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Pass the potassium, please - Some Midwestern soils are short of this vital nutrient.

In past years, potassium (K) deficiency was a mystery tucked inside a riddle wrapped inside an enigma.

Vibrant ridge-till corn suddenly succumbed to striped firing on the outer leaf edges — a classic sign of K deficiency. What puzzled ridge tillers were the high soil K levels that accompanied K deficiency symptoms. Although soil K levels were plentiful, plant roots could not access the nutrient because of nutrient stratification in soil layers. To remedy the situation, ridge tillers banded K in the fall on top of ridges so plants could access nutrients.

End of problem, right?

For ridge tillers, it was. But as 2000 showed, K deficiencies are no longer limited to ridge till. Corn in a number of Midwestern fields with more tillage — such as chisel plowing — exhibited K deficiency symptoms.

“It was the first time in 15 years I’d seen such widespread symptoms over much of the state,” says Antonio Mallarino, an Iowa State University soil scientist. “Usually, deficiency symptoms have occurred mainly in eastern Iowa fields.”

Causes. The reasons for these K deficiencies vary. Soil K levels in some Midwestern fields are low. If you’ve had problems with K deficiency in the past, it’s a good idea to soil test in order to detect low K levels. Coarse-textured soils are the most susceptible to K deficiency, says Ron Gelderman, manager of the South Dakota State University Soil Testing Laboratory.

In many other cases, though, weather triggered K deficiency. If that’s the case, relax.

“Last year, we had a dry spring in Iowa,” says Mallarino. “Dry weather limits potassium uptake, especially when it’s dry in the topsoil. There is little you can do about that if the soil is dry 8 to 10 in. or deeper.

If weather patterns change, K deficiencies could decrease, says Kevin Branick, a Pioneer Hi-Bred International agronomist stationed in Sioux Falls, SD. “If we go back to more normal growing conditions, it won’t be as big of a problem as last year.”

What can you do? Potassium deficiency symptoms do not surface until corn enters the V6 growth stage — approximately four to six weeks after planting. Because K demand increases during this time, reduced uptake effects then become visible.

Yellowing starts at the leaf tip and then progresses toward the leaf base. If K deficiency persists, it will reduce plant growth and increase stalk lodging later in the season.

Unfortunately, there is little you can do once symptoms appear. However, advance planning can help you determine if it is worthwhile for you to buy K and the accompanying application.

Switching systems. If you plan to switch to an intensive conservation tillage system like no-till or ridge till, make sure that you first beef up your K levels, says Tony Vyn, a Purdue University agronomist.

“Conservation tillage may impose an additional restriction on K availability to corn, due to stratification in the soil,” Vyn says. That’s why building up high K levels before you convert makes sense. Ridge till. If you’re a ridge tiller, deep banding K2O in the fall is a no-brainer. For the $4 to $10/acre cost of a K application, ridge tillers receive an average 8-bu./acre return from deep banding, Mallarino says. Ridge till application rates of K2O vary from 40 to 140 lbs./acre, depending upon soil tests.

Mallarino recommends banding K 5 to 7 in. deep. “Soils can dry down deeper than that, so this doesn’t always work,” he says. “But going any deeper than that requires tremendous power that isn’t worth it.” Hybrids can differ in their response to K applications, Gelderman says. “But almost all of the hybrids in our trial responded to K applications,” he points out. “So, we can best manage this by K applications, and not by hybrid selection.”

No till. The economics of K applications aren’t so bright on no-till, Mallarino says. On average, deep banding K in no-till garners growers 4 1/2 bu./acre.

“In some years, you can get an 8- to 10-bu./acre response. In some years, you get nothing,” Mallarino says. “The results are more variable.”

Less intensive tillage. On less intensive tillage systems like chisel plowing, save your money. “Unless soil tests indicate K levels are medium or lower, we don’t gain much by applying deep-banding potassium in most cases,” Mallarino says. “In cases where deficiencies do occur, they’re often weather-related ones that you can’t do anything about.”

However, soil test K levels are spotty in some of these fields, regardless of weather, Mallarino adds.

“In some of those areas, the soil test levels are really low,” he says. “We may not be able to fix those now because deficient areas are as small as a few square feet. But once we learn more about variable rate technology, we may be able to apply more K in some very small areas and less in others.”

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