Effect of Broadcast versus Banded Phosphorus Application on Fate of Applied P in Soil and in Snowmelt Water Flow

There is interest in the impact of P fertilizer placement on crop recovery, and fate of the applied P in the soil, and in run-off water. This research will evaluate the influence of methods of placement of mono-ammonium phosphate (MAP) fertilizer on the yield and recovery of phosphorus by soybean, in a typical Western Canadian soil.

A farm field (wheat stubble) was selected in the spring of 2014 in the Dark Brown soil zone near Central Butte, Saskatchewan. Two sites representing contrasting slope positions in the same field were chosen: an upper slope and lower slope. A soybean variety (Moosomin) was selected for the study. Treatments evaluated were no P fertilizer, and P fertilizer added at the recommended rate (20 kg P₂O₅/ha) applied in the seed-row, banded beneath the seed, broadcast with incorporation, and broadcast without incorporation, as well as broadcast without incorporation treatments at rates of 40 and 80 kg P₂O₅/ha. Each treatment was replicated four times at each slope position. Background soil information was obtained from pre-plant, and pre-fertilizer application soil samples. Both sites were considered marginal to deficient in available P with < 10 mg P/kg soil test extractable P.

The soybeans grew well over the 2014 season, and were harvested in the week following the first hard frost (September 11, 2014). Soil cores to a 60 cm depth were obtained from each plot in October and will be analyzed for plant available P content. Intact surface soil slabs were collected in October and were used in a simulated snow melt run-off experiment in January 2015 to assess the impact of treatments on soluble P in the run-off.

Soybean yield responses observed in the 2014 season at the site showed a grain yield response to P placement at the lower slope position, with the highest yields (~3,000 kg/ha) found in the seed-placed and banded treatments, followed by broadcast and incorporated. The yields of all the broadcast treatments, including the higher rates, were similar to the control or no P treatment. These results indicate that sub-soil placement of P is more effective in promoting soybean yield than broadcast application. No significant response of soybean yield to P rate or placement was observed in the upper slope site, which may be explained by the eroded nature of the soil and high variability. Enhanced extractable P concentrations were observed near the soil surface with broadcast applications.