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Fertilizing ’Flowers in '08

Wednesday, April 2, 2008
filed under: Fertility

If ever there was a time when it made good sense (and dollars) to pay particularly close attention to the combination of soil fertility levels and crop nutrient needs, it’s now. With nitrogen and other fertilizer inputs at record-high prices, producers of all crops — including sunflower — need to come as close as possible to marrying an optimum yield with “just enough” applied fertility to achieve that yield.

That’s why numerous agronomists stress the importance of producers going well beyond university fertility recommendation tables in determining their crops’ needs. While the tables are quite valid in outlining the amounts of N, P and K required by a given crop for a specific yield goal, they serve simply as a starting point. From there, growers need to plug in a lot more information to come up with the most cost-effective input strategy.

“Looking at the cost of fertilizer today, there are a lot of variables a grower needs to examine in his own situation to see if the university recommendations make sense,” says Bruce Due, Breckenridge, Minn.-based agronomist with Mycogen Seeds. One of Due’s chief concerns with ’flowers specifically is that standard Upper Midwest fertilizer recommendation tables were developed decades ago when both (1) fertilizer prices and (2) sunflower hybrids were very different from those of today.

Dave Franzen, extension soils specialist with North Dakota State University, agrees the time is right for updated nitrogen formulas in particular. The NDSU-developed sunflower tables were based largely on studies done in the late 1970s and early 1980s, and “there was a reason they came up with those formulas; there were data behind them,” Franzen says. But, he adds, “I think the time is upon us — given the newer hybrids and higher-priced nitrogen — to consider new nitrogen calibration studies on sunflower.” The problem for most universities, however, is resources — human and monetary. In North Dakota, for instance, “we’re really busy on the wheat calibration project right now,” Franzen relates, “and corn is probably next in line.”

Bruce Due believes sunflower can be viewed in a light similar to what’s presently going on with corn nitrogen recommendations. “Over the past four years, the nitrogen recommendation on corn has been dramatically changed across the country,” he says. “We used to work off an old formula developed in the mid-’60s. At that time, N prices were very inexpensive; so the primary yield goal was always associated with the top of the curve — wherever you could add a little fertilizer and get some yield, you did it. That has changed dramatically in the past two years in particular — just because of N costs.”

Substantial progress in plant genetics has also brought the value of old formulas into question. “Yields have continued to go up,” Due points out. “That’s true for most crops — and certainly for sunflower and corn.

“Oftentimes, the way you get increased productivity is from a better root system. That not only gives you better standability; it also provides better uptake of nutrients and moisture. And if you have better nutrient uptake, tables that were designed in the ’60s or ’70s don’t account for the improved genetics that are in the field today.” For example, the old corn formulas recommended about 1.3 lbs of nitrogen per bushel. “Now, a lot of states are using 0.9 or 1.0 lb/bu,” Due relates. “That’s nearly a 25% decrease in the recommended nitrogen rate based on a particular yield goal.”

The seed company agronomist believes the same trend holds true with sunflower. “I put these two crops — corn and sunflower — together because they’re both full-season, later-maturing crops, and the way they utilize nitrogen is very similar.”

So what kinds of questions should today’s sunflower producer be asking himself to best determine his crop’s nitrogen requirements?

The first is field history. If you’ve been farming a given field for a long time, you know its cropping and fertility history; but on newly rented or purchased ground, you may not. “If the fertility program has been good over the past five to seven years, that’s a big consideration,” Due observes. “The more nitrogen, phosphorus, potassium and micros we’ve been routinely putting on the ground, the better chances there’s subsoil N the sunflower can go down and use. Barring a serious hardpan, sunflower can go down four to five feet and secure nutrients. I think there are a lot of fields that — if we’re into a four- or five-year or longer rotation — can have 20 to 25 lbs of N below the soil profile that’s being tested.

“The number I use with the ‘average’ grower, if he’s in a four-year or longer rotation, is a credit of 20 lbs of N below two feet. And if he’s going on ground that hasn’t been in sunflower for even longer, likely he can credit more than 20 lbs.”

Obviously, the only way to know for sure is to soil test. “But most people — especially outside of the Red River Valley — aren’t going to do a two- to five-foot soil probe,” Due points out. “So that’s where some assumptions must be made. You need to go back and analyze (1) what crops have been on that ground, (2) what the fertility program has been, and (3) the sizes of the crops coming off that ground over the past several years.

“If you’ve had a couple near crop failures, for example, a good share of the nitrogen you’ve put down is likely still in that soil profile — albeit probably not in the upper part. On the other hand, the more moisture you’ve had, the better the odds the nitrogen has been driven deeper.”

Due also views “soil bank nitrogen” as a big factor with corn and sunflower, given they are late-season crops. A soil test drawn in late fall or early spring provides a picture of what’s in the soil at the time of that test. If a crop like soybeans, dry edible beans or peas was on that field the prior season, there may be a 40-lb/ac credit. The reason for the credit, Due says, is not because those crops leave nitrogen in the soil. Instead, it’s because “the curve in the carbon-to-nitrogen ratio on such crops is very low. So the next spring, when temperatures warm and soil microbes start working on organic matter, the residue material from those crops can be turned around quickly and put back into the soil in the form of [available nutrients].”

Since microbial activity is tied to soil temperature and moisture, the microbes naturally are much more active in late May, June and early July than in April or early May. “So a crop like wheat, barley or canola — all being relatively early maturing — will not make as much use of those nutrients becoming available as will later-maturing crops like sunflower or corn,” Due points out.

“It’s based on organic matter. With a crop like sunflower, which isn’t using a lot of nutrient until the middle of July, there’s considerable nutrient that has been turned back to us — through the decomposition of organic matter — that the soil test doesn’t account for.

“Over the years, as a rule of thumb, I’ve found we can credit five to 10 lbs of nitrogen per percent of organic matter,” Due continues. “So if we have a 3% organic matter, I can usually count on 15 to 30 lbs of N becoming available to the sunflower, through microbial action in the soil, by the time the crop really needs it.”

Moisture, not nutrients, is most often the biggest yield-limiting factor in sunflower, Due states, illustrating his point with this example: “It’s easy to get a group of growers together and ask them, ‘Have any of you ever established a yield goal — fertility-wise — of 1,800 pounds; then had real good moisture conditions; and taken off 2,400-lb ’flowers?’ Virtually everyone in the room will raise their hand.”

Why? “It was because of the organic matter in the soil. If you get good moisture, the microbes are ‘doing their thing.’ In a wet, good-productivity year, you’ll always exceed the yield goal of the fertilizer you put down — simply because extra nutrition is becoming available through organic matter breakdown.”

Given a scenario where (1) a soil test is showing 35 lbs of nitrogen in the 0-2’ depth and (2) there’s an organic matter level of 3%, Due says he’d feel comfortable giving an additional N credit of 30 lbs based on the soil’s organic matter. “Then I’d look at my rotation. If there hadn’t been ’flowers or another deep-rooted crop on that ground for four or five years, I’d take another 20 lbs, thus accounting for N that is below the depth of the soil sample taken.” — Don Lilleboe

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