ABSTRACT

Two field experiments were used to study the responses of sweet potato to application of nitrogen and potassium fertilizer in a humid tropical ultisol in the 1998 and 1999 cropping seasons. In each year, the experiment was laid out as a 4 x 4 factorial arranged in randomized complete block design. Treatment comprised four nitrogen levels of 0, 40, 80 and 120 kg N/ha and four potassium levels of 0, 50, 100 and 150 kg /ha. There were three replications. Nitrogen fertilizer significantly increased the sweet potato plant size parameters such as dry weights of leaf, stem and tuber fractions, and total dry matter, number of branches and fresh yield of tops. Marketable and total tuber yields were significantly increased with all cases of applied nitrogen compared with where nitrogen was not applied. On the average, tuber yield was increased with incremental application of nitrogen up to 80 kg N/ha in 1998 but not beyond the rate of 40 kg N/ha in 1999. On the average use of 40-80 kg N/ha proved satisfactory. Potassium fertilizer did not enhance leaf and stem dry matter and number of branches per plant but significantly increased marketable and total tuber yields. Marketable and total tuber yields were significantly increased with application of potassium compared with where potassium was not applied. On the average, potassium applied at the rate of 50-100 kg K/ha was considered satisfactory. For moderate fertilizer use, 40 kg N/ha and 50 kg K/ha gave reasonable high tuber yields, and would be recommended for resource poor farmers. For x optimum yields nitrogen at 80 kg N/ha combined with potassium at 50 to100 kg K/ha would be recommended. Higher rates of nitrogen did not result in further yield improvements. High input of fertilizer, especially potassium, significantly increased the proportion of marketable tubers in the overall tuber yield.