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What Do Those Yellow Leaves Mean?

What Do Those Yellow Leaves Mean?
Diagnosis requires walking field.

By Dave Nanda

Telling your crop consultant or Extension educator that there is something wrong with your corn in a certain field isn't very descriptive. It also won't help get a diagnosis any faster if he asks what's wrong with it, and you say it's turning yellow.

Yellow discoloration can result from lots of causes. If the entire field is stunted and early, it may be an indication that the corn has struggled through a cool, wet period. Perhaps the area where corn looks like this needs drainage. Walking fields and careful observation and note taking will help you zero in on spots that need attention.

Today note taking can still be in a simple, small notepad that fits into your shirt pocket very easily. Or you could take geo-referenced notes with any number of handheld devices that run software and allow you to record positions so that you could learn them again. If you're not ready to invest in the $2,000 to $5,000 level these devices and accompanying software total, revert to keeping a legible, notebook

If the yellowing is running down the midrib, that's likely a signal for nitrogen deficiency. However, if the yellow and browning are around the edges, especially of lower leaves first, it might be potash deficiency.

Corn uses lot of potash. For years farmers applied 6-24-24 fertilizer. Phosphate needs for corn on Indiana soils are generally met faster than potash needs. That's why there are many fields with high phosphate soil test levels that still may be low to moderate on phosphate levels.

As a result, about a decade ago consultants and agronomic specialists noted potash deficiency showing up more often, both in corn and soybeans. Sometimes the black ground is where it's deficient first, because that area has produced the most corn and had the highest potash removal level.

Typical symptoms of potassium deficiency are leaves getting yellow on leaf edges first, then spreading toward the middle and tip of the leaves.

In fields where some farmers have relied on high-priced, low volume popup fertilizer, potassium levels have been drawn down over the years. Simply not enough total products have been applied. You can find dark, Brookston soils in Indiana that test 100 pounds per acre or less for potassium due to this factor.

The remedy is relatively simple. Either apply more potassium, soil test more often and adapt variable –rate spreading techniques, or be satisfied with average yields. (Nanda is a crops consultant and director of genetics and technology for Seed Consultants, Inc.)

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