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When N Isn’t the Answer | Listen Up, Especially if You Have Low Soil Organic Matter on Corn Ground


If you have low soil organic matter, you may be prone to sulfur (S) deficiency. This is more likely on the eroded knobs and side hills common in southern Minnesota, northeast Iowa and parts of Wisconsin and Illinois.

Low soil organic matter limits S provided through mineralization from organic sources. That can lead to an S shortage, especially in a corn-corn rotation, according to Daniel Kaiser, a University of Minnesota Extension soil fertility specialist.

Growers ask whether the yellowing [on the knobs] is from a nitrogen (N) deficiency or S deficiency, says Kaiser, noting that some farmers have applied N to green up yellowed corn without getting a yield response.

 “You may have N symptoms, but it’s worth considering some S application. It’s probably going to increase yield the vast majority of the time,” he says.

Kaiser’s field studies, which looked at corn’s response to several starter fertilizer combinations, found that early plant growth was consistently improved with starter applications of phosphorus (P) but that growth did not translate into better yields. Early application of P and S did, however, improve S uptake.

Sulfur produced better yields consistently when soil organic matter was below 2% and sometimes when matter was between 2% and 4%.

“Our data show you don’t need a lot – maybe 10 lbs./acre for corn based on 2009 data,” Kaiser says, reporting that yield improved on average by 20 bu./acre at some experimental sites where sulfur was applied.

“Typically, we get a lot of farmer questions about the timing of application. There is a concern about leaching losses,” Kaiser explains. “If you look at past research, S doesn’t leach as fast as N, so you may apply it in the fall or spring, depending on the form.

“We have seen some evidence of a yield response from the previous year, but if you’re looking for the most availability, application just before planting or at planting is the way to go,” he says.

Corn and especially alfalfa can tip growers off to a possible S shortage, according to Kaiser. “If you’ve got yellow in the field on these soils, it’s worth it to go out and look at the plants and look at some S application.”

You’re likely to get the maximum economic benefit from spot treating knolls and side hills, says Kaiser. However, some areas with significant amounts of target terrain may gain enough yield to justify sulfur application across entire fields.

Kaiser adds that some of the worst S-deficient fields he’s seen are in a corn-corn rotation: “Dent corn in a corn-corn rotation may require 10-15 lbs./acre of S/year. Depending on where you look, that should take care of most needs on fine-textured soils, but probably 25 lbs. is needed on sands and possibly some other circumstances.” 


January 2011

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