We examined the possibility of an environment-friendly slow-release fertilizer (SRF) made of biochar impregnated by anaerobically digested slurry. The biochar materials were produced from three types of feedstocks (orange peel, residual wood, water-treatment sludge) at different temperatures of 300°C, 500°C, and 700°C via pyrolysis. The release behaviors of the water-soluble K+, Ca2+, and Mg2+ were similar for all impregnated biochars and the commercial SRF used. The water-retention capacity was greatly improved by mixing the biochar-SRF with the soil. The yield of lettuce was lower for the biochar-SRF applications of 3.7 to 34.2 t ha-1 than for the commercial SRF application of 51.4 t ha-1. This might be due to excessive increase of soil pH for the biochar-SRF application. Based on these results, the authors concluded that the biochar impregnated with nutrients could become an effective slow-release K+ fertilizer.
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