Influence of acidified-biochar on phosphorus and potassium availability in alkaline sandy soil

Biochar application is recommended to enhance soil fertility, particularly in arid and semi-arid regions. In alkaline soils, acidifying biochar can help overcome high pH constraints and improve phosphorus (P) and potassium (K) availability. This study evaluated the effects of acidified palm frond (PF) and guava branch (GB) biochars on P and K availability in alkaline sandy soil through a controlled batch incubation experiment. Biochars were acidified using acetic or phosphoric acid and characterized for their chemical properties, surface morphology, and functional groups. A 15-week incubation was conducted using sandy alkaline soil pre-treated with 0.48 Mg ha^-1 of superphosphate fertilizer. Acidified and unacidified PF and GB biochars were applied at rates of 2.4 and 4.8 Mg ha^-1. PF biochar showed a higher cation exchange capacity (CEC: 37.9 cmol kg^-1) and greater K enrichment than GB biochar (CEC: 17.6 cmol kg^-1). Phosphoric acid significantly increased the CEC of both biochars, while acetic acid had minimal impact. Application of 4.8 Mg ha^-1 phosphoric acid-treated GB biochar significantly improved soil P availability, reaching 104.02 mg kg^-1 after one week, with no evidence of P fixation over 15 weeks. PF biochar, regardless of acidification, significantly enhanced exchangeable K levels, particularly at the higher application rate. Acidification improved P and K availability in the studied alkaline sandy soil, but such influence depended on biochar type, chemical composition, and incubation time. Selecting the appropriate acid for biochar modification is crucial to optimizing nutrient availability in alkaline soils.