Biochemical and Agronomic Responses of Soybean (Glycine max L. Merrill) to Spent and Deoiled Bleaching Earth of NPK Fertilization on Filler Basis

Spent bleaching earth (SBE) is the largest waste produced by the palm oil industry. However, according to several studies, SBE and its recovery product DBE have the potential as filler materials in NPK fertilizers. This study examines the influence of NPK fertilizer with SBE and DBE as filler materials on soybean plants’ biochemical and agronomic properties. The field-based experiment was done in a single-factor randomized complete block design with 4 replicates. We tested fertilizers of 10% bentonite clay mineral using NPK on a filler basis (control), 5% bentonite clay mineral with 5% SBE of NPK on a filler basis, and 5% bentonite clay mineral with 5% DBE using NPK on a filler basis. The variables observed include soil chemical properties after applying fertilizer, which involves the concentrations of several heavy metals. Biochemical characteristics, including the content of hydrogen peroxide (H2O2) and peroxidase (POD), superoxide dismutase (SOD) activity, malondialdehyde (MDA), relative electrolyte leakage (REL), total phenolic content, and proline content. The agronomic characteristics of soybean plants, including root and shoot dry weight. The data were analyzed using ANOVA and tested using the least significant difference test at a 95% confidence interval. The results indicated that materials of SBE and DBE could partially substitute the filler elements in bentonite clay mineral of NPK fertilizer on a filler basis, and they had the same influence in SOD activity, H2O2 content, POD, MDA, REL, total phenolic, proline and root dry weight and shoot of soybean plants.