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Plant Sunflower, Save on Fertilizer

Sunday, April 15, 2007
filed under: Fertility

You might not need to apply as much nitrogen to your sunflower crop as you think you do.

For one thing, the amount of residual soil nitrogen can be expected to be quite high in areas with dry conditions in 2006. This is illustrated in regional trends for soil nitrate levels from soil samples analyzed by Agvise Laboratories last fall. With laboratories in Northwood, N.D. and Benson, Minn., Agvise provides soil testing and plant analysis for crop consultants, fertilizer retailers and producers.

In the figures, Agvise Laboratories summarized the average fall soil nitrate levels following wheat and corn production (lb/ac 0-24” samples) in the Dakotas, Minnesota and Manitoba. The bold values indicate the areas where the average soil nitrate test was much higher in 2006 compared to 2005.

The comparative results are striking. For example, areas of South Dakota that had average soil N levels of 52, 54, and 58 lb/ac following corn in 2005 had 100, 102, and 107 lb/ac of residual soil N, respectively, following corn in 2006. This is a tremendous amount of residual soil N, and there will be many cases in drought-affected areas where the amount of N needed for the subsequent crop will be much less than a normal application.

Following a drought, soil P and K levels don’t change much compared to what they were the previous year. North Dakota State University Extension soil specialist Dave Franzen, points out that this is usually the case in most years, due to the small amount of these nutrients removed in a single year by even a bumper crop, but especially so in a small, drought-affected yield.

Soil fertility levels following other crops (including sunflower) and a snapshot of other nutrients can be found online at Click on ‘2006 Soil Test Summaries.’ See the excellent information on a variety of fertility topics under ‘technical articles’ as well.

John Lee, soil scientist with Agvise in Northwood, cautions that these numbers are just big picture snapshots to help gauge soil fertility trends. “As averages, you can expect half of fields to be more, and half of the fields less,” he says. While it is interesting to look at the big picture to see the overall trends, the only way to know for sure what nutrients your fields need (or don’t need) is through soil sampling analysis.

However, even a soil sampling analysis doesn’t give the entire picture, especially if you’re not sampling deeper. Mycogen agronomist Bruce Due reviewed Agvise soil zone sampling results for sugarbeets in the Red River Valley several years ago. He wanted to see how much N might be available at deeper levels. “With beets, they test to four feet down, because the beets need to run out of N, otherwise they don’t put sugar on. The numbers I got were unbelievable, 35 to 40 lbs of N, into the hundreds of pounds of N, that sunflower can tap into.”

That much N deep in the soil profile isn’t surprising. Franzen points out a nutrient analysis in a field near Williston, N.D., where N levels from 2-4 feet were over 200 lb/ac – heat and drought resulted in high amounts of residual N left behind in a non-leaching environment. And in the High Plains, on fields that have been on irrigated corn over multiple years, it’s not at all uncommon to find 200 to 400 lbs/ac of nitrate below the corn root zone, typically three or more feet deep in the soil. As one of the deepest rooting crops grown, sunflower will root down and extract that residual N, provided there’s not a compaction zone to inhibit root growth, and if subsoil moisture is adequate to encourage root growth.

Organic matter breakdown matters

Due says more and more data is being gathered that supports backing off from previously recommended N rates. Part of it is better plant genetics – many crop varieties and hybrids on the market today are better than their predecessors, with better root systems and improved nutrient uptake. Fertility recommendations today are also changing to take deep soil N and the breakdown of soil N during the growing season into account. Further, with $400 - $500/ton fertilizer, more attention is being given to economic optimum N rates.

Universities are adjusting their fertility recommendations accordingly. The University of Minnesota revised its nitrogen recommendations for corn last year ( and the University of Nebraska has done the same ( North Dakota State University has revised its fertility recommendations for dry beans, and expects to do the same for other crops as additional site-year data to support the fertility revisions is accumulated.

Due says a soil test in the fall or spring, and traditional nitrogen recommendations, don’t fully account for the biology that goes on during the growing season. Microbial activity is breaking down organic matter, making soil nitrates available to developing crops. A lot of this breakdown in organic matter in May and June becomes available and is used by late-season crops such as corn and sunflower in July and August. The microbial activity is greater in soils with higher organic matter (>3%) and in areas that receive more moisture. This helps explain why crops will overshoot yield goals, like in 2005, when growing conditions for row crops in the Northern Plains was ideal.

“You fertilize for 2,000 lbs, and get 3,000 lb ‘flowers. How does that happen, if you only put down enough N to meet 2,000 lbs? You don’t get good yield unless you have good moisture. If you have good moisture, you’re getting good organic matter breakdown,” says Due. “It’s causing some in the industry to think that maybe the variable rate application thinking is backward. Maybe you need the extra fertilizer on your poor ground, and not as much on your good ground, because if you’re getting good moisture, there’s enough organic N becoming available in the field to take care of extra yield needs.”

In fact, Due says too much N can be detrimental to a crop like sunflower. “I’ve had situations where a grower would apply enough N to plant corn, then the season gets late, and he puts in sunflower. With good moisture and all that N, now you have a sunflower plant that is building a big factory – tall, big leaves. You get 25,000 plants doing that, and you get a stand that’s dense, thick, lush, and prone to disease. So when soil fertility tests suggest there’s enough residual soil N to grow corn without adding much additional N, that’s too much N for sunflower.”

He says that a number of growers with high fertility levels, or on fields that haven’t seen a deep-rooted crop like sunflower in four or more years, they put down a starter fertilizer, and that’s it. “Are they going to pull 3,000 lb ‘flowers? Probably not. Can they pull 2,000? Probably. With the cost of fertilizer the way it is, a lot of growers are trying to figure out how to get by with less, and sunflower is one crop that can scavenge pretty well. And because it uses much of its N later in the season, there’s a pretty good return there in residual soil N that they can draw on that isn’t going to show up in a soil test.” – Tracy Sayler

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