Field experiment and laboratory analyses were carried out to investigate the effects of biochar on soil acidity and pH-buffering capacity of an Ultisol in Umudike, southeastern Nigeria. The treatments were 0 ton per hectare (t/ha) biochar, 1 t/ha biochar, 3 t/ha biochar, 5 t/ha biochar, 1 t biochar + 400kg NPK (15:15:15) /ha, 3 t biochar + 400kg NPK (15:15:15) /ha, 5 t biochar + 400kg NPK (15:15:15) /ha, and 400 kg NPK (15:15:15) /ha. The treatments were replicated 3 times. The incubation studies were carried out to investigate the effect of the treatments on soil pH and exchangeable acidity over a period of four weeks, using the equivalent of the treatments used in the field experiment, 0g biochar, 1.6g biochar, 5g biochar, 8g of biochar, 1.6g biochar + 0.6g NPK (15:15:15), 5g biochar +0.6g NPK (15:15:15), 8g biochar +1.6g of NPK (15:15:15) and 0.6g of NPK (15:15:15). Each of the treatments was added to 50g of soil in plastic containers of equal size and basal diameter and replicated three times. The soil used for the incubation studies was strongly acidic, having a pH (H2O) of 4.38 and exchangeable acidity of 1.84. The soil pH (H2O) and exchangeable acidity were determined on the incubated samples at weekly interval for 4 weeks, using standard laboratory procedures. The effect of the treatments on the pH-buffering capacity of the soil during incubation was also determined using standard procedures. Results obtained showed that 5 t/ha biochar significantly (p<0.05) increased soil pH from 4.38 to 8.1, 8.64, 8.20 and 8.42 from week 1 to week 4 of incubation respectively, while exchangeable acidity was reduced from 1.84 to 0.34, 0.61, 0.56 and 0.37 from week 1 to week 4 respectively throughout the incubation period. The pH-buffering capacity of the incubated soil was also increased by 96%, 97%, 87% and 92% from week 1 to week 4 respectively. It could therefore be concluded that the application of 5 t/ha biochar ameliorated soil acidity and increased soil pH-buffering capacity of the study area. It was therefore recommended.

Keywords: amelioration, soil acidity, pH-buffering capacity, ultisol, biochar