Fall Fertilizer Tips

What to Keep in Mind When Applying Fall Fertilizer and Nutrients

Emily Unglesbee
By  Emily Unglesbee , DTN Staff Reporter
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Anhydrous ammonia tanks become a common sight this time of year in farm country. Remember to watch the weather and soil conditions carefully before making fertilizer applications. (DTN photo by Pamela Smith)

ROCKVILLE, Md. (DTN) -- With some harvests lagging and November afoot, there is a strong temptation to race to the field with fertilizer applications as soon as a crop is out.

Stay calm, watch the weather and prioritize carefully, urged a group of fertilizer specialists during a University of Minnesota Nutrient Management podcast on Oct. 29.

"We sit there and wait to get into the field, and then everybody gets really antsy to get everything done," said Dan Kaiser, University of Minnesota Extension nutrient management specialist. "But there are certain things we can look at where we have more flexibility -- I think P (phosphorus) and K (potassium) applications are one of those, where it's actually probably better to go earlier than later. But with nitrogen, especially areas where we have fall urea as a [best management practice], those are ones where I would delay as late as possible."

The specialists also included Fabian Fernandez, a nutrient management specialist who focuses on nitrogen, Chryseis Modderman, an Extension educator focused on manure management and Melissa Wilson, an Extension specialist in manure management.

They covered a range of topics, including the necessary soil and environmental conditions for nitrogen applications, proper use of nitrification inhibitors and sampling nutrient levels effectively.

Here are the highlights of the talk:


Experts agree that fall fertilizer applications should only begin when soil temperatures drop to 50 degrees, in order to minimize nitrification, the process wherein ammonia converts to nitrites, and then nitrates, which are susceptible to leaching losses.

But keep in mind that nitrification will continue to occur at 50 degrees -- it only fully stops when soils reach freezing temperatures, Fernandez cautioned. "I always encourage people to look at the forecast, and if they can wait a few more days, it's even better, because you get lower temperatures," he said.

Fall temperature swings and late-season warm spells are also becoming more common in the upper Midwest, Kaiser and Fernandez noted. Tile drainage studies at the University of Minnesota used to wrap up reliably by the end of November thanks to consistently frigid conditions, Fernandez noted. Now, with warm spells creeping into December, researchers find tile water continues to flow up until Christmas, he said.

"You know that if that tile is flowing, and if there is ammonium transforming to nitrate, then that nitrate is still subject to leaching losses," he said.

Soil moisture matters, too. Soil that is too wet or dry can prevent applicators from getting a good seal when applying anhydrous ammonia, Fernandez warned. "The best thing do is just do a short pass, and go back, and if you smell ammonia after a few minutes you know that you are losing ammonia," he said.

When fertilizing in the fall with manure -- particularly liquid manures with high ammonia contents -- Wilson and Modderman also stressed the importance of 50-degree soils and fully incorporating manure into the soil or getting a good seal.

Frozen or snow-covered soils are also bad news for fertilizer applications, particularly phosphorus (P) and potassium (K) applications and manure, the specialists noted.

"Some of our research has shown that especially on snow, most of [manure] nutrients will just end up washing off, unless they melt very slowly and get to the soil before massive flash flood events occur," said Wilson.

Beware the soil compaction dangers of fall fertilizer applications, as well, particularly if soils are overly wet. Liquid manure tankers can be especially heavy, Modderman said.

And while proper soil conditions get a lot of attention come fall fertilizer time, don't forget to check wind speeds, too, Fernandez added. "Applying lime or fertilizers when you have really strong winds, you don't know where that fertilizer is ending up -- you're not getting the spread you would want," he warned.


Nitrification inhibitors can help slow nitrification of your fertilizer product, but they are not a silver bullet that allow farmers to make applications before soil temperatures are cool enough, Fernandez said.

"At warmer temperatures, the inhibitor degrades faster, so you have less efficacy," he said. "So waiting until temperatures get cool is not only important to slow down the bacterial activity of the soil that transforms ammonium to nitrate, but it is also is important to protect that inhibitor so it stays active longer."

Nor are all these products created equal. The most common active ingredient in nitrification inhibitors, nitrapyrin, is effective, but formulations designed for use with urea or manure show mixed results, Wilson and Fernandez said.

For urea applications, Fernandez and Kaiser recommend applying in sub-surface bands, with an inhibitor, rather than broadcasting it. For liquid manure, research shows inhibitors can help, but they have limits, Wilson said. "It may delay [nitrification] a little bit compared to just raw manure, but inhibitors only last a couple of weeks," she said. "So if you're applying really early, it's not going to help that much."


When sampling in the fall to help assess your soil nutrient needs, beware of a few pitfalls, the Minnesota specialists said. First, when testing for phosphorus (P), potassium (K) and zinc, sample at 6 to 8 inches, Kaiser said. "If you go any deeper than that, you'll likely get lower results; if you go shallower, you're going to get higher results just because of how the stratification works," he said.

Be careful to sample in many different parts of cornfields, added Fernandez. As corn stalks deteriorate, they leach potassium. The wetter the year, and the longer a mature crop sits in the field, the more leaching will occur, which can create high concentrations of potassium within the rows.

Wilson actually recommends that, if possible, growers sample their manure, too, to understand the total nitrogen, phosphorus, potassium and ammonia content of your fertilizer. "That way when you calculate your application rate, you know you're being as accurate as possible," she said.

Finally, if you run out of time to sample before fertilizer applications, don't head to the field with your soil probe this fall, Kaiser said. "If you sample too soon after an application, you can see some pretty inflated values," he said.

Listen to the podcast here:


See more Nutrient Management articles from Minnesota here:


Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

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Emily Unglesbee