Soybean nutrient requirements are significant, as they are one of the most economically valuable crops in the grain and oil industry. Although soybeans have a high demand for nitrogen, they can fix atmospheric nitrogen through root nodules, typically obtaining 5 to 7.5 kg per mu, which accounts for about 40% to 60% of their total nitrogen needs. For every 100 kg of soybeans produced, the plant requires approximately 1.8 to 10.1 kg of nitrogen from the soil, 1.8 to 3 kg of phosphorus, and 2.9 to 3 kg of potassium. Compared to rice, wheat, and corn, soybeans absorb significantly more nutrients throughout their growth cycle.
The soybean growth cycle consists of several stages: seedling, branching, flowering, pod formation, grain filling, and maturity. The entire growth period lasts between 90 to 130 days. Nutrient absorption follows a specific pattern. Nitrogen uptake is highest during the flowering to grain-filling stage, with 15% absorbed during the early seedling and branching phase, 16.4% between branching and full bloom, 28.3% from flowering to pod development, and 24% during the grain-filling phase. Phosphorus absorption peaks in the mid-growth stage, with 17% taken up during the seedling stage, 70% from early flowering to pod formation, and 13% during the final maturation phase. Potassium is absorbed mainly before flowering (43%) and during the grain-filling stage (39.5%), with an additional 17.2% required after pod formation.
Fertilization strategies for soybeans should include base fertilizer, seed fertilizer, and top-dressing. The goal is to provide sufficient nutrients while supporting rhizobial nitrogen fixation. Excessive nitrogen application at any stage can hinder rhizobia activity or cause lodging. It's also important to correct the misconception that soybeans don't need nitrogen at all. A balanced mix of N, P, K, along with micronutrients like boron and molybdenum, is essential. Combining slow- and fast-release fertilizers ensures steady nutrient availability.
Applying organic manure as a base fertilizer is crucial for boosting soybean yield. Organic materials used on previous crops in crop rotation can benefit subsequent soybean growth by improving soil structure and promoting nodule formation. In low-fertility soils, adding 10 kg of calcium phosphate and potassium chloride as a base fertilizer can greatly enhance productivity.
For seed fertilizer, 10 to 15 kg of superphosphate or 5 kg of diammonium phosphate per acre is recommended. In boron-deficient soils, 0.4 to 0.6 kg of borax should be added. Since soybeans are dicots, it’s best to apply seed dressings to the lower or side areas of the seeds, avoiding direct contact with the fertilizer to prevent damage.
Top-dressing with a small amount of nitrogen during the seedling stage, especially when nodules haven’t fully developed, can help strengthen the plant. Applying 7.5 to 10 kg of urea at the flowering stage is also beneficial. Spraying 0.2% to 0.3% potassium dihydrogen phosphate or superphosphate solution during this time can improve grain quality and increase yield. Additionally, applying 0.1% borax, copper sulfate, and manganese sulfate solutions during flowering can promote better grain filling and enhance oil content.
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