Agricultural productivity in the tropics is constrained by low soil nutrient contents and accelerated mineralization of soil organic matter. This study investigated the effects of biochar particle size on soil chemical properties and nutrient contents of degraded Ultisol in Nsukka, southeastern Nigeria. Three biochar particle sizes; < 0.25 mm, 0.25–0.50 mm and 0.50–1.00 mm were applied at 10 t/ha to 4 kg of soil. The soil-biochar mixture was watered to field capacity for one week after which maize (Orba super 2) was sown. 200 kg/ha of NPK 15–15-15 was added to the pots 2 weeks after germination. Agronomic parameters were collected throughout the experiment that lasted for 8 weeks. Soil samples were collected at the end of the experiment. The experiment was repeated without additional application of biochar and NPK fertilizer. The data obtained were subjected to analysis of variance (ANOVA) using Genstat. Significant (p < 0.05) effect of biochar particle size on soil chemical properties investigated except Magnesium (Mg2+) and Aluminum (Al3+) were obtained. Soil pH improved from strongly acid (4.1) in the original soil to slightly acid (5.3) in soils amended with biochar of different particle sizes. Soil organic matter (SOM) increased by more than 100% in soils amended with biochar of different particle sizes relative to the original soil. Similarly, the results showed a significant (p < 0.05) residual effect of biochar particle size on soil chemical properties except for Al3+, Mg2+, cation exchange capacity (CEC), hydrogen (H+), and organic matter (OM). The residual effects showed that soil properties improved with increasing biochar particle sizes while the main effects of biochar particle sizes showed no consistent trend except for nitrogen. The results also showed a significant (p < 0.05) main and residual effects of biochar particle sizes on plant growth parameters investigated. The study concluded that biochar particle size could influence soil chemical properties and nutrient content, plant growth and productivity of degraded Ultisols especially in southeastern Nigeria.
