Legume Genomics and Genetics 2025, Vol.16, No.6, 253-269 http://cropscipublisher.com/index.php/lgg 256 weight per plant also tends to decrease. For example, the dry weight of grains per plant under high density is about 28.6% lower than that under normal density (Sichilima et al., 2018). Correspondingly, the yield difference between individual plants increases, and the coefficient of variation of grain yield between different individuals can increase by 6%-30% under high density. Although the yield per plant decreases, the total yield per unit area often shows an upward trend due to the increase in population density until the balance point between density effect and individual effect is reached. For example, Han et al. (2021) comprehensively evaluated the density tolerance of 90 soybean germplasms under high density stress and found that the grain weight per plant decreased the most under high density, followed by branch-related traits, but the population yield can still remain stable or even increase within a certain range. When the density exceeds the optimal value, the population yield will decrease due to excessive competition, often accompanied by serious lodging and disease problems. Lodging is one of the main limiting factors for high yield of dense planting: densely planted plants have thin and tall stems, weak lodging resistance, and are prone to lodging and breaking under strong winds and rains, resulting in reduced yield. Studies have shown that once soybeans fall over, yield losses can be as high as 20% or more (Di Mauro and Rotundo, 2025). The closed environment caused by dense planting can also easily lead to increased field humidity and the breeding of pathogens. For example, powdery mildew, downy mildew, and sclerotinia are more likely to occur under dense planting conditions. Therefore, in actual production, it is necessary to select appropriate planting density according to specific varieties and ecological conditions to achieve coordinated optimization of single plant and group yields. On the one hand, by improving the branching and pod-bearing ability of dense-tolerant varieties, the negative impact of dense planting on single plant yield can be reduced; on the other hand, through density management, excessive competition and lodging risks can be avoided to ensure the reliable realization of group yield increases. 3 Fertilization Strategy and Nutrient Absorption and Utilization 3.1 Effect of nitrogen, phosphorus and potassium application ratio on root and aboveground growth Among the various nutrients required for the growth of legume crops, nitrogen (N), phosphorus (P) and potassium (K) are the "three major factors" that determine yield. A reasonable N-P-K supply ratio can coordinate the growth of the underground and aboveground parts of the plant to achieve high and stable yields. The nitrogen supply level has a dual effect on the root development and nitrogen fixation process of legumes: appropriate nitrogen fertilizer is beneficial to the early growth of seedlings and the establishment of the root system, but excessive nitrogen fertilizer will inhibit nodule symbiosis, reduce the number of nodules and nitrogenase activity. Experiments show that without nitrogen application, soybean plants can obtain nitrogen through nitrogen fixation. Although the number of pods per plant is low, the group benefit is high; while the application of excessive chemical nitrogen (such as 60 kg/hectare) significantly reduces the number and quality of nodules, and the dry weight of nodules is reduced by nearly 20% compared with the no nitrogen treatment (Wysokinski et al., 2024). The best strategy is to apply a small amount of nitrogen as a "guide fertilizer" at the time of sowing or in the seedling stage to promote the growth of seedlings, and then rely mainly on biological nitrogen fixation to meet the needs. Phosphorus is an important element for energy metabolism and root development. Adequate phosphorus supply can stimulate the elongation and branching of bean roots and enhance the nitrogen fixation activity of rhizobia. When phosphorus fertilizer is sufficient, the soybean root system's ability to absorb nitrogen is improved, and the aboveground part of the plant grows vigorously; conversely, phosphorus deficiency will lead to underdeveloped root systems, reduced nitrogen fixation and nitrogen absorption capabilities, short plants, and reduced flowering and pod production. Potassium is closely related to carbohydrate transport and enzyme activity, and has an important impact on bean grain filling and stress resistance. Increasing potassium fertilizer often increases pod fullness and grain weight, and enhances the plant's resistance to lodging and disease. For example, appropriately increasing the proportion of potassium fertilizer under dense planting conditions can compensate for the reduction of nitrogen, maintain the physiological balance of the plant, and thus ensure yield. In the field experiment of Zhou et al. (2019), after reducing the conventional nitrogen fertilizer application by 20% and increasing the potassium
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