To improve phosphorus management recommendations for soybeans in Saskatchewan by investigating crop response to monoammonium phosphate rates and placement methods.
A project was initiated in 2015 to investigate soybean response to phosphorus (P) fertilizer rates and placement options under field conditions in Saskatchewan. The second year of field trials has been completed with locations at Indian Head, Outlook, Melfort, and Scott. The treatments are a combination of three P fertilizer rates (22, 45, and 90 kg P2O5 ha-1) and three placement options (seed-placed, side-banded, and pre-seed broadcast) along with a control. The specific field protocols being followed were originally developed at the University of Manitoba and have also been implemented at various locations throughout that province.
Field trials were initiated in 2015 and conducted over three seasons at four Saskatchewan locations (Indian Head, Outlook, Melfort and Scott) to improve provincial phosphorus management recommendations for soybeans. The results suggest the current recommendation of no more than 10-20 kg seed-placed P2O5/ha may be too conservative but side-banding is a preferable method for applying phosphorus to soybeans, especially at high rates. Positive yield responses to phosphorus fertilization were infrequent and modest but detected 25% of the time. Such responses occurred only when soil test phosphorus values were below 15 parts per million and there were no other major yield limiting factors. In the cases where positive responses occurred, they were similar across placement methods with the exception of one site the strongest response was to broadcast phosphorus. The response to broadcast phosphorus was better than expected; however, this is still not a recommended practice due to higher potential for movement in runoff and, over time, conversion into less soluble phosphorus forms. While the results varied amongst sites, on average seed yields were 6% higher when at least 40 kg P2O5/ha was applied; however, with 90 kg P2O5/ha placed in the seed row, yields did not significantly differ from the control. For the three responsive sites specifically, the yield benefit was 11-22% or 415-876 kg/ha (6-13 bu/ac).
Preliminary results showed evidence of reduced plant populations with 45-90 kg ha-1 of seed-placed P at 3/8 site-years (Melfort 2015 and 2016, and Scott 2016) while at Outlook in 2015, effects on emergence were not significant but yields were reduced with 90 kg P2O5 ha-1 of seedplaced P. While negative impacts on emergence were not always detected and were frequently subtle, these results support the recommendation that rates of seed placed P exceeding 22 kg P2O5/ha should be avoided to minimize the potential for seedling injury. While whole plant tissue P concentrations were increased with fertilization to some extent at the majority of sites, seed yield increases with P were only evident at 2/8 sites with the strongest response at Indian Head 2016 and smaller but significant linear yield increases with P fertilizer at Outlook in 2016. The magnitudes of the responses were approximately 18% and 13% at Indian Head (2016) and Outlook (2016), respectively. In contrast, at Outlook in 2015 the only significant P fertilizer effects on yield were negative and specific to the highest rate of seed placed P. Data for seed P concentrations and P exports are not yet available for all sites; however, increased seed P concentrations with fertilizer were observed at Indian Head in both years. This work is continuing at all four locations in 2017 with funding provided by the Saskatchewan Pulse Crop Development Board.