Field Crop 2025, Vol.8, No.6, 258-264 http://cropscipublisher.com/index.php/fc 259 2 Nutrient Demands of Soybean Growth 2.1 Stage-specific nutrient requirements (N, P, K) During the entire growth process, soybeans require a large amount of nitrogen, phosphorus and potassium, and the demand for these elements is different at different growth stages. Generally speaking, modern soybean varieties absorb about 275 kg of nitrogen, 21 kg of phosphorus and 172 kg of potassium per hectare to achieve a high yield. Among them, the utilization rate of nitrogen and phosphorus is relatively high, indicating that they are particularly important for grain formation (Bender et al., 2015). Potassium and iron are mainly absorbed in the later stage, while nitrogen and phosphorus are slowly absorbed from the early stage and are used until the filling stage. In order for soybeans to grow well and have high yields, these nutrients need to be supplied in place at the critical stage. 2.2 Complementarity between rhizobial nitrogen fixation and applied nitrogen Although soybeans can cooperate with rhizobia to obtain some nitrogen through biological nitrogen fixation, this is usually not enough. In order to have high yields and high protein content, some additional nitrogen fertilizers must be applied. If nitrogen fertilizer is used together with inoculants such as bradyrhizobium, soybeans can grow faster, photosynthesize more efficiently, and produce higher yields. Combining organic fertilizers, chemical fertilizers, and inoculants is now a recommended nutrient management method, which allows nutrients to be better absorbed by plants and helps crop health and yield. 2.3 Long-term chemical fertilizer use and soil health impacts If only chemical fertilizers are used for a long time, the nutrients in the soil will become unbalanced, the microbial activity will deteriorate, and it will easily cause nutrient loss and environmental pollution, which will eventually affect the overall health of the soil (Singh et al., 2024). Therefore, many people now advocate the use of organic fertilizers, such as compost or manure, to replace part of the chemical fertilizers. These organic fertilizers can improve soil structure, allow more microorganisms in the soil, and promote nutrient circulation. This practice not only maintains soybean yields, but also benefits the long-term fertility and ecological environment of the soil. 3 Fertilization Strategies with Partial Organic Substitution 3.1 Setting replacement ratios The common practice now is to replace part of chemical fertilizer with 25%, 50% or 75% organic fertilizer. Studies have found that when the replacement ratio is around 45% to 50%, the soybean yield and soil quality are better, and it can also reduce the emission of some harmful gases such as nitrous oxide (Xu et al., 2024). If the replacement ratio is lower, such as 15% to 30%, it is also good for the environment, but the long-term improvement effect on the soil may not be obvious. If the ratio is too high, such as more than 75%, it may not only fail to increase the yield, but it may also easily cause problems such as uneven nutrients and increased costs (Hou et al., 2022). 3.2 Types of organic fertilizers and nutrient profiles There are many types of organic fertilizers that can replace chemical fertilizers, such as pig manure, cow manure, compost, straw, and urban sludge compost. They contain different nutrients and have different effects on the soil (Tang et al., 2021; Yang et al., 2024a). Manure, for example, has a more appropriate carbon-nitrogen ratio and can provide a variety of nutrients; while compost and straw are more conducive to increasing organic carbon and microbial species in the soil. The choice of organic fertilizer will affect the enzyme activity and microbial composition in the soil, which will affect the growth of crops and the sustainable planting capacity of the field. 3.3 Optimization of timing and methods of application When and how to use organic fertilizer is also critical. It is best to apply it when soybeans are just planted or are still in the growth period, so that they are more easily absorbed (Zhai et al., 2022). In addition, organic fertilizer should be turned into the soil instead of being spread on the surface, which can reduce nutrient loss and make it more usable by crops. Usually, we should pay more attention to the nutrient status of the soil and adjust the amount of fertilizer according to the situation. Only by persisting in the long term and managing according to local conditions can soybeans grow well without damaging the land.
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