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Things to consider before sidedressing

Bim/Getty Images aerial view of tractor spraying fertilizer on field
SIDEDRESS DEBATE: There may not be a perfect answer to when you should do a sidedressing. Lots of factors go into it, but start by walking your fields and seeing how your corn looks.
Ask the Experts: Early-season application and the weather are two of the biggest factors that should be guiding your decision to sidedress.

“There isn’t one ‘correct’ time to sidedress,” says Kurt Steinke, associate professor and Extension soil specialist at Michigan State University.

There are lots of factors to consider, from how much you applied earlier in the season to how much the weather has affected nitrogen loss, and even current fertilizer and crop prices. The first step, though, is to see how the crop is doing. Is your corn a little shorter than you’d like for this time of year? Could it use some more juice to get it supercharged for tasseling?

Steinke; Harold Watters, Extension field specialist in agronomic systems at Ohio State University; and Charlie White, assistant professor of soil fertility and nutrient management at Penn State, give their takes on sidedressing nitrogen given current growing conditions, and rising fertilizer and crop prices.

Here’s what they had to say:

Kurt Steinke. Corn is showing a lot of variability across many fields right now with some plants still emerging and other plants, even just a few feet away in the same field, already at V4 or later. 

For growers that applied most of the N preplant incorporated, we really haven’t experienced N loss conditions, so no adjustments may be required. For those who planned on sidedressing, take a look at the corn stand and try to determine if yield potentials may have shifted due to the extreme dry conditions, or establishment and emergence issues. If so, sidedressing may allow the opportunity to adjust those N rates.

Charlie White. Nitrogen management is one of the most challenging aspects of agronomy because there are so many different loss pathways for N, and these loss pathways are all affected by the weather.

The weather can also affect the rate of N mineralization, an important source of available N from soil organic matter, manure and cover crop residues. Finally, you can factor in changes in the cost of N fertilizer and the value of the crop to adjust application rates to be as economically efficient as possible.

If you averaged out the temperature and moisture swings we’ve experienced this spring, we might be on track for an “average" weather year. But underneath this average are several weeks of hot and dry weather followed by the recent week of cool, wet weather.

The warm, dry weather we experienced early in the growing season may have affected N in various ways. If you applied a urea-containing fertilizer during the dry period without a urease inhibitor and without incorporation into the soil, the risks for N losses through volatilization are relatively high. Under worst-case scenarios, about 30% of untreated, surface-applied urea can volatize into the atmosphere.

UAN that is applied in a concentrated band usually soaks into the soil and has less risk of volatilization. UAN applied as droplets is at a very high risk of volatilization without a urease inhibitor.

Urease inhibitors usually protect the urea from volatilization for about two weeks, during which time you hope to receive at least a half-inch of rain to soak it into the soil. Even if you treated urea or UAN with a urease inhibitor, if you did not receive a half-inch of rain within two weeks of application, the risk of N volatilization would start to go back up again.

The dry weather may have also affected N mineralization rates. Soil that dries out significantly will start to have reduced microbial activity and slower mineralization rates. However, this slowdown could have been counteracted by the warmer weather, which speeds up mineralization rates.

Farms that were able to maintain soil moisture through crop residue cover may have benefited from overall increased mineralization due to the warmer temperatures, while farms that had soils drying out probably experienced no significant difference from normal mineralization patterns.

The recent week of cool, wet weather probably had a negligible overall effect on N mineralization and N losses. Although cooler weather slows down microbial activity, soil temperatures are somewhat buffered from swings in air temperature due to the thermal mass of the soil. The rain accompanying the cool weather also likely stimulated microbial activity after the dry period.

You may be wondering how the high crop and nitrogen fertilizer prices might be affecting the economics of N fertilization. In Pennsylvania, our N recommendations given in the Agronomy Guide do not factor in real-time variation in corn grain prices or fertilizer prices, as is done in many Midwest states.

One of the reasons for this is because we do not have as rich of a database of N response curves to develop recommendations from, and our N response curves tend to be more variable than in the Midwest due to greater variability in soil types and farming systems.

In analyzing a few N response curves from recent corn studies in Pennsylvania, I have found that updating the price of grain and N fertilizer from last year to this year reduced the economic optimum N rate by about 10 pounds of nitrogen per acre in most locations. Given the margin of error in our N recommendations, this level of reduction in N fertilizer suggested by current prices is probably not significant enough to act on.

It also tells me that if you suspect you experienced any early-season N losses from volatilization — unless those N losses were extremely major — it probably won’t pay to try and replace those losses with any additional N.

So, despite the gyrations in weather and the crazy behavior of the markets, from an N management perspective, I would encourage you to stay the course.

Harold Watters. In Ohio, the planting season required a little less dancing between rains with the soils being less wet than previous years. The hope is we get some roots down in the ground deeper for a change, and growers will be out to sidedress around V6.

For nitrogen application, growers should use the Corn Nitrogen Rate Calculator at cnrc.agron.iastate.edu. They should make sure to subtract off what has already been applied with an herbicide, with the planter or with their DAP (diammonium phosphate 11-52-0). I have found that this could be 50 to 60 pounds of N, so they only need 100 to 120 pounds of N as sidedress, for example.

TAGS: Fertilizer
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