An evaluation of nutritional constraints on sweet potato (Ipomoea batatas) production in the central highlands of Papua New Guinea

Sweet potato (Ipomoea batatas) is the staple food crop in the highlands of Papua New Guinea (PNG). Declining crop productivity, however, appears to be threatening the sustainability of sweet potato-based farming systems within the region, a probable cause being the exhaustion of soil nutrient reserves in continuously cultivated sweet potato gardens. To assess the extent of the problem, a survey of sweet potato gardens was conducted across four of the highlands provinces and information on soil and crop variables was obtained for old gardens (cultivated over many seasons) and new gardens (newly brought into cultivation) on soils of volcanic and non-volcanic origin. Crop leaf nutrient data collected in the survey were interpreted using the Diagnosis and Recommendation Integrated System (DRIS), to try to identify the main nutritional constraints on tuber production in different garden types on soils of volcanic or non-volcanic origin. The results suggested that K deficiency was the primary cause of poor crop production in almost a third of sweet potato gardens, but was more of a problem in old gardens than in new. Phosphorus deficiency was also a problem on volcanic soils, and S deficiency on non-volcanic soils. These latter deficiencies, however, were at least as prevalent in new gardens as in old. Important factors contributing to K and S depletion from garden systems were the removal of K and S-rich vines from cultivation areas, the shortening of fallow periods and the burning of weed and crop residues, the latter releasing S (SO(2)) to the atmosphere. Correction of K and S deficiencies may require the recycling of old vines back to sweet potato cultivation areas and the adoption of a zero-burn policy for fallow management. Correction of P deficiency may necessitate the use of P-accumulating fallow species, e.g. wild Mexican sunflower (Tithonia diversifolia), to extract the P fixed by sesquioxide and allophanic minerals.