STATISTICAL MODELLING OF THE INFLUENCE OF SOLUTION pH ON MACRONUTRIENT ADSORPTION BY FORTIFIED WATERMELON RIND FOR SUSTAINABLE SOIL AMENDMENT

This study investigated the effect of pH on the adsorption capacity of nitrogen (N), phosphorus (P), and potassium (K) during the fortification of watermelon rind (Citrullus lanatus) using potassium ammonium phosphate KNH₄PO₄) for potential application as a soil amendment. Watermelon rind was processed into a powdered adsorbent and subjected to batch adsorption experiments at different pH levels (2, 4, 6, 8, and 10). The adsorption capacities of N, P, and K were determined after equilibration using standard analytical methods. Results showed that pH significantly influenced nutrient adsorption, with adsorption capacity decreasing as pH increased. The highest adsorption capacities were recorded at pH 2, with values of 5.81 mg g⁻¹, 5.87 mg g⁻¹, and 5.91 mg g⁻¹ for nitrogen, phosphorus, and potassium, respectively. The lowest adsorption capacities were observed at pH 10, where values decreased to 3.74 mg g⁻¹, 3.48 mg g⁻¹, and 3.67 mg g⁻¹ for N, P, and K, respectively. A formal two way Analysis of Variance (ANOVA) demonstrated that pH exerted a statistically highly significant influence on nutrient loading (p < 0.001), whereas the variation among nutrient types was not significant (p > 0.05). The enhanced adsorption under acidic conditions was attributed to the protonation of surface functional groups and increased electrostatic attraction between nutrient ions and adsorption sites on the watermelon rind. The findings indicate that acidic conditions favor nutrient loading and retention, thereby enhancing the suitability of fortified watermelon rind as a slow-release soil amendment for sustainable agriculture.