Field Crop 2025, Vol.8, No.4, 176-186 http://cropscipublisher.com/index.php/fc 179 controlled-release fertilizer or foliar spraying, both can reduce nutrient loss and greenhouse gas emissions as a whole, which is a good direction for climate-friendly agriculture (Hasukawa et al., 2021). 4 Irrigation Management in Soybean Cultivation 4.1 Critical growth stages for irrigation: flowering and pod filling phases It's not about keeping a close eye on the water throughout the entire growing season, but rather seizing several key opportunities (Li et al., 2024). After soybeans enter the reproductive period, especially from the flowering stage (R1-R2) to the fruiting stage (R3-R6), they become particularly sensitive to water. Once water is scarce at this time, not only will the pods be produced less, but the seeds will also be lighter, and the yield will naturally decline. If drought occurs during flowering or podding, the yield loss can even reach 73% to 82%, and the weight per 100 grains during the fruiting stage will also decrease significantly. But it doesn't mean that the entire process must be filled with water. Studies have pointed out that even replenishing water only during these "critical window periods" can maximize yield and water use efficiency (Kanade et al., 2019; Heatherly, 2022; Tang et al., 2024). Sometimes, even if irrigation is delayed until the pods are just beginning to form or are about to fill up, as long as there is no water shortage during that "sensitive period", the effect is not much different from that of full-season irrigation. 4.2 Irrigation scheduling: use of evapotranspiration models and soil moisture sensors Ultimately, how to use water and when to use it really can't be done without watching. Effective irrigation plans often rely on models and tools to "assist in decision-making". Climate-based evapotranspiration (ET) models, soil moisture sensors (such as substrate potential sensors), and some physiological signals of plants themselves, such as canopy temperature or crop water stress index, are all available means. Many studies have shown that methods that trigger irrigation based on soil moisture thresholds are more stable in increasing yields and improving the physiological state of crops than those that merely rely on weather conditions or plant characteristics (Franca et al., 2024; Xie et al., 2024). Especially when the drip irrigation system is used in conjunction with precise planning, the dual improvement of water use efficiency and yield becomes more significant (Morbidini et al., 2023). In addition, some models, such as InfoCrop-Soybean or AquaCrop, can also simulate the best irrigation timing very well after being tuned with local data. When dealing with years with variable climates, such tools can also be regarded as good assistants for "stabilizing water supply and production" (Jha et al., 2018; Morales-Santos et al., 2023). 4.3 Impact of water stress and waterlogging on yield and physiological functions Too little water is not good, and too much water can also cause problems. Especially once soybeans enter the reproductive period, the lack of sufficient water directly affects their height, leaf area and total aboveground biomass, and ultimately leads to a loss in seed yield (Wei et al., 2018). Among them, the flowering and podding stages are the most vulnerable to "injury". Drought not only inhibits the external growth of plants, but also causes a simultaneous decline in internal photosynthesis, stomatal conductance and chlorophyll levels. It is also prone to oxidative stress and cell membrane damage (Dong et al., 2019). Although some functions can be restored after replenishing water, the extent to which they can be restored depends on the severity and duration of the drought at that time (Cui et al., 2020). On the other hand, although the impact of waterlogging is not as frequently mentioned as that of drought, it can still damage yields and physiological processes. If such extreme situations happen to occur at a critical stage, they can still completely disrupt the previous efforts. In conclusion, whether it is due to water shortage or overwatering, the result may lead to "reduced yield", especially during the period from flowering to podding. Maintaining a stable input of water is of vital importance. 5 Interactions Between Irrigation and Fertilization in Soybean Cultivation 5.1 Synergistic effects on nutrient uptake and root zone dynamics Soybeans often perform better when water and fertilizer are well combined. It's not that all technical means must be employed. Instead, in many cases, such as in arid regions or calcareous soil conditions, as long as water management and fertilization are carried out in the right rhythm, the root system can play a greater role, and the activity of soil microorganisms will also increase (Rajanna et al., 2022; Amine et al., 2024; Khalili et al., 2024.
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