There's no magic bullet to pin down the potassium deficiency issue Southeast cotton growers face. The K-problem has been a moving target for several years, varying from field to field, within a single field and year to year.
"Potassium deficiency is probably our biggest nutrient problem on cotton in Georgia and in the Southeast. We do a pretty good job with nitrogen, and we don't seem to have a lot of phosphorus problems," said Glen Harris, University of Georgia crop fertility specialist.
Harris and other fertility specialists across the region have addressed the issue for growers for many years. He talked about it during the 2020 Alabama Row Crop Short Course. The annual course wasn't held in person in 2020 but is available now as an online video series.
Current commercial cotton varieties fruit faster and mature earlier than older ones. This can open the gate to K deficiency, he said, but that's not the whole story.
Growers, scouts and county agents know the signs: Leaves lose their green (chlorosis) and maybe turn reddish and cup. In tough cases, defoliation happens. The K-problem is a problem when and where it's a problem, and, yes, 2020 was a problem year for it.
"A lot of people think we had issues with potassium (in 2020) because we had a lot of rainfall. I'm not sure that was the exact reason. And, yes, we received a lot of rainfall, but we've had rainy years before. And in fact, a lot of people are asking, 'Does our potassium leach more than it used to?'" he said.
Unlike nitrogen, which can readily move in soil, potassium is more stagnant and available if the soil pH and calcium levels are good.
The sandy soils of much of the Southeastern Cotton Belt typically pose a problem of low pH for growers to correct, but in recent years, he said, some growers are pushing pH too high. A high pH, or around 7.0, which he's seen, combined with calcium levels at 2,000 or 3,000 can interfere with the soil's ability to hold onto K on the cation exchange capacity. “Think of it as musical chairs, there are only so many seats and if they are largely occupied by calcium then K doesn’t have a seat and can leach out."
Growers who use standard soil sampling, or one composite sample per field, should shoot for soil pH between 6.0 and 6.3. Grid sampling, typically on 2.5-acre grids, and variable rate liming are encouraged, especially in large fields traditionally sampled and diagnosed with soil pH problems, he says.
All K requirements should be applied preplant on all soil types such as Piedmont, Coastal Plain and Deep Sand soils.
"Split applications are getting a lot of attention, but recent research hasn’t shown any benefits to yields between putting all your potash out at planting verses splitting it up between planting and sidedress or even later applications," he said.
Recent field trials in Georgia looked at additional soil-applied K during N sidedressing versus foliar K applications during peak bloom. The studies conducted on Coastal Plain soils show foliar K may be more effective than sidedress K in improving yields. Harris said more research is needed on Deep Sands to see which practices benefits yields the most.
Looking ahead to the 2021 growing season, Harris recommends K foliar applications on:
- Deep sands, or more than 18 inches to the clay subsoil clay.
- Low-K soils.
- High-yielding conditions.
- Short-season varieties.
- Severe K deficiency where leafspot symptoms have been seen. For this situation, two foliar applications with 5 pounds to 10 pounds of K2O in each application during first thru fourth week of bloom.
Commercial additives are available and designed to improve K and P uptake. In Georgia cotton studies, consistent benefits from the additives hasn't panned out.