Field Crop 2025, Vol.8, No.5, 231-237 http://cropscipublisher.com/index.php/fc 233 (Adekiya, 2022). If the seeds are inoculated with rhizobia and supplemented with phosphorus at the same time, the yield can be further improved: some experiments have increased the yield from 1 097 kg/hectare to 1 674 kg/hectare (Kyei-Boahen et al., 2017). Using water-soluble fertilizers with drip irrigation and supplementing NPK on demand can also increase the yield of grains and stems. 4.2 Nutritional content and flavor indices Precise fertilization can also make beans more nutritious. Reasonable combination of NPK and some organic materials can increase the protein, crude protein and trace elements such as iron, manganese and zinc in the grain (Goswami et al., 2022; Iqbal et al., 2024). The test showed that the grain protein can reach 223-252 g/kg, and it is higher after adding formula fertilizer and inoculation. With a good nutrient balance, there will be fewer anti-nutritional factors, and the flavor will naturally be better. However, it should be noted that when the yield is particularly high, the protein content may be "diluted", resulting in a phenomenon of high yield but slightly lower protein. 4.3 Commercial traits and market acceptability Mix fertilizer according to soil test results to make the pods larger and more uniform, and the market is more willing to buy them. When nitrogen, phosphorus and potassium are used together with organic fertilizers, the pod yield can reach up to 71.6 kg/hectare, and the quality is also better (Gaden et al., 2023). The healthier the soil, the more uniform the product specifications, and it is easier to sell at a good price. In short, applying fertilizers according to the actual needs of the soil and crops can not only stabilize yields and improve quality, but also make more money and reduce waste (Iseki et al., 2023). 5 Soil Nutrient Dynamics and Ecological Feedback 5.1 Trends of available nutrients in soil Fertilizer allocation based on soil test results can keep nitrogen, phosphorus, and potassium at appropriate levels, with neither deficiency nor excess. Research models show that as long as precise fertilization is followed, combined with practices such as mulching or returning straw to the field, the nutrients in the soil will be more stable and more useful in the long run (Turner and Kodali, 2020). If leguminous crops were planted the previous year, more nitrogen can be retained to help save fertilizer for the next crop (Koyama et al., 2021). 5.2 Changes in soil enzyme activity and microbial community The amount of fertilizer applied is based on soil conditions, and the enzymes and microbial communities in the soil will also change accordingly. For example, after phosphorus supplementation and lime addition, alkaline phosphatase activity will increase; when no fertilizer is applied, acid phosphatase will become more active (Ndabankulu et al., 2022). In the long run, microorganisms will tend to be those that "eat fast and grow fast"; changes in enzyme activity also indicate that nutrients in the soil are more useful - when there is more nitrogen, the activity of nitrogen-producing enzymes in the soil will be "suppressed" (Nieland et al., 2024). These changes directly determine whether nutrients can be quickly used by plants, and are also related to the overall health of the soil. 5.3 Residual effects and long-term soil fertility The benefits of fertilizer matching according to soil testing are not only effective in the current season, but also leave nutrient "deposits". The positive feedback brought by legume rotation makes the soil structure better, the nutrient cycle smoother, and the subsequent crops save fertilizer (De Long et al., 2023). After years of persistence, the organic matter in the soil has increased, the microbial diversity has stabilized, and the ecosystem has become more resistant to shocks. In this way, cowpea cultivation can maintain high yields and environmental protection for a long time. 6 Fertilizer Use Efficiency and Environmental Friendliness 6.1. Nutrient use efficiency and leaching control Applying fertilizer according to the actual soil conditions can significantly improve the efficiency of nutrient use. The soil testing and formula method allows crops to "eat" according to their needs, neither too much nor too little.
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