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Predicting sidedress nitrogen rates

Pre-sidedress N tests can be a fairly good predictor of a corn crop’s N fertilizer needs.  FILE PHOTO.

Applying some nitrogen (N) to your corn crop at planting time and some as a sidedress at about the six-leaf stage often makes more economic sense than a single application at planting. That’s especially true if the later application allows a more accurate estimate of how much nitrogen to apply.

“The principal advantage of applying nitrogen at sidedressing time is being able to have a better look at the situation to help you in deciding whether to apply the average sidedress rate – or less, or more – depending on what the crop’s yield potential looks like and what the weather has done from April to the first part of June,” Greg Stewart, corn industry program lead with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), says.

“If you have to decide in April how much nitrogen to apply in a one-shot preplant deal, you really have only one option. That is to use a historical calculator, like the Ontario Corn Nitrogen Calculator. With that calculator, you describe your field, where it is, what the previous crop was, what you think the current crop will yield, etc. Then the calculator provides a nitrogen recommendation based on how much nitrogen a field like that has needed historically. On average that prediction will be pretty close.”

But in years when the weather is distinctly different from the average, Ontario research indicates it is better to assess a crop’s N needs at sidedressing time. “For instance, in a cool, wet year like 2011, we needed significantly more nitrogen, and we really couldn’t have made that decision until June 10,” Stewart notes. “In 2012, we were in our T-shirts at March break, April was hot, and rainfall was less than average, so the weather would have dictated that we needed less sidedress nitrogen. Then there are years like 2013 or 2014 where perhaps you don’t change the nitrogen rate that you would have applied in April because the weather is sort of average.”

The other key advantage of sidedress applications is there’s a lower risk of N loss. A corn crop’s maximum N uptake occurs between about the V9 stage and just before tasselling, so N applied at around V6 isn’t at risk for as long a period as N applied at planting.

“If you apply all your nitrogen fertilizer in April, you are putting more of it at risk,” Stewart says. “It converts to nitrate, and nitrate can leach out of the soil because it is soluble. [The amount of leaching will be higher when rainfall amounts are higher and when soils are sandier.] Nitrate can also denitrify [become lost as a gas] if it is sitting in saturated conditions, so sidedress nitrogen also makes sense for
growers with heavier, poorer drained soils.”

Of course, N requirements not only vary over time, they also vary across a field as characteristics like soil texture vary. So another aspect of improving estimates of sidedress N rates is to evaluate the crop’s N needs in ways that take in-field variability into account.

Soil nitrate tests, rainfall data and sensors
Stewart explains that you can look in several places for clues as to how much sidedress N your corn crop needs.

One place to look at is the soil’s nitrate levels. Stewart and Ian McDonald, applied research coordinator for field crops with OMAFRA, have conducted various studies that show pre-sidedress N tests (PSNT) are a fairly good predictor of the crop’s N needs.

This test measures the amount of nitrate-nitrogen, in parts per million (ppm), in a soil sample. OMAFRA recommends PSNT soil samples should be collected when the corn is six to 12 inches tall and should be taken to a soil depth of 12 inches.

Stewart and his colleagues are currently working on some strategies to make PSNT a better N rate predictor. For example, they recently recalibrated OMAFRA’s sidedress N rate recommendations. Previously, these recommendations were based solely on the PSNT results. So the researchers have gone back into the original dataset, added more data and revised the approach so the rate recommendations now take into account both the PSNT results and the field’s expected corn yield.

“Together, the soil nitrate test and the yield expectation give you a better estimate of how much nitrogen you need,” Stewart says, adding the recalibrated recommendations will soon be available at
An additional possibility is to collect the PSNT soil samples based on yield maps, sampling the high-, medium- and low-yielding areas separately, so the PSNT results would be specific to each of those areas.
Stewart will also be talking to growers about ways to improve PSNT sampling procedures. For example, he advises growers to avoid collecting the samples in soggy weather. “If it rains all weekend, it is not a good time to take soil nitrate samples because those damp soils seem to respond inaccurately in the soil nitrate testing process. For accurate results, soil needs to be drained out, not freshly-rained-on.”
Stewart and his colleagues will also be investigating whether there are benefits from collecting more than one nitrate sample before sidedressing. 

