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Soybean Phosphate Fertility

Phosphate Drives Soybean Growth
Phosphate is essential for soybean growth. A 55-bushel per acre soybean crop takes up 53 pounds per acre of P2O5 per acre, and for every bushel of soybean harvested, 0.80 pounds of P2O5 per acre is removed from the field.

Research done in Indiana, over a 10 year period, showed that the average soybean yield increased from 44 bushels per acre to 54 bushels per acre — a yield increase of 18.5% when phosphate fertilizer was used3.

Phosphate is Crucial to Nitrogen Fixation
Research shows that phosphate nutrition has a significant positive impact on nitrogen fixation. Good phosphate nutrition results in more nodules being formed and more active nitrogen fixation.

• Phosphorous is critical for photosynthesis, which is basically developed energy in the leaves. Plant energy is important because nitrogen fixation requires an enormous amount of energy. Thus, the more energy the plant has, the more nitrogen fixation the plant can support.
• Phosphorous helps move the energy from photosynthesis to the roots, where it is needed to fuel nitrogen fixation.
• More extensive root growth — provides greater opportunity for the development of nitrogen-fixing nodules.
• Faster development of active nodules — results in greater nitrogen fixation.
• More active nitrogen fixation means more nitrogen to the plant, which means higher yield.
• Increases the number and size of nodules, and the amount of nitrogen fixed by the plant.
• Higher population of Bradyrhizobia bacteria in the soil surrounding the root.
• Increases the percent and total nitrogen in the harvested portion of the crop.

Phosphate Fertility Challenges
Phosphate fertility programs in soybeans must deal with several challenges that may limit phosphate availability
and uptake:

• Soybean seed is very sensitive to fertilizer salt injury, therefore all fertilizer must be placed away from the seed- broadcast before planting, deep banded, or banded in a 2 x 2 inch side band while seeding. However, placing the phosphate fertilizer away from the seed prevents optimum early season uptake.
• Under no-till conditions, if broadcast phosphate is commonly used, the phosphate level in the rooting zone may be lower than the surface layers.
• When soil surface residues are high and soil is cold and wet, conditions common under early seeding or reduced tillage programs, phosphate availability may be limited.
  
  3 Potash and Phosphate Institute; Yield and Economic Responses to Phosphorus; Better Crops/Vol. 83 (1999, No.1).