ABSTRACT

Incubation studies and field trial were carried out at the Michael Okpara University of Agriculture, Umudike to evaluate the effect of biochar rates on some soil chemical properties, yield and nutrient content of sweet potato (Ipomoea batatas) in an Ultisol of Umudike, southeastern Nigeria. The incubation study was aimed at determining the rate of nutrient release as well as the rate of acid neutralization by biochar and NPK (15:15:15) fertilizer. The treatments comprised of control (0 t/ha), biochar at 2.5t/ha, 5t/ha, 7.5t/ha and 10t/ha and NPK (15:15:15) fertilizer at 400kg/ha with the equivalent rates of biochar at 3.3g, 6.6g, 10g and 13.3g respectively and NPK (15:15:15) fertilizer at 0.53g. These were replicated four times in Randomized Complete Block Design (RCBD). The second experiment which was the field experiment was carried out to investigate the effect of treatments on the root tuber yield of sweet potato in t/ha of the rates mentioned above. Sweet potato variety UmuSpo1 was the test crop. Analysis of the nutrient content of sweet potato was also carried out after harvest to test for the nitrogen, phosphorus and potassium contents of the harvested sweet potato root tubers. From the incubation study, biochar rates significantly (p<0.05) increased soil pH from 4.87 to 13.92 and as the rates were increasing, pH increased with time following these pattern 10t/ha>7.5t/ha > 5t/ha > 2.5t/ha> and 400kg/ha having these values 13.92>13.50>10.21>7.85>5.49>4.53 respectively. The application rate of 2.5t/ha of biochar reduced soil exchangeable acidity at 4weeks of incubation from 1.92cmol/kg to 0.62cmol/kg. At 12weeks of the incubation study, 10t/ha of biochar gave an increase in the level of soil exchangeable potassium from 0.13cmol/kg to 0.83cmol/kg. Results from the field study showed that 400kg/ha of NPK (15:15:15) fertilizer gave the highest value for vine length at 16WAP (150.4cm) over the control 117.2cm. Biochar at 5t/ha gave the highest number of leaves per plant (235.6) at 12WAP and this was significantly (p<0.05) different from control and 7.5t/ha of biochar gave the highest value of stem girth (0.45cm) at 16WAP. Results on yield of sweet potato root tubers showed that biochar at 5t/ha produced the highest saleable sweet potato root weight (7.84t/ha), 7.5t/ha of biochar produced the least non-saleable sweet potato root weight (0.21t/ha) and 5t/ha of biochar gave the highest total root weight (8.11t/ha). 7.5t/ha of biochar recorded the highest percentage saleable root number (87.20) and also the least non-saleable root number (13.70). What this implies is that, there would be no need applying beyond 5t/ha of biochar in the study area in order to obtain optimum yield. Also in terms of non-saleable root yield, 7.5t/ha is better as it is not economical for non-saleable root yield to be in abundance. For the nutrient contents of sweet potato root tubers, 2.5t/ha of biochar gave the highest nitrogen content (0.947%), while 10t/ha of biochar gave the highest phosphorus and potassium contents (0.532% and 0.984%) respectively. Conclusively, therefore biochar at the rates of 5t/ha, 7.5t/ha and 10t/ha is ideal for improving yield and nutrient contents of sweet potato production in the study area.