Don't be in too big a hurry to apply fall nitrogen

Don't be in too big a hurry to apply fall nitrogen

Iowa State agronomist offers tips on fall tillage, cover crops and application of nutrients. By

While farmers across Iowa are still plenty busy harvesting corn and soybeans this week, some are starting to do fieldwork after their fields are harvested. "Don't get in too big of a hurry on anhydrous application. wait until the ground gets cold enough and stays cold. And don't get in too big of a hurry on tillage. You may want to consider planting a cover crop," says Mark Johnson, Iowa State University Extension field agronomist in central Iowa. "Remember, you can still plant cover crops in October."

DON'T GO UNTIL 50 OR BELOW: Don't inject anhydrous ammonia in the fall until soil temperature at 4-inch depth reaches 50 degrees F or below and the weather is in a downward cooling trend, advise ISU agronomists.

First, he reminds farmers that weeds are a consideration: what you are seeing in your field at harvest this fall can help you plan next year's weed management program for your farming operation. "As you harvest your fields pay attention to where you may have had some weeds that survived that are resistant to the herbicide program you used this year," says Johnson. "Keep a record of which fields showed a lot of weeds at harvest. Glyphosate resistance, in particular, has spread throughout the Corn Belt."

Fall fieldwork is already starting for some farmers
"I'm starting to see fall fieldwork being done closely following the combines," says Johnson. "I'm reminding farmers that cover crops can still be established during October. The sooner you plant them, the better. You don't have to wait to apply manure to the land. Go ahead and plant the ground to the cover crop. Then knife-in the manure application. It works just fine to slice manure into established cover crops."

And don't go overboard on tillage this fall. "In the interest of saving soil from wind and water erosion from now to when next year's crop is well established next spring, you should do as little fall tillage as you can," he advises.

Fall application of nutrients; get most bang from your buck
With projected 2016 crop margins looking tight, "you want to get the most bang for your bucks spent on crop inputs such as fertilizer. That's true whether the nutrients come from application of manure or from commercial fertilizer," says Johnson.

Whether applying manure or anhydrous ammonia, remember to wait until the soil temperature at the 4-inch depth is below 50 degrees F and the weather forecast indicates the soil temperature will stay below 50 degrees. You can monitor soil temperatures for each county in Iowa at this site


Be sure to follow the thumbrule: "Don't go until its 50 or below." Johnson says, "This is critical because the conversion to nitrate is much more rapid above 50 than below 50. Nitrate is the form of N that is subject to leaching and denitrification." Here are the October 5 soil temperatures at the 4-inch depth as found at the above link.

"As you can see, the soil is still too warm currently to be applying any fertilizer product with nitrogen," says Johnson. Use of nitrification inhibitors may be especially important when N is applied in the fall, he adds. "They slow the conversion of ammonium to nitrate and reduce the risk of loss. They do not, however, eliminate the risk."

Apply nitrogen only on part of your 2016 corn acres
Consider applying nitrogen on only a portion of your next year's corn acres, says Johnson. Keep these guidelines in mind when considering where to apply anhydrous ammonia in the fall to minimize the risk of N loss.

•The fields with the least chance of N loss due to leaching and/or denitrification are the medium texture soils (very fine sandy loam, loam, silt loam, silt) with pH below 7.2.

•Course texture soils (sand or loamy sand) are more prone to leaching.

•Fine textured soils (sandy clay, silty clay, clay) are more prone to denitrification.

•Soils above pH of 7.2 convert nitrogen to the nitrate form more rapidly.

Applying less than the full amount of nitrogen to any given field is also a good strategy to help reduce N losses, says Johnson. The balance can be applied preplant next spring with the planter or it can be side-dressed.

Using manure as part of your soil fertility program
"There are a few things we need to keep in mind as we use manure for crop nutrients," says Johnson. "When planning your fertility program, keep in mind the value of any manure you happen to apply." Here are some nutrient values that came out of research done by ISU.

Try to avoid applying manure too early in the fall. Same as with anhydrous ammonia, you want to apply manure when the soil is cold enough so you don't risk losing the nitrogen that's in the manure. "If you have to apply manure early in the fall, especially liquid swine manure, consider applying only enough to alleviate manure storage concerns," says Johnson. "Then you can resume application of the manure after the 4-inch soil temperature drops below 50 degrees F."

TAGS: USDA Extension
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