Co-Pyrolysis of Maize Stover and Igneous Phosphate Rock to Produce Potential Biochar-Based Phosphate Fertilizer with Improved Carbon Retention and Liming Value
2020
Tumbure, Akinson | Bishop, Peter | Bretherton, Mike | Hedley, Michael
A biochar product that can supply the phosphorus (P) needs of growing crops will be highly advantageous for smallholder farmers in acidic, P-fixing, and P-deficient soils. In this study, nonreactive Dorowa phosphate rock (DPR) from Zimbabwe was added to maize residue (stems + leaves) at ratios of 1:2, 1:4, 1:6, and 1:8 (w/w) and pyrolyzed at 450 °C for 30 or 60 min to produce a suite of biochar-based fertilizers (BBFs). The total phosphorus, carbon, and nitrogen retention, biochar yields, liming value, and the amount of citric-soluble phosphate for these BBFs were assessed. After 60 min pyrolysis residence time, the 1:4, 1:6, and 1:8 mixtures yielded at least 26% more biochar yield, retained at least 43% more carbon, and retained up to 26% more nitrogen in the biochar compared to the pyrolysis of maize residues only. The citric soluble P of DPR increased up to 34 and 37% of the total P in the BBFs produced from the 1:6 and 1:8 DPR + maize mixes. This translated to more than a 30% increase in citric-soluble P compared to the unamended DPR. The BBFs show promising potential for supplying the P needs of growing crops, with about 2.1 t of the BBF (1:6 initial mix) being needed to supply 40 kg of citric-soluble P. The alkaline BBFs (pH > 10.2) had calcium carbonate equivalent values that ranged from 9.19 to 19.14 and are more suited for application to P-deficient, acidic soils. Glasshouse and field agronomic testing are recommended.
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