ABSTRACT

Two field experiments were used to study the effects of nitrogen fertilization on light interception, weed biomass, growth and productivity of two white and two orange-fleshed sweet potato varieties in 2005 and 2006 cropping seasons in a tropical ultisol of southeastern Nigeria. In each year, the experiment was laid out as a 4 x 4 factorial arranged in randomized complete block design with three replications. Treatments comprised all possible combinations of four nitrogen levels of 0, 40. 80, 120 kg N/ha and our varieties of sweet potato - white-fleshed TIS 87/0087 and TIS 8164, as well as orange-fleshed Ex-Igbariam and CIP Tanzania. Nitrogen fertilizer significantly (p

<0.05 incrcicd w'et potato leaf area index, crop growth rate, shoot dry matter, light interception and storage root dry matter but decreased net assimilation rate and weed dry matter. On the average, storage root yield was increased with incremental application of nitrogen up to 80 kg N/ha when initial soil nitrogen was 0.056%N in 2005, but not beyond the rate of40 kg N/ha when initial soil nitrogen was 0.104%N in 2006. The use of 40 or 80 kg N/ha proved satisfactory, depending on the soil nitrogen status. Application of nitrogen above 80 kg N/ha did not result in further yield improvement but rather reduced storage root yield significantly. The white-fleshed TIS 87/0087 followed by orange- fleshed Ex-Ighariam piodiaced other varieties and intercepted over 70% of the incident radiation. On the average, storage root yield of TIS 87/0087 was higher than the yields of Ex-Igbariam, TIS Xl 64 un; I by 22, 55 and 160%, respectively. White-fleshed TIS 8164 showed low response to nitrogen, with yield not increased beyond the 40 kg N/ha rate. The high nitrogen-response varieties, white-fleshed TIS 87/0087 and orange-fleshed Ex-Igbariam gave higher shoot dry matter and storage root yields than the low nitrogen-response variety. >