Low Potassium Symptoms Show in Soybeans

Seed rep says it's number one symptom so far.

You'll hear talk about soybean aphids and rust down at the fertilizer dealer or even your local coffee shop. And while aphids haven't shown up yet in Indiana, at least not in many numbers, it's worth watching fro them. It's also worth keeping an eye and ear out for rust, although again, as in the past two seasons, it doesn't appear to be shaping up to be a season that would favor rust moving this far north early enough to cause damage.

But you likely won't hear are farmers talking much about potassium deficiency. It may seem like an old-time problem and rather mundane compared to the newer set of issues facing soybean growers. But at least one seed rep says it's very real, and believes farmers ought to be paying attention to it.

"The number one symptom I've seen in soybeans so far in soybeans is potassium deficiency," asserts Bill Cobbler, head sales rep for Stewart Seeds, Greensburg, Ind. "I've seen more fields showing signs of potassium deficiency than showing any other symptoms so far this season."

Whether the nature of the season, with a very dry start and sporadic rains since, is contributing to the potassium deficiency symptoms expressed in leaves might be up for debate. But Cobbler says he's definitely seeing more symptoms and hearing more about potassium deficiency in soybeans this year than in the recent past.

Other agronomists, however, have been noting that potassium deficiency is a threat to highest yields for some time. While Indiana soils tend to test relatively high in phosphorus, with some exceptions, of course, they don't rate as high on potash overall. Part of this likely relates to past fertilization practices, going back decades, when bagged 6-24-24 was the fertilizer of choice. In relative terms, that puts on a bigger percentage of phosphorus needs than potash needs.

Signs of potash deficiency in soybeans include browning around the leaf edges. If the browning or discoloration is in the middle of the leaf or veins, it's probably some other problem. But if the edges turn uniformly brown for an inch or so around the outline of the leaf, it's likely potassium deficiency.

If you see such symptoms, it might be another reminder that you need to reevaluate both your soil testing and soil fertilization programs. Are you testing on a regular basis? What about on rented fields- are they tested? How up to date are your latest soil sample records and recommendations for each field? What is the potassium level in each field?

The Purdue University Crops Training Center Field Guide has very descriptive pictures that illustrate potash deficiency. Reach the Purdue Training center for more info at: www.agry.purdue.edu/dtc.)

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