Sweet potato yields and nutrient dynamics after short-term fallows in the humid lowlands of Papua New Guinea

  • A.E. Hartemink
Keywords: improved fallow, natural fallow, <i>Piper aduncum</i>, <i>Gliricidia sepium</i>, <i>Imperara cylindrica</I>, crop yield, nutrient budgets


Shifting cultivation is common in the humid lowlands of Papua New Guinea but little is known about the effect of different fallows on sweet potato (Ipomoea batatas) yield and nutrient flows and pools in these systems. An experiment was conducted in which two woody fallow species (Piper aduncum and Gliricidia sepium) and a non-woody fallow species (Imperata cylindrica) were planted and slashed after one year. Sweet potato was grown for two consecutive seasons (1 year) after which the fallows and yields were compared with yields from continuously cropped plots. The experiment was conducted on a high base status soil (Typic Eutropepts). In the first season, marketable sweet potato yield after piper and imperata was about 11 t ha-1 but yields after gliricidia and under continuous cropping were significantly lower. Vine yield was similar for the continuously cropped plots and for the sweet potato after piper and gliricidia, but significantly lower than after imperata. The effects of the fallows on sweet potato yield lasted only one season. In the second season after the fallow, sweet potato yields were higher, which was contributed to lower rainfall. Nutrient budgets showed that the three fallow species (piper, gliricidia and imperata) added insufficient amounts of nitrogen, phosphorus and potassium for the removal of these nutrients by two consecutive seasons of sweet potato. From a yield point of view there seems no benefit in having a nitrogen-fixing fallow species like Gliricidia sepium in sweet potato based systems on high base status soils.