▶ Rice straw biochars were charred at temperatures from 250 to 450 °C for 2, 4 and 8 h. ▶ The chemical, physical, morphological and spectral properties of biochar are largely influenced by charring temperature and duration. ▶ Biochar has very low aggregation, dislike other organic materials. ▶ Biochar amendment of a typical Ultisol was proved to have two functions: biochar as a conditioner plays a much more important role in improving crop growth than as a fertilizer itself. Applying biochar to soils may cause a win–win situation resulting in C sequestration and soil fertility improvement. The effect may be more evident in highly weathered and infertile tropical soils, but will be dependent on biochar quality. An Ultisol, typical to southern China, was used to evaluate amendment with biochars produced by a range of temperatures and durations, to investigate its effects on soil properties and plant growth. Rice straw-derived biochars were charred at temperatures from 250 to 450 °C for between 2 and 8 h. The increase of temperature caused smaller less structured (as viewed by SEM) fragments to form with less O, H and aliphatic C functional groups, but more aromatic C as indicated by infrared spectroscopy. The mean residence time of biochars under controlled conditions (25 °C, 40% field capacity) was estimated from 244 to 1700 years, generally increasing with charring temperature and duration. Amendment of 1% biochar increased pH by 0.1–0.46 ( P < 0.01) and CEC by 3.9–17.3% ( P < 0.05), but had no effect on aggregate stability. In pot trials maize biomass was increased by 64% (without NPK) to 146% (with NPK) after biochar amendment. The study emphasizes that amendment with biochar can improve soil fertility at least in the short term. Future studies focusing on the persistence of biochar fertility in the field must explicitly take into account additional factors to transfer this technology.