Estimating The Optimum Dose of Fertilizer to Enhance Soybean Productivity Under Kayu Putih (Melaleuca cajuputi) Stands

Optimum doses of nitrogen, phosphorus and potassium obtained through fertilizing are necessary to increase crop productivity. An excess of or lack of nitrogen, phosphorus and potassium obtained through fertilizing will cause plants responsiveness to decrease. This research estimated the response surface, i.e. a step to determine the optimum doses of urea, SP-36, and KCl fertilizers. It employed a split-split plot design. The main plot was intended for the urea dose while the subplot and sub-subplotwere intended for SP-36 and KCl, respectively. Theadministration of urea was divided into two categories, i.e. 0 kg. ha-1, 25 kg. ha-1 and 50 kg. ha-1, while the administration of SP-36 and KCl was divided into three categories, i.e. 0 kg. ha-1, 150 kg. ha-1, 300 kg. ha-1; and 0 kg. ha-1, 75 kg. ha-1, and 150 kg. ha-1, respectively. Based on the response surface, it is revealed that the optimum combined dose for ureais equal to 26.64 kg. ha-1 while for SP-36 and KCl, the optimum combined doses are 292.61 kg. ha-1 and 97.71 kg. ha-1, respectively. The combination of these three substances can produce a maximum weight of soybean by 2.07 ton. ha-1.Keywords: Nitrogen, Phosphorus, Potassium, Soybean Productivity, Kayu Putih Stand