Since the 1990s, Oklahoma State University’s stance has been that plant analysis alone shouldn’t be used to make fertilizer recommendations for crops.

Yet Oklahoma State nutrient management specialist Brian Arnall says he’s been fielding calls from many growers who are spraying foliar nutrients on in-season winter canola as a result of tissue tests taken in early spring.

Arnall says this development is due to the increase in canola acres in the state, and the resulting influx of new crop consultants. But in the last 5 years he’s been managing canola plots for the university and following this method, he’s never seen canola gain an ounce of grain, no matter which nutrient was added.

So Arnall investigated this issue a little further. In a 4-by-20-foot plot in Stillwater, he took tissue tests in winter canola in three growth stages: fall (rosette), early spring after dormancy break, and late spring (pre-bolt). For each growth stage, he took samples three times a week, and on each of those days he took sub-samples at morning, noon and evening.

Arnall says the results confirmed his suspicions: Levels for most nutrients were all over the board, and there was little or no predicable pattern upon which an accurate nutrient application could be based. You can view his presentation by clicking here.

While there was some stability in phosphorus, potassium and iron within a day’s time, within 3 days time only phosphorus and copper showed any consistency in sampling.

For example, in iron he saw one reading plummet from 450 ppm at 8 a.m. to 190 ppm at noon — a span of only 4 hours. An early-season test of sulfur showed levels that were “in the tank” one day had changed to excessive in only 15 days.

“Not one nutrient was the same across all three growth stages, which tells me if we’re going to use tissue tests to do nutrient recommendations for winter canola, we have to have a different value for every growth stage,” Arnall told attendees at last weeks’ No-Till Oklahoma conference in Norman.

He also found the weather environment — previous nighttime and daytime temperatures, cloud clover, relative humidity, and soil temperatures and moisture — significantly impacted nutrient recommendations from these tests. Arnall also questioned where the nutrient recommendations being shared with growers came from.

In states where tissue testing is more established and used on corn and cotton, crop consultants and agronomists know better than to use these tests in early-season plants to decide on fertilizer applications, Arnall explained.

“I’m not dismissing tissue testing that is used as a diagnostic tool. But farmers should not use in-season tissue testing to make fertilizer recommendations for winter canola,” he says. “Even if it does tell you you’re low on something, it doesn’t tell you how much to apply.”

I think the takeaway here is that no-tillers considering fertilizer in-season applications to winter canola should ask their crop consultant some tough questions about how they came up with their recommendations. It’s often said the dumbest questions are those that are not asked.