Biochars have been proposed as soil amendments based on research indicating beneficial effects of biochar addition on various processes such as trace element retention, mitigating greenhouse gas emission, and increasing carbon sequestration in soil. In particular, acidic soil exhibiting low cation exchange capacities and low base saturation percentages were found to be ameliorated by biochar additions. In tropical regions, strongly weathered acidic soil such as ferralsols and acrisols are very abundant. In Southeast Asia, acidic soils are often used for growing oil palms, which are fairly tolerant to acidic conditions and elevated Al concentrations in soil solution. Often acidic soils are also used for rice production, being the staple food in Southeast Asia.

In this study, we aimed at assessing the suitability of biochars produced from economic crop residues as soil amendments for acidic soils in Thailand. Rice and oil palm residues were selected as the feedstock materials for biochar production. Dialysis and acid-washing were applied to reveal the contribution of the charred carbonaceous solids in biochars to their reactivity as compared to the untreated biochars containing inorganic solids. Two well-characterized biochars produced from rice straw and chestnut wood were included as reference materials. Titration experiments revealed a trend in reactivity of the biochars with respect to cation binding of the charred carbonaceous solids in the following order: rice straw>oil palm fruit fibre>rice chaff>>oil palm shell. The reactivities were highly related to the amounts of carboxylic carbon of the biochars as determined with FTIR spectroscopy. Acid-soluble solid phases contained in the non-treated biochars substantially contributed to the total reactivity. Their share was strongly related to the intensities of C-O out of plane deformation IR band of carbonates indicating that solid carbonates are major reactant phases in biochars. Compared to reactivities reported for humic materials, the values measured for all biochars investigated here were low. Consequently, the investigated biochars have to be applied in high quantities to make the biochars substantially contribute to trace element sorption in soil. As the carbonates contained in biochars can additionally support trace element retention in soils by increasing soil pH and by acting as a sorbent we conclude that carbonate-rich biochars are preferably suited as amendments for acidic tropical soils contaminated with cationic trace elements such as Cd²⁺, Cu²⁺, and Pb²⁺.