Get more bang for your fertilizer buck by focusing on rates

It’s not easy to determine the optimum rate, but start with post-harvest soil testing, says soil scientist

The fertilizer bill can be the biggest single cost in growing a crop, but how do you know if you’re overdoing it?
Reading Time: 4 minutes

A bumper crop of canola or wheat is bound to put a smile on your face.

But writing a cheque for replacing all the nitrogen used by that crop? Not so much.

And those cheques are only going to get bigger if the trend for N use continues on the same track it’s been on since the mid ’90s, said Mario Tenuta, a University of Manitoba soil scientist and one of the country’s leading experts on fertilizer use and 4R nutrient management.

Farmers in his province have been upping their N application by 1.2 pounds per acre for spring wheat, 1.1 lbs./acre for canola, and 1.5 lbs./acre for corn every year for the past quarter-century, he said during a recent webinar.

“Do a projection to the end of this century… we’re going to be well into the 200 pounds per acre for canola and 300 (lbs./acre) for grain corn,” Tenuta said with a laugh, adding that prospect is highly unlikely.

Nevertheless, the trend is a big factor why fertilizer tops the list of production costs. In his province, fertilizer accounts for 32 per cent of the cost of growing canola ($81 of $253), and 28 per cent for corn ($99 of $348) — with nitrogen making up most of that nutrient bill, he said.

It’s also why application rates need to be examined more closely, he said, calling rate “the forgotten R” in 4R nutrient management (which is putting the right rate of the right fertilizer on at the right time with the right placement).

“It’s something, particularly as researchers, that we’re not paying as much attention to as we should be,” said Tenuta, the senior Canada research chair in 4R nutrient management.

The rate dilemma

But determining the correct rate is literally tricky ground — it depends on the soil type, the amount of soil moisture at the time, how (and how deep) the fertilizer is placed, the crop, the weather after placement, how quickly the crop gets established, and many other factors.

However, there’s a payoff for producers who can successfully navigate that maze — the potential to use less nitrogen without sacrificing yield, Tenuta said during the webinar, which was presented by Glacier FarmMedia (the parent company of this paper).

He illustrated that point by outlining some of his latest research, which is focused on enhanced-efficiency fertilizers (EEFs). There are an increasing number of these products using a variety of methods to thwart the big four nitrogen thieves: Volatilization, nitrification, denitrification, and nitrate leaching.

“If we look at the environmental benefits of using these products, it’s really clear — it’s crystal clear — that these products work in reducing losses,” said Tenuta.

But evaluating the products on the agronomic side — because of the factors cited above — is much more complicated.

One way is to look at how much additional yield would be needed to justify the extra cost of EEF. Tenuta ran some numbers for a theoretical canola crop where 130 pounds of N per acre was applied. The cost for urea was $61 per acre, $70.20 for urease-treated urea, $72.80 for double-inhibitor urea, and $76.70 for polymer-coated urea.

If the canola fetched $12.70 per bushel, only “modest” additional yield would be needed — an extra three-quarters of a bushel for urease-treated urea, just under a bushel for double-inhibitor urea, and 1.23 additional bushels for polymer-coated urea.

But there’s a problem here as the recommended rates are designed to ensure a crop doesn’t lose yield because there’s not enough N.

“The recommendations for the rates are designed to give insurance,” said Tenuta. “You need to tailor your rates to your 4R use.”

But that’s easier said than done.

Using the same break-even analysis, he calculated that keeping the N cost at $61 for an acre of canola (that is, the cost of 130 pounds of urea), a producer could only apply 113 pounds of urease-treated urea, 109 pounds of double-inhibitor urea, and 104 pounds of polymer-coated urea.

“Most farmers would be very antsy about reducing their N rates by 20 per cent,” he said. “I don’t know too many who would say, ‘Yep, I’m going to do that.’”

Reducing the rate

But Tenuta also offered a third way — soil test post-harvest to see what’s left in the ground.

“If we’re using the 4Rs but we don’t change our rate and we don’t change our yield, we should be looking for higher residual nitrates,” he said. “You can use that as a tool. If you see high nitrates, you can say, ‘Hey maybe let’s do some adjustment here. Maybe I could have changed my rates.’

“So look at that. See if your fields are responding to that 4R. That will give you guidance.”

That was borne out by a 2019 study by one of Tenuta’s grad students. The trials found virtually no significant yield difference between plots given urea, ESN (a polymer-coated product) and SuperU (which has both urease and nitrification inhibitors).

But the two enhanced-efficiency fertilizers both left more residual nitrates in the soil.

“We used ESN, we used SuperU. We didn’t get better yields because of that but we get more nitrogen left in the ground because of that,” he said. “We probably could have reduced our nitrogen rates and not affected yield. That’s pretty cool.”

However, it’s just one study and, again, a host of factors come into play and many are specific to individual farms. For example, dry conditions and soils that take longer to warm in spring can affect available N from an enhanced-efficiency fertilizer, so blending it with straight urea might be needed.

And while there are many questions to be answered, Tenuta predicted that producers will be writing more cheques for enhanced-efficiency fertilizers — but also finding ways to make them pay.

“I really do see these products as being the way of the future,” he said. “I see more innovation happening with products and new inhibitors coming online. They’re going to be used more and more.”

The presentation can be found at youtube.ca.

About the author

Editor

Glenn Cheater

Glenn Cheater is a veteran journalist who has covered agriculture for more than two decades. His mission is to showcase the ideas, passions, and stories of Alberta farmers and ranchers.

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