A 15-year study in China found that using a combination of NPK fertilizer and farmyard manure led to higher wheat and corn yields compared to unfertilized or single fertilizer treatments. The soil pH declined in most treatments, but the NPKM treatment helped maintain soil fertility and buffering capacity. This approach also increased soil organic C, available P, and essential nutrients like Ca2+, Mg2+, Cu2+, and Zn2+. Overall, combining NPK fertilizer with manure is crucial for sustaining crop yields and soil health in red soil regions.