49-9 Phosphorus Management for Soybean Production in Manitoba: Effects of Soil P and P Fertilization on Seed Yield and Plant Stand.
Plant stand reduction was extremely rare at typical agronomic rates of 22.5-45 kg P2O5/ha. However, at 90 kg P2O5/ha, seed-placed P reduced stands at 4 sites, most frequently in coarse textured soils and/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.
Phosphorus fertilization did not increase biomass or seed yield at any of the 18 site years, regardless of P rate, P placement or soil P level. Even in soils testing as low as 3 ppm Olsen P soybeans were able to acquire enough soil P to produce high yields without responding to P fertilizer.
Some studies have shown that soybean may respond to soil P despite the lack of response to freshly applied fertilizer P. Soybean yield response to different levels of soil test P was measured for four site-years on a long term trial where incremental rates of P fertilizer were applied from 2002 to 2009 to create a range of soil P from 14 to 186 ppm Olsen P.
No significant yield differences were observed. However, the lowest soil test P was still greater than 10 ppm Olsen P, which could be enough to supply the requirements of soybeans.