Effect of Five Levels and Three Sources of N on Sweetpotato Yields on an Ultisol

Abstract

The effect of N levels and sources on sweetpotatoes (Ipomoea batatas) was investigated on an Ultisol in the central, hilly, interior region of Puerto Rico at about 450 m above mean sea level. Treatment differentials were: Broadcast, preplant applications of 0, 10, 20, 40 and 50 kg/ha of N from urea and 40 kg/ha of N from two sulfur-coated urea compounds (slow-releasing N sources) containing 39.6 and 37% N in each case. The soil had a pH of 4.7 in the top 25 cm of the profile but it apparently supplied enough Ca for a sweetpotato crop. There was no evidence of Al injury to the crop even though exchangeable Al levels were high, i.e., more than 50% of the sum of cations in the 25-50 cm layer. Maximum marketable yields were 14.6 tons/ha (130 cwt/acre). These were obtained with the application of 40 kg/ha (35.6 lb/acre) of N as ordinary urea. Evidently there was no advantage in using slow-releasing N sources. In fact, a slight yield depression was observed. The Capó fertilizer-yield equation was applied to the mean yield data obtained from the urea treatments, and a coefficient of determination of 0.99 was obtained. This equation appeared to be useful in predicting sweet potato yields in terms of the N fertilizer applied in this experiment.