Field Crop 2025, Vol.8, No.4, 176-186 http://cropscipublisher.com/index.php/fc 177 balanced application of major and micronutrients, as well as the combined use of organic conditioners. The aim is all to achieve higher yields, more efficient resource utilization, and less environmental impact. It is worth mentioning that the article also emphasizes the importance of "adapting measures to local conditions". Different regions and different varieties should adopt different methods. At the same time, we also explored possible future directions, including the development of precision agriculture and the breeding potential of stress-resistant varieties. 2 Nutrient Requirements of Soybean 2.1 Macronutrient demands: nitrogen fixation, phosphorus uptake, and potassium balance Soybeans, as a crop, have a relatively high demand for nitrogen, phosphorus and potassium, all of which are crucial for their healthy growth, good fruiting and high yield. However, unlike other crops, soybeans' dependence on nitrogen is somewhat "special"-mainly not through application, but through their own "cliques" to cooperate with rhizobia to fix nitrogen from the air. This symbiotic interaction can sometimes bring 50 to 300 kilograms of nitrogen per hectare to the soil, which is why it is particularly important in the protein synthesis and vegetative growth stages (Dass et al., 2022). But then again, nitrogen alone is not enough. If phosphorus supply is insufficient, there will be many problems-root development will be affected and seed formation may also go wrong. Some research indicates that up to 81% of the phosphorus at harvest is actually concentrated in the grains (Bagale, 2021), which suggests that it plays a significant role in the formation of grains. What about potassium? Although often overlooked, it is actually crucial for enzyme activity, water regulation, and pod fullness, especially when the plant enters the later stage of growth and just begins to form pods, its absorption is the most active (De Almeida et al., 2017). Even the absence of any one of these three elements will affect the development of the plant. Especially nitrogen and potassium, once the supply is insufficient, not only will the plants grow slowly, but the final yield will also shrink significantly. 2.2 Role of micronutrients: boron, zinc, and molybdenum in reproductive development Although trace elements like boron, zinc and molybdenum may not sound so "hardcore", their role in soybeans is by no means significant. Take boron for example. The formation of cell walls cannot do without it, and its absence during the reproductive period directly affects the podding rate and final yield (Tarar et al., 2022). Zinc is often sprayed on the leaf surface during the flowering and podding period. Experiments have shown that doing so can indeed significantly increase the number of pods and seeds. Molybdenum, on the other hand, is somewhat like a "behind-the-scenes hero". It participates in the fixation of nitrogen and the synthesis of proteins. Although the amount used is not large, without it, nitrogen cannot be utilized at all. During critical growth periods, such as from flowering to the fruiting stage, foliar spraying of these micronutrients not only helps increase yield but also enhances the plant's stress resistance and nutrient utilization efficiency (Figure 1) (Dimkpa et al., 2017). So, although these elements may seem insignificant at first glance, if they were to be lacking, the problem would be much greater. 2.3 Stage-specific nutrient uptake and its implications for timing of application Soybeans are not the kind that can be eaten from beginning to end. Their nutrient absorption is rhythmic and can be viewed in stages. For instance, potassium and iron are mostly absorbed in the later stage of vegetative growth. However, substances like nitrogen, phosphorus, calcium, magnesium, sulfur, zinc, manganese, boron and copper have a more uniform absorption time and are quite crucial both during the vegetative growth period and the filling period (Bender et al., 2015). However, it should be noted that the absorption of many trace elements before flowering is actually not high, even less than 17% of the total amount for the entire season (Gaspar et al., 2018). This indicates that the reproductive growth stage is the "key supply period". Therefore, foliar spraying at stages like R3 (pod formation stage) and R5 (grain swelling stage) has been proven to be a relatively effective strategy. When it comes to fertilizing, "timing the right time" is more important than "applying a large amount". Only by rationally arranging the application timing and ensuring that plants can absorb sufficient nutrients when needed can both yield and resource utilization efficiency be achieved.
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