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Converting abundant agricultural residues to valuable products, such as biochar, is a pressing challenge for society. Here, our objective has been to produce biochar-based fertilizers (BBFs) with high carbon stability, high yield, and characteristics favorable for their use in soils. Thus, H 3 PO 4 with and without MgO was co-pyrolyzed with coffee husk and poultry litter. Proximate analysis, total phosphorus and magnesium, biochar yield, carbon retention, and thermal and chemical carbon stability were assessed. The BBFs were slightly acidic (pH ∼6.0) and showed high contents of total P (146−206 g kg −1 ) and total Mg (96−98 g kg −1 ), similar to commercial fertilizers. Biochar yield increased up to 65% when treated with H 3 PO 4 and H 3 PO 4 −MgO, resulting in up to 78% greater carbon retention. BBFs showed higher thermal stability, by thermogravimetric analysis and muffle oxidation, when compared to corresponding pristine biochars. Conversely, the addition of H 3 PO 4 −MgO decreased the chemical stability (higher H 2 O 2 and K 2 Cr 2 O 7 oxidation), compared to the biochars, because of the increase in surface area and reactivity. We concluded that the co-pyrolysis of biomasses with H 3 PO 4 and MgO promoted thermal stabilization (increased yield) and increased chemical oxidation, because of increased surface area and reduced crystallinity of the BBFs.

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