Most farmers and agronomists should be very aware of the extensive Manitoba research that applied phosphorus fertilizer does not increase soybean yield. However, phosphorus replenishment of soil with P is important.
Some farmers may still will wish to add some starter fertilizer with soybeans, since in this study (Phosphorus Fertilization Beneficial Management Practices for Soybeans in Manitoba), the seed placement of phosphorus only reduced stands significantly in 7 of 28 sites (1 in 4 cases). Before growers jump to the conclusion that soybeans tolerate high rates of fertilizer with the seed, let’s review these cases:
- In almost all cases, soybeans were seeded in moist to wetter soils – as soon as soils were fit to allow traffic. Row spacing ranged from 7-12” with knife or disk openers.
- The 7 cases of stand damage highlighted the risk factors:
- 3/7 = disk openers with row spacings of 12” (so low seedbed utilization)
- 3/7 = sandy soils (less water holding capacity and buffering)
- 1/7 = dry seedbed (higher risk)
Our traditional safe seedplaced fertilizer guideline for soybeans is no fertilizer for rows wider than 15”, and up to 10 lb P2O5/ac when seeded in narrower rows. These limits may even be too high when soils are drying as at present. We do not expect any advantage to this seedplaced P, so the safest option is to avoid any with the seed.
Again – a review of risk factors for seedplaced P:
- Soil moisture: dry = riskier
- Sol texture: sandy = riskier
- Seed opener: narrow or disk = riskier
- Row spacing: wider = riskier
- Fertilizer rate: more = riskier
These factors are all considered in the Seed Placed Fertilizer Decision Aid posted at https://www.ipni.net/toolbox
Submitted by: John Heard, Crop Nutrition Specialist, Manitoba Agriculture
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