Effect of acid modification of biochar on nutrient availability and maize growth in a calcareous soil

Abstract The nutritional quality of acid‐modified poultry manure biochar is unclear, so a better understanding of its properties and agronomic potential is needed. The biochar was modified with phosphoric acid (H 3 PO 4 ) and nitric acid ( HNO 3 ) and a combination of the two, before and after pyrolysis. After characterization of biochar samples and determination of total and water‐soluble mineral element concentrations, biochars were applied to a calcareous soil at a rate of 0.5% (w/w) to find out their effects on the mineral nutrition and growth of maize. Treatment with H 3 PO 4 + HNO 3 enriches the biochar with phosphate, nitro groups and nitrate. The experimental results suggested that biochar modified with HNO 3 and/or HNO 3 +H 3 PO 4 after pyrolysis increased the water‐soluble phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), zinc (Zn), copper (Cu) and manganese (Mn) concentrations and increased plant‐available nutrients and plant growth by positively affecting the absorption of plant nutrients. The highest plant dry weight was obtained from biochar post‐treated with HNO 3 +H 3 PO 4, and this was followed by HNO 3 ‐ and H 3 PO 4 ‐modified biochars. Premodification with HNO 3 +H 3 PO 4 also significantly increased plant dry weights. The results of this study revealed that poultry manure biochar modified with HNO 3 and H 3 PO 4 can be used effectively in calcareous soil for the improvement of plant mineral nutrition.