Phosphorus Management for Soybean Production in Manitoba: Effects of Soil P and P Fertilization on Seed Yield and Plant Stand.

Very little is known about 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, 22.5, 45 and 90 kg P₂O₅/ha) and different fertilizer placements (seed-placed, broadcast and side-banded) on soybean plant stand and seed yield.

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.