This paper summarizes a series of field experiments that investigated the effects of organic and inorganic nutrients on sweet potato tuber yield in the humid lowlands of Papua New Guinea. In the first experiment, plots were planted with Piper aduncum, Gliricidia sepium and Imperata cylindrica, which were slashed after one year, whereafter sweet potato was planted. Sweet potato yield was lowest after Gliricidia fallow, but no yield differences were found after piper and imperata fallow. In the second season, there was no significant difference in sweet potato yields. The second experiment consisted of a factorial fertilizer trial with four levels of N (0, 50, 100, 150 kg ha–1) and two levels of K (0, 50 kg ha–1). Nitrogen fertilizers increased yield in the first season, but depressed tuber yields in the second and third seasons. Potassium fertilizer had no effect on marketable tuber yield. The third experiment consisted of a comparison between N from inorganic fertilizer and poultry litter at four rates (0, 50, 100, 150 kg ha–1). No difference was found between the inorganic fertilizer and poultry litter, and the highest yields were found at 100 kg N ha–1. In the second season no significant response was observed. Although yield variation was considerable, this series of experiments has shown that sweet potato yield can be significantly increased by inorganic or organic N applications. Sweet potato yields after fallows were less variable than after inorganic nutrient inputs. Inputs of inorganic fertilizer or poultry litter may strongly increase or decrease tuber yields.
Kinetics of potassium release in sweet potato cropped soils: a case study in the highlands of Papua New Guinea
