Phosphorus management for soybeans in Manitoba
Very little research has been conducted to determine the best rate, source, placement, and timing of P fertilizer for modern soybean cultivars grown in the Canadian Prairies. In 2013 and 2014, 18 sites were established in Manitoba to measure the effect of different rates of monoammonium phosphate (0, 20, 40 and 80 lb P2O5/ac) and different fertilizer placements (seed-placed, broadcast, side-banded or mid-row banded) on soybean plant stand and seed yield. Stand reduction was rare and did not occur at typical agronomic rates of 20-40 lb P2O5/ac. However, at 80 lb P2O5/ac, stand reductions occurred at 4 sites, most frequently in medium to coarse textured soils or when wide row spacing and low seed bed utilization increased fertilizer concentration in the seed row. Nevertheless, seed yield was reduced below that of the unfertilized treatment at only 2 of the 4 site-years where there was a stand reduction. In both cases where seed yield was reduced, the high rate of seedrow P reduced the stand to less than 100,000 plants/acre, the traditional replant threshold for soybean. Phosphorus fertilization regardless of soil P level, P rate and P placement did not increase biomass or seed yield at any of the 18 site years. Even in soils with very low soil P levels (as low as 3 ppm Olsen P) soybeans were able to explore the soil in order to take up enough P to produce high yields without responding to P fertilizer rate and placement.
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