Abstract

East and Central Java are among the major cassava producers in Indonesia. Assuming that potassium (K) availability is a limiting factor for cassava cropping under the given conditions, the effects of K fertilizer at six seasonal doses (0, 30, 60, 90, 120, and 180 kg K₂O ha^-1) applied twice (one and three months after planting), and one treatment attributed to farmers’ practice, were examined at four locations: Malang, Tulungagung, Wonogiri, and Karanganyar districts. The soils of the different regions vary from neutral (pH 6.2-6.8) silt loam to acid (pH 4.6-5.1) clay, and from high to very low exchangeable K (exch-K) contents. All K Research Findings fertilizer treatments were combined with nitrogen (N) - 135 kg N ha^-1, and phosphorus (P) fertilizers - 36 kg P₂O₅ ha^-1, except one treatment with 200 kg N ha-1, and 60 kg P₂O₅ ha^-1. Urea (46% N), SP36 (36 kg P₂O₅), and KCl (60% K₂O) were used as the source of N, P, and K fertilizer, respectively. Potassium doses hardly affected soil properties at harvest. Crop response to K dose was small to negligible at three sites, and significant only at Tulungagung, where tuber yield increased from 19 to 35 Mg ha^-1. The highest yields, 40-50 Mg ha^-1, were obtained at Malang on a rather fertile soil, but this is still below the recognized cassava yield potential. Nevertheless, some evidence indicates that there is considerable potential for K fertilization and other means to improve cassava production in these regions. The major problem of K nutrition, common to all four regions at varying significance, seemed to be the rapid depletion of the soluble K pool, including the applied fertilizer, from the rhizosphere before reaching the uptake zone of the roots. The tropical precipitation regime that promotes soil weathering and nutrient leaching must be taken into account. Measures such as division of the seasonal K dose into many frequent applications and supplementation of composted organic matter in order to enhance soil fertility and cassava crop performance are discussed.