Soybean nutrient requirements are essential for maximizing yield and quality. As one of the most valuable grain and oil crops, soybeans have a high demand for nutrients, particularly nitrogen. While soybeans can fix atmospheric nitrogen through root nodules—typically absorbing 5 to 7.5 kg per mu, which accounts for 40% to 60% of their total nitrogen needs—they still require additional nitrogen from the soil. For every 100 kg of soybeans produced, about 1.8 to 10.1 kg of nitrogen, 1.8 to 3 kg of phosphorus, and 2.9 to 3 kg of potassium are needed. Compared to rice, wheat, and corn, soybeans absorb significantly more nutrients.
The growth stages of soybeans include the seedling stage, branching stage, flowering stage, pod-setting stage, grain-filling stage, and maturity stage. The entire growth cycle lasts between 90 to 130 days. Nitrogen uptake is highest during the flowering to grain-filling period, accounting for 24% of total nitrogen absorption. Phosphorus is most needed during the mid-growth phase, with 70% absorbed between early flowering and pod setting. Potassium is mainly taken up before flowering (43%) and during the grain-filling stage (39.5%). This highlights that the flowering to grain-filling period is critical for both dry matter accumulation and nutrient uptake.
Fertilization strategies for soybeans typically involve base fertilizer, seed fertilizer, and top-dressing. The key principle is to provide sufficient nutrients while supporting rhizobial nitrogen fixation. Excessive nitrogen at any stage can hinder nodule formation or cause lodging, so it's important to avoid over-application. It’s also crucial to correct the misconception that rhizobia alone can supply all nitrogen needs. A balanced mix of N, P, K, and micronutrients like boron and molybdenum should be used, along with slow- and quick-release fertilizers.
Applying organic manure as a base fertilizer is an effective way to boost soybean yields. Organic materials can be incorporated into previous crops in a rotation system, benefiting soybeans in the following season by improving soil structure, promoting nodulation, and enhancing nitrogen fixation. On low-fertility soils, adding 10 kg of calcium phosphate and potassium chloride as a base fertilizer can further support plant growth.
For seed fertilizer, 10 to 15 kg of superphosphate or 5 kg of diammonium phosphate per acre is commonly used. In boron-deficient soils, 0.4 to 0.6 kg of borax should be added. Since soybeans are dicots, seedlings may struggle to push through the soil, so seed treatments should be applied to the lower or side parts of the seeds, avoiding direct contact with fertilizer.
Top-dressing is beneficial during the seedling stage when nodules haven’t fully developed or are less active. Applying 7.5 to 10 kg of urea at this time can strengthen plant growth. At flowering, a small amount of nitrogen fertilizer is also recommended. Spraying 0.2% to 0.3% potassium dihydrogen phosphate or superphosphate solution can improve grain nitrogen content and increase yield. Additionally, applying 0.1% borax, copper sulfate, and manganese sulfate solutions during flowering can enhance grain filling and boost oil content.
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