Distribution of potassium fractions in sweet potato (Ipomoea batatas) garden soils in the Central Highlands of Papua New Guinea and management implications

Previous studies indicated that potassium (K) deficiency is an important soil-related factor for yield decline of the sweet potato gardens in the Central Highlands of Papua New Guinea, where sweet potato is an important staple food crop. An effort was made to characterize various fractions of K in the diverse soils of this region under sweet potato, to ascertain the probable reasons behind the observed K deficiency and its relationship to decreasing yield trends. Soils from two depths (0-10 cm) and (10-20 cm) in two types of gardens (old and new gardens) were assessed for different fractions of soil potassium in volcanic and non-volcanic soil groups. Volcanic soils (Hydrandepts and Andaquepts) were significantly lower (P < 0.05) in exchangeable K than the non-volcanic soils (Dystropepts, Tropoqualfs and Eutropepts). Mean exchangeable K content of the non-volcanic soils was 95.5 mg/kg, whereas that of volcanic soils was 72.4 mg/kg. Similarly, new gardens had an average exchangeable K content of 94.1 mg/kg, which was significantly greater than 71.6 mg/kg soil of older gardens. Non-exchangeable K content differed significantly (P < 0.001) between the soil types; mean K content was 85.9 mg/kg for the volcanic soils, whereas in non-volcanic soils, it was 184.9 mg/kg. Garden types also differed significantly (P < 0.05) with respect to non-exchangeable K content; new gardens registering higher average values (by almost 20%) than the older gardens. Multiple regression analysis showed that variability in the tuber yield was as a result of variability of water soluble and exchangeable K (up to 22%), non-exchangeable K (2%), mineral K (4%) and leaf K concentrations (10%). Older gardens, which are in volcanic soil groupings, are more susceptible to the K depletion problem because of continuous sweet potato cultivation, possibly owing to their lower K reserves. Such gardens should be managed either with sufficient fallow periods for regeneration of soil fertility or with suitable application of mineral K fertilizers to enhance productivity.