Another place to look for clues to help in predicting sidedress N rates is the local weather. “For instance, if the rainfall has been way above average from April 10 to June 10, and the corn crop looks pretty good, then you probably need more nitrogen. If rainfall has been 75 per cent of normal and the corn crop looks pretty good, then you probably are in better shape and need less nitrogen,” Stewart says.

He would also like to see growers improve their rainfall data. “If you have accurate soil nitrate data and rainfall records, and then you add in the soil type, maybe you can start to draw a pretty nice picture of the crop’s nitrogen needs. For instance, if you know you’ve had a lot of rain, and the soil nitrate tests seem low, particularly low on your clay soils, then you could decide to put more nitrogen on the clay areas.”
A further place to look for guidance in estimating N rates is at the crop. In various studies, Stewart and his colleagues have been evaluating plant sensors like the GreenSeeker technology for determining in-crop N needs. In the coming months, Stewart will be analyzing the results from their 2014 GreenSeeker work.

“The GreenSeeker probably can do a decent job of determining this part of the field needs more nitrogen and that part needs less, but where in the spectrum between 50 pounds and 200 pounds should that ‘more’ and that ‘less’ be? That’s our challenge: finding the proper calibration for the GreenSeeker or other sensors.”

Optical sensors like GreenSeeker do not directly detect a crop’s N needs. They emit specific wavelengths of light onto the crop canopy, and the canopy reflects some of that light back to the sensors. The sensors determine the difference between the emitted and reflected light. They are able to detect how much biomass the crop has produced, which allows an estimate of the crop’s yield potential and how responsive it might be to N.

One advantage of sensors like GreenSeeker, CropSpec and OptRx is they can be used for on-the-go variable rate N applications. The sensors are attached in front of the fertilizer applicator unit and are integrated with the application system. Most of these sensors require an N-rich strip as a standard for comparison with the rest of the field. If the sensor readings in another part of the field are close to the readings in the N-rich area, then the fertilizer system doesn’t apply any N to that part of the field. If readings are quite different, then the system uses a formula to calculate what N rate to apply. The formula, or “algorithm,” varies depending on such factors as the climate, tillage practice, soil texture, crop type, crop growth stage and sensor type. Researchers in various parts of North America have developed algorithms for their own regions.

Stewart and his colleagues have also started to explore the use of in-field soil sensors, like SoilOptix. This particular sensor is able to map a wide range of soil characteristics, but it needs to be calibrated for local conditions.

“This tool can estimate soil moisture-holding capacity, soil texture, organic matter and many other characteristics based on scanning the soil. So the idea is that you could perhaps divide a field up into zones that would make sense to be managed differently in terms of nitrogen,” Stewart explains. “And possibly you could combine that map with some of the other technologies. For instance, you might do your soil nitrate sampling based on the SoilOptix management zones to get more meaningful results from the soil nitrate tests.”

He adds, “You could argue that yield maps, or soil organic matter maps or other maps might capture those management zones, too. We’re looking at the research to see whether something like SoilOptix might do a better job of dividing a field up into management zones and so give better direction on how to manage nitrogen within those zones.”

A possible approach
For corn growers who want to aim for more accurate sidedress N rates, Stewart suggests the following steps.

“Step one, I would open the Ontario Corn Nitrogen Calculator [on], and describe the current situation: my field’s soil texture, previous crop, expected yield, the current cost for nitrogen, the expected price for corn and so on. The calculator will tell me that, historically, research suggests this field needs X amount of nitrogen. So that gives me a benchmark of about what the range should be.”

Step two is to apply some portion of the total N application at or near seeding time. “To keep this example simple, I’ll say I’m going to put down 30 pounds of nitrogen per acre through my planter.” Thirty pounds should be enough N to take the corn crop up to sidedressing time.

“Step three, I am going to use the new and improved soil nitrate test to evaluate how much nitrogen I should put down as my sidedress rate. And I’m going to do the nitrate sampling based on some sort of map – maybe a yield map, or an aerial image, or a topographic map – that helps identify areas with about the same yield potential.”

Step four is to input that map with the N recommendations into a variable rate N application system and apply the fertilizer as a variable rate sidedress.

May 21, 2015  By Carolyn King


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