Açaí biochar reduced soil acidity and enhanced potassium and micronutrient availability in Amazon Ferralsol under cowpea cultivation

Soil acidity and aluminium toxicity are major constraints to crop productivity in tropical regions. Although liming effectively increases soil pH and enhances nutrient availability, its high cost limits adoption by smallholder farmers, especially in the Amazon region. Biochar has emerged as a sustainable alternative. However, its effectiveness depends on pyrolysis conditions and interactions with soil properties. This study evaluated the effects of açaí biochar ( Euterpe oleracea Mart.), applied alone or in combination with reduced lime rates, on the chemical properties of a clayey Ferralsol and on nutrient uptake and biomass production in cowpea ( Vigna unguiculata [L.] Walp.). Two experiments—soil incubation and greenhouse trials—were conducted in a completely randomized design with a 5 × 5 factorial arrangement. Treatments consisted of five biochar rates (0, 5, 10, 20, and 40 Mg ha −1 ) and five lime rates (ranging from 100% to 0% of the recommended dose to achieve 60% base saturation). A 40 Mg ha −1 biochar application increased soil pH by 0.52 units, reduced exchangeable Al by up to 53%, and enhanced plant K accumulation by 55%, even without lime. Intermediate to high biochar rates combined with 25–50% lime improved the availability of key nutrients, particularly K, B, Cu, and Zn. Although shoot biomass showed limited response, Cu and Zn uptake increased with biochar application. Açaí biochar, applied alone or with reduced lime, is a promising amendment to improve soil fertility and micronutrient uptake, thereby contributing to more sustainable crop production in low-fertility tropical soils. • Açaí biochar increased soil pH and reduced exchangeable Al in Ferralsol. • Biochar plus 25–50% lime enhanced soil K, B, Zn, and Cu availability. • Biochar increased K and micronutrient uptake, especially Cu and Zn. • Açaí biochar is a viable alternative to reduce lime use in tropical soils.