soil health indicators and to develop an evidence-based remediation framework that integrates agronomic, ecological, and socioeconomic considerations for sustainable plantation management. Specific objectives are (1) to characterize baseline soil physico-chemical properties (pH, electrical conductivity, texture, bulk density, soil organic matter, total nitrogen, available phosphorus, exchangeable potassium, cations exchange capacity) across 60 representative plots in three coastal districts; (2) to assess soil biological health via microbial biomass carbon,-respiration, soil enzyme activities (dehydrogenase, urease, phosphatase) and nematode community structure; (3) to quantify heavy metal and trace element concentrations (e.g., cadmium, lead, arsenic) and their correlations with management history; (4) to evaluate remediation options, including organic amendments, cover crops, and reduced-tillage practices, through a randomized complete block design on 18 demonstration plots over two growing seasons; (5) to model drivers of soil health using multiple regression and structural equation modeling within the framework of the Soil Health Theory and the Ecosystem Services concept; and (6) to translate findings into a practical remediation framework for Sabah’s coastal oil palm operations. A mixed-methods approach will be employed. Quantitative data will be collected through soil sampling at 0–20 cm, 20–40 cm, and 40–60 cm depths, with triplicate samples per plot, analyzed using standard soil analytical methods (pH in water and KCl, EC, particle size analysis, Walkley-Black organic carbon, Kjeldahl nitrogen, Olsen phosphorus, exchangeable bases, CEC). Microbial analyses will include substrate-induced respiration, microbial biomass carbon via chloroform fumigation-extraction, and phospholipid fatty acid (PLFA) profiling. Enzyme assays will target dehydrogenase, urease, and acid and alkaline phosphatase activities. Heavy metals will be determined by inductively coupled plasma mass spectrometry (ICP-MS) following acid digestion. Plant- and management-related data will be collected through farmer surveys and field records to contextualize results. Data will be analyzed using ANOVA to compare treatment effects, followed by Tukey’s HSD for post hoc comparisons. Regression analyses will identify key predictors of soil health indicators, while structural equation modeling will test hypothesized pathways linking management practices, soil properties, microbial activity, and ecosystem services. Temporal trends will be examined using repeated-measures analyses across seasons. A GIS-based spatial analysis will map soil health gradients and remediation potential across the landscape. Expected findings include (i) quantification of the extent of soil acidification, salinity, and organic matter decline in degraded zones; (ii) identification of microbial and enzymatic bottlenecks limiting nutrient cycling; (iii) evidence on the effectiveness of organic amendments (e.g., palm oil mill effluent compost, biochar, green manure) and cover crops in restoring soil structure, fertility, and biological activity; (iv) elucidation of risk factors associated with heavy metal accumulation and their management implications; and (v) a transfer-ready remediation framework outlining recommended amendment rates, rotation schemes, and monitoring indicators tailored to Sabah’s coastal conditions. The study contributes to knowledge by integrating soil physicochemical, biological, and ecological service perspectives within a tropical plantation context, offering empirical guidance for remediation strategies that reconcile productivity with environmental stewardship. The anticipated conclusion emphasizes the feasibility of low-cost, farmer-acceptable remediation practices that improve soil health indicators, reduce environmental risk, and enhance resilience to climate variability. Recommendations include policy support for routine soil health monitoring, adoption of organic amendments and cover cropping in coastal palm oil systems, capacity-building programs for plantation managers, and the development of Sabah-specific soil health thresholds to guide management decisions.