The mineralization processes of plant residues in the soil vary depending not only on many factors such as climate and soil properties but also significantly on the C/N ratios. Increasing the surface area of these materials can also affect the mineralization process. This study increased the surface area of sunflower and maize harvest residues by fine grinding and applied 0.5%, 1%, 2%, and 4% doses by weight to soil with high clay content. After the application, the ammonium and nitrate nitrogen contents of the soils brought to the field capacity water content were measured on the 30th, 60th, 90th, and 180th days during the 180-day incubation period. The NH4 and NO3 nitrogen amounts of the starting soil were 12.76 and 13.99 mg kg−1, respectively. The highest NH4 and NO3 nitrogen were observed as 28.29 mg kg−1 and 96.53 mg kg−1 in the M4 application on days 90, respectively. The lowest nitrogen contents were determined as 3.74 mg kg−1 in the M2 application on the 180th day and 8.65 mg kg−1 in the control application on the 90th day. Applications have remarkably increased the mineral nitrogen content of the soil and hence soil MNSR. As a result, it has been determined that establishing more controlled processes in stubble management, converting the nitrogen from stubble materials into a useful form for the plant, and reducing chemical fertilizer costs can contribute to the improvement of agricultural sustainability.