Chemical Evaluation of Nutrient Supply From Fly Ash–Biosolids Mixtures

Abstract

Prediction of plant nutrient supply from fly ash and biosolids (sewage sludge and poultry manure) may enhance their agricultural use as crop fertilizer. Two mild extraction methods (42‐d equilibration with ion‐exchange resins; 2‐d equilibration with pH 4.8 buffered nutrient solution) and analysis of nutrient data by the Diagnosis and Recommendation Integrated System (DRIS) were tested with 29 fly ash samples, four biosolids samples, and their mixtures. The resin method was useful for major nutrient (N, P, K, Ca, Mg, S) extraction from fly ashes and organic materials, particularly where mineralizable fractions of N and P under aerobic conditions are required. However, resins were inefficient in extracting P from high‐Fe sewage sludges because organic waste samples caused premature failure of semipermeable membranes and fouling of resins. Extraction of fly ash with dilute buffered nutrient solution was more successful because micronutrient recovery was improved, major nutrients were correlated to the resin method, both addition and removal of nutrients were recorded, DRIS analysis was possible, and equilibration was rapid (2 d). The overall nutrient supply from these extremely variable fly ashes was: (high micronutrient, low major nutrient supply). For biosolids, the major nutrients ranked: P > N ≈ Ca > S > Mg > K (sewage sludges), and N > Ca ≈ K > P > Mg > S (poultry manures). In mixtures of fly ash with 26% sewage sludge the order was: Ca > S > N > Mg > P > K, while in mixtures of fly ash and 13% poultry manure, the nutrients ranked: Ca > K ≈ N ≈ S > Mg > P. Optimal plant nutrition (especially N–P–K balancing) should be possible by mixing these three waste materials.