The aim of this study was to determine the efficiency of a Leonardite-based slow-release organic-mineral fertilizer for corn nutrition in comparison with common fertilizers in two experiments, with a completely randomized block design. The first experiment included corn fertilizer requirement plus (B=0, T1=300, T2=500, and T3=800 kg/ha) of slow-release fertilizer. The second experiment included 500 kg/ha of slow-release fertilizer plus (B+=0, R1=50, R2=75, and R3=100) percent of corn NPK requirement, in 4 replications and a total of 32 plots in the field. The results showed that the efficiency of nitrogen, phosphorus, and potassium in the second experiment was higher than their efficiency in the first experiment. The higher efficiency of nitrogen, phosphorus, and potassium when using slow-release fertilizer, led to increase the end- season yield. The use of slow-release fertilizer, in addition to increasing the efficiency of NPK and yield, also caused a significant decrease in pH, which probably helped improve the availability of trace nutrients. The efficiency of nitrogen, phosphorus, and potassium in the mid season of both experiments, when the plant is not fully established, was very low, but in the end season, the efficiency of nitrogen, phosphorus, and potassium increased sharply. Overall, the highest efficiency of nitrogen (N=66.68%) was obtained in the R3, phosphorus (P=85.19%) in the B+, and potassium in the R3 (K=8.87%) treatments of the second experiment. As a general conclusion, it can be stated that the tested slow-release fertilizer can be used as a complete or auxiliary fertilizer in corn nutrition with appropriate yield.