Application of Organic Fertilization and Chemical Fertilization Enhances the Coupled and Coordinated Degree of Soil Fertility and Economic Benefits in Corn Farmland

Combined application of organic (M) and chemical fertilizer (C) is a significant measure to enhance soil fertility and ensure food security. In 2023 and 2024, we established six treatments: T1 (no fertilization), T2 (100% C), T3 (75% C + 25% M), T4 (50% C + 50% M), T5 (25% C +75% M), and T6 (100% M), with three replicates for each treatment. The total amount of nitrogen applied to the soil for T2–T6 was the same, and the organic fertilizer was compost sourced from cow dung. The aims of this study were to explore the effects of organic fertilizer combined with chemical fertilizer on soil fertility, and apparent nutrient balance, to investigate its possible economic benefits. We also analyzed the influence of the combined application of organic and chemical fertilizers on the degree of coupling and coordination (D) between soil fertility and economic benefits. The total phosphorus, total potassium, available phosphorus, available potassium, and organic matter in the soil all showed an increasing trend with an increase in the proportion of organic fertilizer applied. T2 reduced the soil pH by 7.41–8.94% compared with T1, while applying organic fertilizers (T3–T6) increased the soil pH by 0.72–8.62% compared with T2. T4 is conducive to the balance of income and expenditure of nitrogen, phosphorus, and potassium elements. The corn yield, net income, and input–output ratio all showed an initial increase followed by a decrease with an increase in the proportion of organic fertilizer applied, and their values all reached the maximum under T4. Based on the CRITIC-TOPSIS method and the coupling coordination degree model, it was determined that the fertilization strategy with the highest comprehensive score and D under the conditions of this experiment was 50% C +50% M (T4), which not only improved soil fertility but also achieved the highest economic benefit. The research results were of great significance for promoting sustainable agricultural development.