Integrated Nutrient Management Research on Sweet Potato at Hobu, Morobe Province

In order to investigate the effect of organic and inorganic nutrient sources on sweet potato tuber yield, we carried out a series of experiments at Hobu, Morobe Province, PNG. In the first experiment, plots were planted with Piper aduncum , Gliricidia sepium and Imperata cylindrica . After one year, these plants were slashed and sweet potato was planted. Sweet potato yield was lowest in plots with previous gliricidia, but there were no differences in yield between previous piper and imperata. In the second season, there was no significant difference in sweet potato yields between the plots. The second experiment consisted of a factorial fertiliser trial with four levels of nitrogen (0, 50, 100 or 150 kilograms per hectare) and two levels of potassium (0 or 50 kilograms per hectare). Nitrogen fertilisers increased tuber yield in the first season, but depressed tuber yields in the second and third seasons. Nitrogen fertiliser significantly increased vine yields in all three seasons. Potassium fertiliser had no effect on marketable tuber yield, but increased nonmarketable tuber yields. The third experiment compared nitrogen provided by inorganic fertiliser or by poultry litter at four rates (0, 50, 100 or 150 kilograms per hectare). No difference was found between the inorganic fertiliser and poultry litter, and highest yields were found at 100 kilograms of nitrogen per hectare. In the second season, no significant response to nitrogen was observed. This research indicates that sweet potato yield can be significantly increased by either inorganic or organic nitrogen applications, although yield variation is considerable. Sweet potato yields after fallow were moderate but less variable than yields following inorganic nutrient inputs. Fallowing seems the safest way to obtain steady sweet potato yields; with extra inputs through inorganic fertiliser or poultry litter, tuber yields may be strongly increased or decreased. U NTIL the 1980s, it was widely perceived that inorganic fertilisers were a viable means of increasing land productivity in the low fertility soils of the humid tropics. This line of thought was adopted by, among others, the Food and Agriculture Organization (FAO) Freedom from Hunger Campaign and its Fertiliser Program, which began in the 1960s. Organic fertilisers (e.g. compost or farmyard manure) were regarded as important, but it was obvious that they were not available in sufficient quantity to drastically increase food production. In the early 1980s, various reports showed that the use of inorganic fertilisers in the tropics had stagnated, and this was explained by poor marketing and inadequate profitability. From that time on, integrated nutrient management has been advocated. Essentially, this involves the combination of both inorganic and organic fertilisers to increase crop production (Janssen 1993). In this paper, we present the results of integrated nutrient management research with sweet potato ( Ipomoea batatas (L.) Lam.) in the humid lowlands of Morobe Province, PNG. Despite the fact that sweet * International Soil Reference and Information Centre, PO Box 353, 6700 AJ, Wageningen, The Netherlands. Email: [email protected] † Department of Agriculture, PNG University of Technology, PMB, Lae, Morobe Province, PNG. ‡ Department of Agriculture, The University of Queensland, Brisbane, Queensland 4072, Australia. Proceedings.book Page 698 Monday, September 17, 2001 11:30 AM