Reuse of agricultural waste materials is a smart solution for reducing their environmental impacts and increase economic value. Spent coffee grounds (SCG) is worldwide generated in tremendous amounts. The objectives of this study were to optimize a method to extract humic-like substances (HLS) from SCG using KOH extractant, prepare a liquid organic-mineral fertilizer enriched with N and P in addition to K, and to evaluate this fertilizer on growing maize (Zea mays L.). HLS extracted from SCG increased with KOH concentration up to 3 N and with extraction ratio up to 1:10 at room temperature. Increasing the temperature to 50 and 80 °C and contact time up to 3 h, significantly enhanced HLS recovery. Therefore, the optimum conditions for maximum HLS extraction were; 2 N KOH, SCG-to-extractant ratio of 1:10, 3 h of contact time, and 80 °C. Adjusting the pH of the alkaline K-HLS supernatant to pH 6 was achieved using a mixture of HNO3 and H3PO4. The prepared NPK mineral-organic fertilizer (NPK-HLS) was dark brown containing 2.0/5.8/8.6 as N/P2O5/K2O and 5.1 %w/v HLS.
The produced fertilizer was evaluated for its effect on maize plants grown in a calcareous soil low in available nutrient using a CRBD pot experiment. Two application rates of NPK-HLS (NPK-HLS1 and NPK-HLS2) based on P rates, 125 and 250 mg P pot-1, with and without a basic fertilizer (BF) were applied. Both NPK-HLS1 and NPK-HLS2 rates significantly increased shoot dry matter, NPK uptake and availability in soil compared to the control, but did not significantly affect root dry matter. Increasing the application rate to NPK-HLS2 did not further increase dry matter or NPK uptake. No significant effect was found for BF for all measured variables. Results of this study showed that the extraction of HLS from SCG and its incorporation into a liquid fertilizer could be an alternative solution to the reuse of SCG.
Key words: Spent coffee grounds, alkali extracts, mineral-organic fertilizers, maize growth, NPK uptake.