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The ultimate N rate for canola

If we could predict the weather perfectly, it might be different, 
but in the real world, the best tool to determine nitrogen rates is 
the soil test. It’s a great starting point for 2015

canola seed in hand

One lesson from the Ultimate Canola Challenge is that it’s a wise economic decision to follow your soil test recommendations for nitrogen.

“We have always recommended soil tests to get an idea of the nutrient situation in a field, and what really pleased us about the Ultimate Canola Challenge results is how strongly they confirmed this practice,” says Dan Orchard, agronomy specialist for the Canola Council of Canada (CCC).

The CCC has run the Ultimate Canola Challenge (UCC) the past two years to see what products and practices could further improve farm yields over and above those achieved by following standard best practices. Results from 17 sites across the Prairies (eight in 2014 and nine in 2013) show that best management practices still rule.

“After two years of trials at random locations across the Prairies, additional products and practices have not shown they enhance results or increase profitability,” Orchard says. “The next step with UCC is to test products and practices more regionally. We hope to understand how these products and practices — including micronutrients and primers, for example — work under varying soil and environmental conditions.”

two men inspecting a canola crop
Dan Orchard (l), and Murray Hartman diagnose field nitrogen status in a project to identify optimum rates. photo: Gord Pyzer

Best management practices used in UCC trials were recommended by a panel of experts, including provincial extension staff, Agriculture and Agri-Food Canada (AAFC) researchers and CCC agronomists.

“When it came to setting fertilizer rates for UCC, our general nitrogen and sulphur rates were not significantly different from the lab recommendations in most cases,” Orchard says. “I will add that while nitrogen and sulphur soil test recommendations were most often determined to be accurate, members of the panel debated soil analysis recommendations for phosphorus, potassium and micronutrients and were more likely to adjust these rates significantly to suit their assessment of need at each site.”

Once the UCC panel determined the best nitrogen rate for each site, they set up trials with nitrogen applied at 75, 100 and 125 per cent of that recommended rate. Results for 2013 and 2014, averaged across all 17 sites, were 56 bu./ac. for the 75 per cent rate, 59 bu./ac. for the 100 per cent rate and 60 bu./ac. for the 125 per cent rate.

The 125 per cent rate was achieved by two methods — either with 25 per cent more added at time of seeding or 25 per cent more as foliar top dress. “There was no advantage for top dress over additional fertilizer at seeding, and neither provided a statistical or economical yield increase over the 100 per cent rate, on average,” says Murray Hartman, provincial oilseed specialist for Alberta Agriculture and Rural Development and member of the UCC panel.

A statistical and economic disadvantage was evident, however, when nitrogen was decreased to 75 per cent of recommended rate. Going from 75 to 100 per cent will cost growers an extra $15 to $20 per acre, based on typical N rates, but in UCC trials this investment produced a three bu./ac. increase in yield, on average.

Hartman adds that the majority of the sites (but not all) were low in soil test nitrogen and were therefore responsive to nitrogen fertilizer. This is the situation most likely to produce an economic gain when increasing nitrogen from 75 per cent of the recommended rate up to the full recommended rate.

“I would stress that achieving more yield per acre with additional nitrogen fertilizer is most likely to occur with low-yielding fields that have been fertilized less than the soil test recommendation,” Hartman says, adding, “Trying to push high yields even higher with more than recommended rates of nitrogen is not supported by this data and thus isn’t a wise strategy.”

Orchard notes that the UCC was not designed to test lab recommendations. “But the results reassure us that current lab recommendations are a great starting point for achieving target yields.”

How much N does canola need?

Nutrient uptake and removal tables used by the Canadian Fertilizer Institute suggest that canola needs 3.0 to 3.5 pounds per acre of available nitrogen per bushel of seed yield. (Of that, about 60 per cent is removed in the seed and about 40 per cent is in plant material left in the field.) This amount includes nitrogen from all sources, including soil reserves and mineralized plant material. Fertilizer supplies part of this need, not all of it.

sign in a canola field
UCC site at Beaverlodge, Alta.

Nitrogen rate recommendations from soil test analyses include consideration for crop rotation, soil organic matter, previous yields and, of course, nitrate levels in the soil sample. Recommendations are generally around 2.0 pounds of nitrogen applied for every bushel produced when yield targets are at the top end of the range, and around 2.5 pounds per bushel when yield targets are mid-range.

“Growers know that this is very dependent on application timing, the amount of moisture through the growing season, heat during flowering, and other environmental factors that can influence yields — no matter how much nitrogen is applied,” Orchard says. “But grower experience seems to support these numbers.”

The CCC and AAFC conducted a survey of 996 canola growers from across the Prairies in 2011. Based on grower responses, nitrogen rates averaged 2.3 pounds of actual N for each bushel of canola when yields were in the mid-range (mid- to high 30s) and as low as 1.6 pounds of N per bushel for top end yields (in the 60s).

“Top yields tend to come in high organic matter soils and in good moisture situations. This combination of factors will result in high levels of mineralization — which can explain the lower N-to-yield ratio in high yield situations,” Orchard says. “Top yields are also hard to predict and plan for. So top yields aside, these numbers do support the 2.0 to 2.5 pounds of nitrogen per bushel start point when setting nitrogen rates for average to above average yields.”

Winter and spring soil samples

Soil tests in late fall provide a close representation of nutrient levels in the spring, and give growers time to process samples and get results and recommendations.

Soil sampling in winter is an option, although it helps to have a layer of snow to keep soil from freezing solid. Clear the snow, then probe. The key is to keep the sample cool — 5 C to 10 C, not frozen — and get it to the lab within a couple of days. Don’t put it in a tight sealed container. The Soil Analysis Handbook states that “field-moist soil, when placed in an air-tight container, can undergo significant biological changes at room or elevated temperatures” within days.

“If you do plan to take a winter soil sample, talk to your lab first to see what specific requirements they have,” Orchard says.

Spring tests are the most accurate in predicting the soil nutrient situation at seeding time. Labs may be able to provide results within a few days or a week, so spring tests can be done without holding up the seeding process.

For more on soil sampling, go to the Canola Encyclopedia and find the Fertilizer Management chapter and click on “Identifying fertilizer requirements.”

Jay Whetter is communications manager with the Canola Council of Canada.

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Jay Whetter is communications manager for the Canola Council of Canada.

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