The quality and effective fertilizers support site-specific nutrient management of paddy fields, which can increase yields and efficiency of fertilizer. Fertilizer formulas should be based on soil nutrient status and crop requirements. This study aims to examine the reformulation of compound NPK fertilizers for lowland rice. The study was conducted in Cibungbulang, Bogor Regency from October 2020 - March 2021. The experimental design was carried out using a randomized complete block design with 10 treatments, and 3 replications. The treatments consisted of five levels of NPK 15-10-12 fertilizer doses, plus control treatment, NPK 15-15-15 and single NPK as standard, and additional treatment with the addition of straw compost. The plots were made measuring 5 m x 5 m. The results showed that statistically, the application of NPK 15-10-12 fertilizer gave the same effect on plant height, the number of tillers, weight of dry grain harvested, the weight of dry milled grain, and weight of dry straw compared to single NPK fertilizer and NPK 15-15-15. The optimum dose of NPK 15-10-12 fertilizer for lowland rice is 220 kg ha-1 combined with Urea at a dose of 225 kg ha-1. At the same dose (300 kg ha-1) the efficiency of NPK 15-10-12 fertilizer (9.70 kg grain kg-1 fertilizer) was higher than NPK 15-15-15 fertilizer (8.47 kg grain kg-1 fertilizer). Higher efficiency is indicated by lower fertilization doses. The RAE value of NPK 15-10-12 (142%) was higher than that of NPK 15-15-15 at the same dose and single NPK. This research implies that the formula for compound NPK 15-10-12 fertilizer can be used as a substitute for compound NPK 15-15-15 fertilizer.

Potassium is one of the primary macro nutrients along with N and P nutrients, which plants need in large quantities. Lack of K nutrients, one of which can inhibit photosynthetic transport to plant seeds. The availability of K nutrients in paddy soil varies greatly depending on the chemical properties of other soils. This paper aims to evaluate the nutrient K characteristics of paddy fields, the K contribution from irrigation water and harvested straw, and the response of fertilization to rice yields. The availability of K nutrients, apart from being influenced by clay mineral composition, clay content, organic C and soil CEC, is also influenced by N fertilization, soil pH, exchangeable of Ca and Mg, and N-total. The content of potential K and exchangeable of K increases in paddy fields which are further east of Java Island because they have soil with parent material that is rich in potassium minerals. The content of exchangeable K is very closely related to the potential K with R2 = 0.53 (n = 73). Based on the map of the nutrient status of K rice fields with a scale of 1:250,000 in 23 provinces in Indonesia, it is known that most of the paddy fields have moderate K status at 37% and high 48%. The K content in paddy soil can be contributed by irrigation water as much as 7-74 kg K/ha/season. If all of the remaining rice straw from crop yields is returned to the field, the K nutrient requirements for rice plants in the medium and high K nutrient status fields have been fulfilled. The response to K fertilization in rice plants was only visible when the exchangeable K content < 0.10 cmol(+)/ kg. The policy implication of this paper is that K fertilization is focused on paddy fields with low K content, the effectiveness of K fertilization can be increased by returning rice straw from crop residues and proper N fertilization.