Biochar as a Soil Amendment in Tropical Acidic Soils: Effects on Nutrient Retention and Crop Yield

Abstract

Biochar, the charred residue of biomass heated with little oxygen, is often proposed as a way to repair acidic, weathered tropical soils: it may raise pH, bind aluminium, hold nutrients that would otherwise leach and, in the end, lift crop yield. This narrative review asks how far the published evidence supports that chain of claims, and where it breaks. It draws on work on soil acidity, on how feedstock and pyrolysis temperature shape biochar, on mechanisms of nutrient retention, on field and pot experiments in the tropics, and on ageing, economics and life-cycle assessment, and it reports no new data. The main complication is variability. Effects depend on feedstock, pyrolysis temperature, soil and application rate, and yield responses reported in the literature range from clearly positive to neutral to negative. Much of the benefit seen in acidic soils looks like a liming and nutrient-supply effect that fades or changes as the material ages, nutrient retention measured in columns is not always matched by better yield, and cost, ageing and field-scale evidence remain thin. The review concludes that biochar is a plausible but conditional amendment, suggests how trials should be reported and compared with lime and fertiliser, and lists the evidence needed before broad recommendations can be made.