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Crop Advisor’s Casebook: Yellowing soybeans a mystery

A Crop Advisor's Solution from the February 3, 2015 issue of Grainews

Ryan Turnbull

Ryan Turnbull
photo: Supplied

An experienced hand at producing canola, wheat and barley on the sandy loam soils of his 4,200-acre farm east of Dauphin, Man., Brent was brand new at growing soybeans. Just after Labour Day, Brent gave me call to talk about a concern he had with his soybean crop as he prepared for harvest.

“I think the plants in some parts of the field might be ripening faster than others,” he said. “I don’t know if this is normal or not, so I’d appreciate it if you could come out and have a look.”

After arriving at the farm, I was shown the plants in Brent’s soybean field that appeared to be ripening much more quickly than others. Their leaves were turning yellow and brown, and in some extreme cases, looked crispy and even burnt.

I scanned the field for a pattern for the affected plants, and it appeared the yellowing was greatest at the approach and headland areas. The field was generally fairly level, but wherever there were small ridges or hilltops, some yellowing was occurring there as well.

I checked for insects but could not detect any aphids or spider mites that might be causing problems. Brent told me he hadn’t applied any fertilizer to the crop, since soybeans can fix their own nitrogen and previous soil tests confirmed adequate levels of other nutrients.

I had seen this kind of issue with soybeans before, and had a pretty good idea was going on in Brent’s field. I took plant tissue and soil samples in both the affected and unaffected areas, and subsequent testing confirmed my suspicions.

Crop Advisor’s Solution: Potassium deficiency

As I mentioned, I’d seen this type of yellowing pattern in soybean crops before, and I had a pretty strong suspicion what was causing it. Test results from plant tissue and soil samples taken from affected and unaffected areas within Brent’s field confirmed my diagnosis: a potassium deficiency was the culprit.

Brent said he hadn’t applied fertilizer to the field because previous soil tests had indicated nutrient levels were adequate. However, I explained that at this point in early September the growing season might be over for cereal or canola crops but the soybean plants were continuing to develop. They still required a fair amount of potassium during their critical seed fill stage.

Potassium moves very little in the soil, especially when it is dry or compacted as was the case at this late stage of the growing season. Compounding the problem was that sandy loam soils like that in Brent’s soybean field tend to be low in potassium.

The result of all this was that in the earth underlying the high points in the field — the headlands, ridgelines and hilltops — there was simply not enough potassium available to match crop demand. Plant tissue and soil samples from unaffected areas of the field indicated potassium levels were low but still sufficient for growth. This wasn’t the case in the poorer areas of the field, where potassium levels were in the deficient range.

If the problem had turned up earlier in the year, it’s likely that a foliar potassium application may have helped. However, with only a short time before harvest, there wasn’t much that could be done for Brent’s struggling soybeans, which were smaller, had fewer pods and finished earlier than the rest of the crop. Yields were lower as a result, but fortunately for Brent, the affected areas comprised only a small portion of the field.

Going forward, Brent will benefit from a revised fertility program tailored for his new crop rotation that now includes soybeans. With just 130 parts per million of potassium in the top six inches, Brent’s field is sufficient in potassium but only just — the plan is to build up the potassium level in the soil so that this essential nutrient is there when the plant really needs it. For lighter soils, like that in Brent’s field, it is particularly important to address the long-term nutrient needs of the whole crop rotation, especially for potassium but for phosphorus as well.

Ryan Turnbull is a sales agronomist with Richardson Pioneer Ltd. at Dauphin, Man.

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