Impact of Substituting Organic Fertilizer for Chemical Fertilizer on Soil Fertility and Yield of Four Crops

The partial replacement of chemical fertilizers with organic fertilizers is regarded as one of the effective ways to achieve sustainable agricultural development. However, further research is needed to understand how different cropping systems respond to the substitution of organic fertilizers and the underlying mechanisms in the context of calcareous purple soils in the same region. In this study, four field trials involving soybean, wheat, rice, and corn were conducted at different sites within the same regional calcareous purple-soil area to investigate the comprehensive effects of partially replacing chemical fertilizers with organic fertilizers on soil fertility. The initial soil pH at the experimental site ranged from 6.25 to 8.60. The experiment established four treatments, which were CK (sole conventional chemical fertilizer), alongside OC1, OC2, and OC3, in which 10%, 20% and 30% of chemical fertilizer were substituted by a commercial organic fertilizer (45% organic matter, C/N = 25). The results indicate that replacing chemical fertilizers with organic amendments generally led to significant improvements in soil organic matter, available phosphorus, and quick-acting potassium levels. However, the impact on alkaline-hydrolyzable nitrogen varied depending on the crop type. In terms of crop responses, organic fertilizer substitution had a notable effect on nutrient accumulation in grains and overall yield. Soybeans and rice showed positive trends in both nutrient uptake and yield enhancement, while wheat primarily benefited from increased total nitrogen content in its grains. In contrast, corn achieved the best balance between yield and nutrient absorption under the OC2 treatment. An equal-weight membership function evaluation, based on within-system normalization of six soil and crop indicators, ranked the treatments as OC2 > OC3 > OC1 > CK. OC2 therefore showed the highest integrated performance among the four crop systems examined in this study. Under OC2, soil organic matter increased by 1.21–31.24% relative to CK across the four field systems, while significant yield increases of 4.2% and 6.5% were observed in the rice and corn systems, respectively. This study suggests that substituting 20% of chemical fertilizers with organic amendments can effectively enhance soil fertility and boost crop productivity, making it a promising regional fertilization strategy that balances ecological benefits with agricultural output.