Rainfed rice fields are suboptimal wetlands that can be used as land for rice cultivation but have several problems, one of which is the availability of nutrients and water. The System of Rice Intensification (SRI) method emphasizes soil and water management. Nitrogen is needed by rice plants in large quantities, but its availability in paddy fields is easily lost due to several factors. Fertilization with NPK combined with rice straw compost is expected to increase the N element in the soil. This study aims to determine the interaction effect of NPK fertilizer and rice straw compost on the dynamics of ammonium (NH₄⁺) and nitrate (NO₃^-) in rainfed lowland rice at several rice age stages. This research was conducted for seven months (May 2022 - November 2022), taking place in a rain-fed rice field in Jalan Gunung Kupang, Cempaka District, Banjarbaru City, South Kalimantan. The method in this study was prepared using a 2-factor randomized block design (RBD). The first factor is the dose of NPK fertilizer: a0 = without NPK fertilizer (0 kg ha^-1); a1 = ½ NPK recommendation (150 kg ha^-1); a2 = recommended NPK (300 kg ha^-1), and the second factor is the dose of rice straw compost, k1 = 5 t ha^-1, k2 = 7.5 t ha^-1, k3 = 10 t ha^-1, k4 = 12.5 t ha^-1, k5 = 15 t ha^-1, k6 = 20 t ha^-1. As many as 18 treatment combinations were produced in two experimental blocks so that 36 experimental units were obtained. The results of the study showed that the a2k6 treatment combination was the best in increasing ammonium and nitrate in the vegetative, generative, and maturation stages.