Soybean responds infrequently to phosphorus fertilization in Manitoba

Soybean ( Glycine max (L.) Merr.) production in Manitoba has increased substantially over the last 20 years, raising questions about phosphorus (P) fertilization in this region. Between 2013 and 2015, a study was conducted across 28 sites in Manitoba to evaluate the effect of P fertilizer rate and placement on soybean plant stand and seed yield. Treatments were 22.5, 45, and 90 kg P 2 O 5 ha −1 applied as monoammonium phosphate, seed-placed, side-banded or broadcast, plus an unfertilized control treatment. Plant stand reduction due to seed-placed fertilizer toxicity was observed at five of 28 site-years, typically at the rate of 90 kg P 2 O 5 ha −1 . Stand reduction was most frequent on medium- to coarse-textured soils, dry soils or when seeding equipment had low seedbed utilization. Seed yield was reduced at two site-years due to seed placing 90 kg P 2 O 5 ha −1 , which reduced plant stands below the recommended threshold of 247 000 plants ha −1 . Phosphorus fertilization did not increase seed yield, regardless of P rate, P placement, or Olsen soil test P level, except for one site-year where 45 and 90 kg P 2 O 5 ha −1 increased seed yield by 343 and 430 kg ha −1 , respectively. The extremely infrequent response to P fertilizer in combination with the high rate of P removal indicates that soybeans can use soil P reservoirs that are less available to other crops. Nevertheless, soybean growers in Manitoba should consider strategies for applying supplemental P to soybean or other crops in their rotation to maintain P fertility in soil.