Soil test labs don’t factor in nitrogen losses when making recommendations. So if you take steps to reduce losses, you may want to rethink the recommended rates

Whether you are dealing with a lab directly or with an advisor, Karamanos says it’s important to ask whether fertility recommendations can be adjusted to reflect production practices.

Farmers need to question their soil test lab or crop advisor to get a better handle on what their nitrogen (N) fertilizer recommendation is based on, says a long time soil fertility specialist.

Many labs base their recommendations on some general assumptions, says Rigas Karamanos, agronomy manager for Viterra in Calgary, Alta. Those recommendations often don’t —and in some aspects can’t — take into consideration specific farming practices, but it doesn’t hurt to ask, he says. If tell them what you do to limit nitrogen losses, they may be able to set a rate that suits your system more accurately.

“Most labs, in developing their recommendations, follow a set of curves that represent excellent, average or dry conditions,” says Karamanos, who for many years was involved with both public and private soil testing facilities. “But in general those recommendations likely don’t consider what type of fertilizer you are using or how it is applied.”

Karamanos was responding to a question about whether fertilizer recommendations reflect the potential for nitrogen losses. If a farmer takes steps to reduce nitrogen losses — for example, banding nitrogen instead of broadcasting it — does the lab lower its N rate recommendations accordingly? His short answer is “no.”

“There is no real provision for N losses in most lab recommendations,” he says.

With more farmers now using the services of crop consultants or advisors, Karamanos says many labs now provide the basic soil test analysis and it is left to the advisor to do the interpretation.

Whether you are dealing with a lab directly or with an advisor, Karamanos says it’s important to ask whether fertility recommendations can be adjusted to reflect production practices.

Nitrogen losses when fertilizer is broadcast applied, can be significant says Karamanos. “Probably the worst situation is if the fertilizer is broadcast applied under dry conditions,” he says. “Nitrogen losses can be as high as 20 and 30 per cent. And losses can be greater too if the fertilizer is applied in fall versus the spring.”

If there is good moisture and decent rainfall just after fertilizer is broadcast applied, Karamanos says there probably isn’t much difference in whether the fertilizer is on the soil surface or banded. The nutrients will be taken into the soil. But don’t assume that dewy mornings will help draw nitrogen into dry soil, he says. In fact, a heavy dew creates a “perfect environment” for gassing off and nitrogen being lost to the atmosphere.

Because it is difficult to predict rainfall timing, “in general anything you can do to prevent nitrogen fertilizer from coming into contact with the air will benefit in reducing nitrogen losses,” he says. Banding fertilizer in a narrow row is probably the best way to reduce losses, although the high concentration of nitrogen will temporarily inhibit the activity of microorganisms in breaking down nitrogen in the soil.

While it is fairly common for producers to broadcast apply nitrogen just ahead of the seeding operation, he says that doesn’t necessarily mean the air seeder will incorporate all fertilizer into the soil. “If there is a lot of crop residue on the soil, some of the nitrogen will sit on the straw and some of it will be consumed by microorganisms working to break down the straw and it won’t make its way to the crop.

“Another factor that has to be considered is how crop residue is handled,” he says. “Is it left on the soil surface, is it baled off, is it burned? Depending on how you handle crop residue will also affect nitrogen losses, especially if fertilizer is broadcast applied.”

ORGANIC MATTER CONTRIBUTION NOT INCLUDED

While a soil analysis often indicates the amount of organic matter in the soil, Karamanos says usually the value of that organic matter isn’t reflected in fertilizer recommendations.

“The soil analysis says there is “x” amount of organic matter, but it doesn’t consider how much nitrogen that organic matter will contribute to the crop,” he says. “I have written a whole chapter in a book on this, so it is fairly complex, but in very general terms under average conditions, for every one percentage point of organic matter you can figure 10 pounds of nitrogen will be released. And that will vary. If you have fairly wet conditions it could be 12 pounds of N for each percentage point.”

In working with a soil test lab or a crop advisor, the producer should ask if recommendations reflect his farming practices and moisture conditions. The following factors need to be considered, says Karamanos: What type of fertilizer are you using and is it being banded or broadcast applied? How has crop residue been managed? How much organic matter is in the soil and what value does it have? And what are the soil moisture conditions?

Lee Hart is field editor of Grainews, based in Calgary. Contact him at 403-592-1964 or by email at [email protected]

About the author

Field Editor

Lee Hart

Lee Hart is editor of Cattleman’s Corner based in Calgary.

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