Quadratic equation, maximum and optimum dose of N, maximum yield in research on response to N fertilization in lowland and irrigated rice fields

Without N fertilization, rice yields in lowland rice were not optimal, ranging from 1.85 to 5.41 t ha-1. The relationship between N doses and rice yields was linear quadratic, with an R2 value ranging from 0.61 to 0.97. The highest doses of N fertilizer for rice in lowlands ranged from 110 to 303 kg N ha-1, resulting in maximum yields between 7.28 and 7.93 t ha-1. The optimum N dose for rainfed rice fields ranged from 90 to 140 kg N ha-1, corresponding to the dose of urea fertilization based on productivity per sub-district. The dose of urea fertilizer on rice fields with productivity levels of ˂ 5, 5 to 6, and ˃ 6 t ha-1 was 200, 250, and 300 kg Urea ha-1, respectively.Rice plants in technically irrigated fields were very responsive to N fertilizer, and average yields in unfertilized fields ranged from 1.90 to 5.65 t ha-1. The lowest rice yields occurred in unfertilised fields in Kediri Sub District, East Java Province, where the hybrid variety H6444 was cultivated. Meanwhile, the highest result of unfertilized rice was produced in Cianjur, West Java Province.