Legume Genomics and Genetics 2025, Vol.16, No.6, 253-269 http://cropscipublisher.com/index.php/lgg 259 4 Analysis of the Interactive Effect of Dense Planting and Fertilization 4.1 Identification of the optimal combination: medium density + balanced fertilization mode In actual production, the degree of dense planting and the level of fertilization need to be coordinated and matched to achieve the best yield increase effect. Too low density will lead to waste of resources, and even if more fertilizer is applied, it will be difficult to fully play its role; on the contrary, if high-density planting is insufficient in nutrient supply, the growth of crops will be inhibited due to fierce competition, and it will be difficult to produce high yields. Therefore, different regions and different seasons should screen out the optimal combination of planting density and fertilization amount according to specific varieties and ecological conditions. In recent years, many studies have determined the optimal density-fertilization scheme for crops such as soybeans through two-factor field experiments or model simulations. For example, Hao et al. (2023) used a quadratic regression design in Northeast China, and simultaneously examined the planting density (300,000-600,000 plants/ha) and NPK fertilization level to optimize the yield response surface. The results showed that the theoretical optimal combination was: density 45.37×104 plants/ha, nitrogen application 98.4 kg, phosphorus application 218.96 kg, potassium application 47.62 kg, and the expected yield could reach 3816.7 kg/ha. This result quantitatively provides a reference model for high soybean yield in the local area. Of course, the optimal values in different regions vary and need to be adjusted according to soil fertility and variety characteristics. But overall, "medium to high density + balanced and sufficient nutrients" is one of the common characteristic patterns of high bean yield. Through interaction analysis, it can be found that reasonable fertilization can expand the response range of crops to planting density. For example, under low fertility conditions, soybean yield reaches a plateau earlier with increasing density, while under sufficient fertilization conditions, the yield plateau is delayed, and crops can continue to benefit from higher density. This shows that fertilization management can improve the density tolerance of crops. Conversely, close planting measures can also increase the marginal return of fertilizers. Within a certain range, high-density groups have higher utilization efficiency for each unit of fertilizer nutrients. Therefore, the optimization of the two should be carried out in a coordinated manner. For example, for densely planted varieties with the potential for dense planting, the fertilizer input can be increased accordingly to give full play to its yield potential; while for creeping or large-sized varieties that require thin planting, the amount of fertilizer should be properly controlled to avoid closure and leggy growth. 4.2 Synergistic improvement of biomass accumulation and resource allocation The goal of dense planting and fertilization optimization is to simultaneously promote the overall biomass accumulation of crops and the allocation to economic output, and improve the harvest index. Under suitable density and sufficient nutrient conditions, legume crops can maintain a higher group photosynthetic productivity and a longer functional period, so that vegetative growth and reproductive growth can be coordinated. The study found that the plasticity of individual plants enables them to adjust resource allocation strategies under different planting densities and nutrient levels: in sparse planting and low nutrient conditions, plants tend to increase root input (increase root-crown ratio) to obtain more resources; in dense planting and nutrient-sufficient environments, plants allocate more dry matter to the aboveground parts, especially pods and grains, to improve competitiveness and reproductive success rate. Therefore, the combined optimization treatment often presents the characteristics of both strong roots and aboveground parts and sufficient reproductive allocation. The study by Szpunar-Krok et al. (2023) showed that in the treatment of 30 kg nitrogen application and rhizobium inoculation, the number of pods and grains per soybean plant increased significantly, and the number of nodules and dry weight were also higher than those in the treatment without nitrogen application, achieving the simultaneous enhancement of underground nitrogen fixation capacity and aboveground pod-forming capacity. This proves that reasonable nutrient supply can promote plants to accumulate more assimilated products and effectively allocate them to grains. In the experiment, the total dry matter mass of soybean populations treated with the best combination increased by 14% compared with the control, of which the proportion allocated to grains also increased by about 5 percentage points, and the harvest index was significantly improved.
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