Biochar, because of its porous nature, calcium carbonate (CaCO3) equivalent, surface functional groups, and other properties, could serve as an acid soil amendment. To investigate the liming potential of biochars, laboratory characterization and greenhouse and field experiments were conducted in Hawaii and West Java, Indonesia, respectively. Six wood-derived biochars were characterized and amended to a Hawaiian acid soil (pH 4.6, exchangeable aluminum [Al] 1.8 cmolc kg−1) at 2% and 4% alone or in combination with 2 cmolc kg−1 of lime and then planted with Desmodium intortum (a forage legume sensitive to Al) twice in a greenhouse trial. To the Indonesian acid soils (pH 3.9–4.0, exchangeable Al 8–14 cmolc kg−1), a rice husk and a lac tree biochars at 4% and 8% alone or in combination with lime at 4 and 8 cmolc kg−1 and compost at 0.1 and 0.2% were applied and then planted with soybean (Glycine max) cv. Anjasmoro twice in field trials. Biochar effects on soil properties and plant growth were measured. The results indicated that the six biochars varied in pH, ash content, CaCO3 equivalent, total basic cations, cation exchange capacity, and other properties (pore size, surface functional groups). Soil pH was increased, soil exchangeable Al was reduced, and plant nutrients were enriched to different degrees upon additions of biochars. Total dry weights of Desmodium were increased 2- to 4-fold over the control or lime treatment upon applications of biochar. Similar effects on soils and soybean were obtained for the Indonesian field trials. It was concluded that CaCO3 equivalent and total basic cations were among the most important properties of biochar responsible for improving acid soil productivity and plant growth.