When applying nitrogen to corn or wheat, use efficiency is much more important than simply the amount being applied, says Greg Schwab, University of Kentucky soil fertility specialist.
“It’s not just the amount of fertilizer nitrogen you apply, but it’s the amount that actually makes it into the plant and becomes part of yield,” says Schwab.
If a producer grows corn, wheat or any other crop without fertilizer, he’ll still get yield, he says. “And we still have nitrogen uptake in that crop. That’s the nitrogen that comes from the soil. If we subtract that from our fertilizer plots, then we can tell exactly how much fertilizer and nitrogen has made it into the plant,” he says.
Unfortunately, adds Schwab, nitrogen use efficiency in most crops is not as high as most people think. Worldwide, nitrogen use efficiency for cereal crops is about 33 percent, meaning that about 33 percent of the total fertilizer nitrogen applied worldwide actually makes it into the crop.
“In the United States, we’re running better than that, but we’re still at 50 to 60 percent nitrogen use efficiency when we’re at maximum economic yield. There’s good news and bad news in that. The bad news is that you think nitrogen prices are high, but half of what you apply has not made it into the crop.
“The good news is that there’s a lot of room for improvement. We can change practices in the system and improve the nitrogen use efficiency,” says Schwab.
But it doesn’t make sense to worry about nitrogen use efficiency if your soil pH is 4.5, he says. “We once advised soil sampling every three or four years. This past year, we changed that to every two years. We made that change because soil testing is cheap relative to the price of the fertilizer that you’ll either save or need to apply. It does no good to worry about nitrogen use efficiency and managing your nitrogen properly if your soil test potassium or phosphorus is low,” says Schwab.
In Kentucky, there has been a downward trend in soil-test potassium levels in the past four or five years, he says. “I think it’s due to the price of nitrogen. Farmers know they have to put out nitrogen, and they have a limited fertilizer budget. They know they’ll get a lot from their nitrogen investment. So with potassium, they tend to slack off. But you shouldn’t think nitrogen management is going to get you the 200-bushel yields if you haven’t been concentrating on everything else in the system,” he says.
There are several keys or decisions that have to be made in nitrogen management, says Schwab, and the first is rate — what rate should be used?
“Other factors must be considered or you can’t answer the rate question very well. Rates are important, but they aren’t the only factor. If you ask me about rate, I’ll ask you which source you’re using or where you’re putting it on. You need to think about all of those things before you make a decision about rate,” he says.
By far, the best method of improving nitrogen use efficiency is by putting on the correct rate, says Schwab. “If you put on more nitrogen than you need, your nitrogen use efficiency will drop like a rock. The best thing is to use the recommended rates for your state — don’t look at Midwestern recommendations. When you consider rate, keep in mind the previous crop. In Kentucky, we make no corrections — no additional fertilizer is needed for corn after corn compared to corn after soybeans. We don’t make an increased recommendation for corn after corn because we see a parallel yield response curve. When we look at all the data together, we see very parallel yield responses.
“You’ll get a lot higher yield with corn after soybeans or wheat, but that higher yield takes just a little bit extra nitrogen. If we’re following a forage legume, then the nitrogen recommendation will be different. So look at the data and decide how much of a credit you should give to the previous crop,” he says.
Some growers, says Schwab, complain that the university never recommends enough for their high yields conditions. They believe, he says, that university recommendations are suited for average yields — about 200 bushels per acre.
However, looking at Kentucky’s state yield contest from a couple of years ago, Schwab says the winning grower was in the 150 to 200-pound-per-acre nitrogen range, making 231 bushels per acre with 160 to 170 pounds of nitrogen. In some cases, the more nitrogen that was applied, the lower the yield.
“In some cases, that can happen. If you use up your water, you’ll get a lower yield with higher rates of nitrogen,” he says.
To improve nitrogen use efficiency, growers need to understand the entire “system” or science, says Schwab. This includes the three ways of losing nitrogen — denitrification, volatilization and leaching.
Leaching, he explains, goes below the rooting zone and requires that nitrogen be in the nitrate form. “Denitrification is when your wet spots in the field appear light green. It’s when nitrogen is converted back into a gas and leaves the soil surface. Volatilization occurs when urea fertilizer converts to ammonia, leaves the soil and returns to the atmosphere.”
Understanding these reactions, he says, helps growers better understand how to manage fertilizer. “If we’re not using urea, we shouldn’t worry about volatilization. If we’re using anhydrous and it was just applied, we don’t have to worry about leaching or denitrification. But when one of these reactions occurs, we need to start thinking about how we manage our nitrogen.”
Denitrification occurs, says Schwab, when the oil is water logged, temperatures are warm, and the soil pH is high. To prevent denitrification, fertilizer applications should not be made until the soil is no longer water logged, he says.
“This is easier said than done, and it’s why we recommend split applications. If we apply a side-dress and split our nitrogen, we will reduce the amount of denitrification losses that potentially are in our fields. If we wait to side-dress, the corn plant really doesn’t need much nitrogen early in the season.
“So we can get by with a small amount early, wait until the soil is not so prone to saturation, and then apply the rest. In Kentucky, we say we can subtract 35 pounds from the recommendation if we side-dress nitrogen. You wouldn’t want to make this effort unless your soils are prone to being saturated.”
In addition, growers can use nitrogen fertilizers that start out in the ammonium form, since they are not subject to denitrification losses. They can also use nitrification inhibitors.
For surface-applied fertilizers, growers need to be concerned about ammonia volatilization, says Schwab. This easy way to stop this reaction is to stop the enzyme that does the initial conversion. A urea inhibitor targets this enzyme and stops this reaction from occurring, he adds.
“If we want to prevent ammonia volatilization, we can incorporate or inject the urea, think about alternate sources such as ammonium nitrate, or use a urea inhibitor,” he says.
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