Potassium deficiency in cotton can severely restrict yield and reduce grades. In 2006, potassium deficiency was a widespread problem in parts of Virginia.
Early signs of potassium deficiency, primarily yellowing between the veins of the cotton leaf, showed up in Virginia in late July. Once it occurs, it is too late to do much about it.
Virginia Cotton Specialist Joel Faircloth says there are numerous reasons for potassium deficiency in Virginia.
The first is a result of 900-plus pound per acre cotton yields in 2004 and 2005 and a high boll load in 2006. Successive high yielding cotton crops have placed a high demand on all nutrients, including potassium.
In 2006, early spring rains restricted root growth in early planted cotton throughout the Virginia cotton-producing belt. The cotton crop was set up with a small root mass, then the drought came, and there was no extended root system to go out and get nutrients — those in short supply were the hardest to absorb for the cotton plant, Faircloth explains.
“We have seen more potassium deficiency this year, and it tends to show up more in sandy soils than in heavier soils,” Faircloth says. At the Tidewater Area Research and Extension Center in Holland, Va., potassium deficiency showed up about mid-season, according to Faircloth.
“Once the margin of the leaves start to crinkle from potassium deficiency, leafspot sets in, ethylene production increases, and leaves start to fall off. Then, growers don't have to worry about defoliation, but they will see some significant yield losses, Faircloth contends.
In USDA testing, a 60 pound per acre drop in uptake of potassium resulted in a 500 pound per acre yield loss. Most of the yield losses caused by potassium deficiency can be avoided by proper soil sampling.
Advanced grid sampling and zone sampling techniques can make fertilizer application very precise.
Likewise detecting the problem early can give growers some options. In the early stages of potassium deficiency, cotton plants begin to turn a darker shade of green. The top leaves then will become mottled with light green to gold mottling between the veins.
If leaf petiole analysis is taken at this stage, these leaves will likely be in the 1-2 percent range.
As potassium deficiency continues to develop, top leaves continue to turn yellowish-green to gold and leaf edges begin turning brown, but bottom leaves usually appear normal.
Within 7-8 weeks of the onset of potassium deficiency, leaves turn a reddish-brown in color and are shriveled and curled. Most bottom leaves still appear normal.
Potassium deficiency is sometimes confused with verticillium wilt in cotton. With verticillium wilt, the vascular tissue of the stem is discolored and light to dark brown in color.
To determine if verticillium wilt is present, the stem should be sliced with a knife and examined for vascular discoloration.
In Georgia, growers have had major problems with early-occurring potassium deficiency. In some cases cotton has been totally defoliated by the fourth week of bloom. Severe drought throughout the growing season in 2006 over-shadowed all production problems, including potassium deficiency.
In the northern Delta potassium deficiency is an ongoing problem. Researchers there have determined that in some cases spraying potassium on cotton during late-season can correct the problem and significantly reduce yield losses.
A shorter growing season in Virginia may be contributing to the increase in potassium deficiency. Increases in potassium deficiency are partly due to a premature end of reproductive growth. Potassium deficient cotton plants also tend to set more flowers, over 10 percent in some tests, adding to the plant's demand for nutrients.
Though early maturing cotton varieties have been linked to potassium deficiency, there is little research data to support this association.
Cotton continues to be a popular crop, and a new crop, for some Virginia growers who have stopped growing peanuts and tobacco. Growing cotton requires some special care. “It is clear we need to pay more attention to potassium deficiency in Virginia,” Faircloth says.