Phosphorus Fertilization Beneficial Management Practices for Soybeans in Manitoba

Field studies were conducted at 10 locations across southern Manitoba from 2013 to 2015. Olsen extractable phosphorus (P) concentrations at these sites varied between 3 and 44 ppm. Phosphorus fertilizer was applied as monoammonium phosphate (11-52-0) at 20, 40 and 80 lbs of P₂O₅/ac in the seed-row, broadcast or sid-banded. Plant stands were assessed at 4 weeks after planting and biomass was harvested and analyzed for P uptake at R3 stage. Stand reduction was rare and did not occur at typical agronomic rates of 20-40 lbs P₂O₅/ac. However, at 80 lbs P₂O₅/ac, stand reductions were most likely to occur in medium to coarse textured soils or when wide row spacing and low seed bed utilization increased fertilizer concentration in the seed row. Even in soils with very low soil P levels (3 ppm Olsen P), soybeans were able take up enough soil P to produce high yields without responding to any P fertilizer rate and placement. Phosphorus fertilization regardless of soil P level, P rate and P placement did not increase biomass or seed yield at any of the site years.