Roles and optimisation rate of potassium fertiliser for immature oil palm (Elaeis guineensis Jacq.) on an Ultisol soil in Indonesia

Potassium (K) is an essential macronutrient needed in large amounts by oil palm as it is directly involved in physiological processes. This research focused on the influence of K fertiliser on the vegetative growth of oil palm and determined the optimum rate of K fertiliser for immature plants (aged 1 to 3 years). The study was conducted at IPB-Cargill Oil Palm Teaching Farm, Jonggol, Bogor, Indonesia, from March 2013 to March 2016. The application rates were 0, 196, 392, 588, and 784 g K_2O plant^(−1) year^(−1) during the first year; 0, 384, 768, 1152, and 1536 g K_2O plant^(−1) year^(−1) during the second year; and 0, 450, 900, 1350, and 1800 g K_2O plant^(−1) year^(−1) during the third year of the immature oil palms. This experiment used a randomized complete block design with three replications. The optimum fertiliser rate was calculated by differentiating the regression equation of the quadratic response curve for variable growth. Potassium fertiliser significantly affected plant morphology, and increased plant height, stem girth, frond number, frond length, and leaf area of frond 17. Potassium application also significantly increased chlorophyll content, stomatal density, and K nutrient content of the leaves of immature oil palm. The optimum K fertilizer rate for 1-, 2-, and 3-year-old immature oil palm was 512, 966, and 1430 g K_2O plant^(−1) year^(−1), respectively. Application of K fertiliser provided the amount of K needed to support oil palm growth