Understanding Nitrogen and Phosphorus

Developing recommendations for Saskatchewan soybeans

by Kim Waalderbos

Soybeans are gaining ground in Saskatchewan crop rotations thanks to shorter-season genetics and stronger yields. Yet, questions remain about the agronomic considerations for this nitrogen-fixing crop.

Chris Holzapfel, Research Manager for the Indian Head Agricultural Research Foundation (IHARF) is leading two research projects aiming to understand the nitrogen and phosphorus requirements for soybeansin Saskatchewan.

“With this research we plan to establish best management practices to ensure adequate nitrogen and phosphorus supply to help producers grow this crop in the most economically, agronomically, and environmentally sound manner possible,”he says.

Nitrogen Management

“Proper inoculation is critical in areas where soybeans are new to crop rotations,” says Holzapfel. “The question will not be so much whether to inoculate, but how much to inoculate.”

Most soybeans in Saskatchewan are purchased pre-treated with a seed treatment and a liquid inoculant. “Growers need to know whether they should be applying granular inoculant on top of that and, if so, at what rate,” he says.

Seeking answers, Holzapfel and his team are investigating the effects of nitrogen fertilization strategies combined with varying granular inoculant rates. Three plot sites in Indian Head, Outlook, and Melfort are being used.

“Most of our trials are conducted under no-till and on fields that either have not previously been seeded to soybeans or have had no more than one previous soybean crop,” Holzapfel explains.

Four nitrogen fertilizer options are being investigated: zero pounds per acre (lb/ac) or 49 lb/ac as side-banded urea, side-banded environmentally smart nitrogen (ESN), or post-emergent surface dribble-banded urea ammonium-nitrate (UAN).

Also, four granular inoculant rates (zero, one, two, and four times the label recommended rate) are being studied. All the seed is treated with a full rate of commerciallyapplied liquid inoculant.The various nitrogen applications are being studied for their effect on yield, emergence, and plant tissue, seed, and soil concentrations.

To date, this work is showing a strong response to dual inoculation (liquid plus granular), Holzapfel says. “Growers should always aim to ensure adequate nodulation by properly inoculating their crop,” he says. “On sites, such as ours, with limited or no history of soybeans, dual inoculation is likely to be economical.”

In 2016, the additional granular inoculant increased yields at all locations except Outlook (that site has a history of soybeans in rotation). At the locations where a response was observed with dual inoculation, yield increases averaged 29 per cent, and ranged from 12 to 53 per cent, compared with plots where only a liquid inoculant was applied. “Dual inoculation also consistently increased tissue and seed nitrogen, or protein.”

In the growing conditions experienced through the first two years of this research, the label recommended rate for granular inoculant has been sufficient to maximize seed yield, Holzapfel says. Under less favourable conditions for inoculant survival Holzapfel has observed yield benefits with
higher-than-label recommended rates of granular inoculant, even when used in combination with a seed-applied product. 

Applying additional nitrogen fertilizer does not appear to be beneficial for soybeans under normal growing conditions, this study is finding. However, there can be benefits to supplemental fertilizer, potentially, under very poor growing conditions where nodulation is likely inhibited. In which case, the largest and most consistent responses have been observed with a late-season surface application of nitrogen to rescue crops suffering from poor inoculation. “However, yields will not likely be recovered to what could have been achieved with strong initial nodulation,” Holzapfel cautions. “We have not seen benefits to starter nitrogen at all locations or when granular inoculant rates were adequate for optimizing yield.”

Phosphorus Management

“Soybeans are recognized as large users of phosphorous,” says Holzapfel. “They are also considered sensitive to seed-placed fertilizer, so applying the required amounts of phosphorus fertilizer in or near the seed may reduce plant stands.”

In a second project, Holzapfel and team are investigating soybean response to phosphorous fertilizer rates and placement, to improve grower recommendations. This study involves plot locations in Indian Head, Melfort, Scott, and Outlook. Three rates (22, 45, and 90 kilograms per hectare) and three placement methods (seedplaced, side-banded, and pre-seed broadcast) are being studied. The various phosphorous applications are being studied for their effect on yield, emergence, and plant tissue, seed, and soil phosphorous concentrations.

With only one exception, “we have not seen significant yield responses to phosphorus fertilizer, regardless of application method,” says Holzapfel, noting this is consistent with results coming out of Manitoba. The exception was at Indian Head in 2016, “with very low soil phosphorus and above-average yield potential at that site, we did observe a yield benefit to phosphorus, and the response was similar regardless of application.”

In preliminary results, the effect of phosphorus application rates on reductions in seedling emergence have been modest. However, there was evidence of slight reductions in plant populations or yield at approximately half of the research locations. To be safe, growers are advised to be extremely cautious or avoid seed-row placement for rates beyond 22 kilograms of phosphorus per hectare.

“Although soybeans are not particularly responsive to phosphorus fertilization, from a longer-term soil fertility perspective, it is not advisable to skip phosphorus application with this crop,” he says. Fields that are already high in residual soil phosphorous may be particularly well-suited to soybean production.

This phosphorous and nitrogen research are both three-year projects started in 2015.