Soil Quality Index for the Two Agricultural Areas along the Naboc River in Davao de Oro, Philippines Contaminated with Mercury

Abstract

The study aimed to determine the soil physicochemical conditions, soil quality index (SQI), its relationship to soil total mercury (THg), and their spatial autocorrelation of the two mercury- contaminated agricultural areas in Monkayo, Davao de Oro, Philippines. Soil physicochemical parameters and THg were assessed, and the data underwent principal component analysis (PCA). The minimum data set (MDS) revealed that percent sand, pH, THg, phosphorus (P), and organic carbon (OC) significantly contributed to the SQI expression. The two agricultural areas (Diwata and Naboc) had comparable (P > 0.05) soil quality indices (0.46 ± 0.073 and 0.46 ± 0.11), which were categorized as Grade 3 (moderate SQI). THg was negatively associated with the SQI, with the regression coefficient (R) equal to –0.419 (p = 0.021); however, this association did not mean a causal effect. Hence, mercury (Hg) contamination could not be singled out as the sole factor in soil quality deterioration but rather the synergistic effects of the agricultural practices and the Hg contamination. Spatial autocorrelation analysis revealed that THg spatial distribution in both areas and the SQI of Diwata were clustered; however, the spatial distribution of SQI in Naboc was random. An in-depth study of soil microbiological function and the ecotoxicological effect of mercury on soil quality is deemed important to be studied.