Crop Nutrition

Coastal Plain Cotton Growers: How Long Can You Go with Low K Levels?

eKonomics News Team

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Here is the big question for cotton growers in southeastern states: When should farmers modify their nutrient management plans to ensure proper potassium levels for cotton production?

The answer from agronomists and soil scientists: make the changes soon. Nearly half of the states in the cotton belt have soils below the critical level for potassium. Without better nutrient management based on soil test levels, yield, and lint quality may decline.

In some southeastern states, such as Georgia, the impact is obvious. Plant diseases, such as Stemphylium leaf spot, are more common because potassium-deficient cotton plants are vulnerable to diseases.

“It is well known that K adds strength to plant leaf cells, and the lack of K makes them weak and susceptible to secondary fungal infection,” says Dr. Glen Harris, Professor and Extension Soil and Fertilizer Specialist at the University of Georgia. “Once leaf spot appears, fungicide sprays do not alleviate the condition, since the primary problem is K deficiency.”

Across much of the southeastern states, coastal plain soils do not contain a lot of clay or organic matter, both of which serve as a nutrient reservoir. Clay and organic matter have a net negative charge that holds positively charged nutrients (potassium – K+ being one). This reservoir of net charge is known as Cation Exchange Capacity (CEC).

As an example, the CEC levels in Virginia can be as low as five. That’s according to Dr. William Hunter Frame, Assistant Professor at Virginia Tech, who specializes in cotton crop nutrition and works directly with growers. He explains that the low CEC allows for leaching of K during heavy rains, weather conditions typical of Virginia Costal Plain soils where the majority of cotton is grown.

“The nutrient uptake of cotton is the same as in other states, but what’s unique about growing cotton in the coastal plain is, we’re worried about losing potassium,” Frame says. “Our soils cannot hold K. When we get heavy rainfall, the K can leach out of the root zone, which causes nutrient deficiencies. And the soils inherently are not able to supply enough K for cotton.”

Figure 1: Unique Soil Conditions in the Southeast

Many states in the southeast have coastal plain soils similar to those in Virginia and consequently, also have the majority of soil test below critical potassium levels. Source: nutrien-eKonomics.com Nutrient Balance Analysis; updated March 2018.

Virginia Knows Its K Levels

Among the Cotton Belt states, Virginia is unique. Cotton is grown in just seven counties, all located between Chesapeake Bay to the north and the Albemarle Sound in neighboring North Carolina to the south. Nearly 90% of Virginia cotton growers regularly test soils and apply K and other fertilizers based on the soil nutrient levels.

Virginia Tech agronomists drove the push for soil testing starting in the 1990s. They’ve been teaching growers how to optimize their fertilizer investment by using precision cropping systems based on soil testing. “For a long time now, we’ve had a very forward-looking emphasis on soil fertility as a state,” Frame says.

One third to one half of the Virginia cotton producers who soil test use the results to precisely map and sample their fields, according to Frame. They map fields using GPS coordinates, then sample soil around the specific GPS locations. The test results are used to make decisions about which fertilizers to apply and at what rate, both for entire fields and for areas within fields.

Historically, the standard K application has been a 6-16-36 (N-P-K) blend at 300 to 350 pounds per acre. At 300 pounds of that blend, the total nutrients applied would be 18, 48, and 108 pounds of nitrogen, phosphorus and potassium.

“Cotton producers in Virginia no longer use that standard program for fertilization,” Frame says. Farmers are being smarter about nutrient management and applying fertilizer based on soil test as opposed to the traditional approach.

After observing the growers’ practices, Frame is convinced that K soil levels in the seven Virginia cotton-producing counties are higher than those reported for the entire state. One reason for this is the more intensive soil sampling.

“The crop demand for K during bloom can be very high, and the ability of our soils to supply potassium is not quite where it needs to be,” Frame says. “Virginia cotton producers tend to be very progressive and try innovative ways to improve their cropping systems. We’re doing more research, but going with split application methods to reduce the loss potential from leaching can really help our growers mitigate some of the deficiencies.”

Frame believes that the shift to split potash applications at planting and again during the growing season helps cotton farmers avoid K loss that would otherwise occur in the coastal plain soils. That said, Frame wants more research, along with re-evaluation of crop management, because agronomists and soil specialists still see K deficiencies.

Georgia Encourages Soil Sampling Every Year

Georgia is another southeastern state where cotton growers must produce crops on coarse-textured soils low in organic matter, meaning a low capacity for holding potassium.

Current recommendations suggest a soil test level of 125 ppm should be adequate to ensure proper potassium nutrition for cotton. This recommended level has not changed even as farmers adopted new varieties which produce higher yields and therefore, take up more nutrients than previous cotton cultivars. The Georgia Extension team encourages farmers to base fertilization decisions on regular soil test levels.

Figure 2: Georgia Coastal Plain Soils: K Thresholds and Fertilization*

*For 3 bale per acre yield goal and Melich-1 soil test extractant. Source: University of Georgia Extension research.

Conducting the sampling and analysis every year is important. However, the success of such an intensive approach relies not just on its frequency, but on the number and location of the soil samples. Harris urges farmers to understand the sampling method that a crop adviser or sampling service uses. Some people take cores from five or six locations near each sample point on the grid. However, the best practice is to take a dozen or more cores from random locations near each sample point.

“When a crop doesn’t do well, growers have a tendency to think it’s bugs or diseases when it could very well be potassium deficiency,” Harris says. “People get more excited about killing weeds or killing bugs. When it comes to fertility, it’s almost like they think it’s so simple a monkey can do it. Well, it is basic, but it’s also so important. Farmers should follow the fertilization recommendations based on the testing. Do a good job with potash and let something else be a yield-limiting factor.”

Want to Stay Profitable? Follow the Soil Test Results.

The high yields in recent years can lull cotton growers into a false sense of security, because the declines in K levels in soils in southeastern states leave plants with insufficient nutrition. It’s not a matter of if that will happen, but when, agronomists say. The result is likely to be decreased yields and poorer fiber quality — and a corresponding drop-off in profitability for cotton producers.2

What’s Happened to K Levels in Your State?

Read the updated nutrient balance analysis from eKonomics.

Find Your State

In the end, the best strategy for cotton growers who want to avoid K deficiency remains intensive soil testing — as frequently as once a year for coastal plain soils in southeastern states. Only after soil testing can cost-effective fertilization decisions be made. Based on soil nutrient levels, K can be applied both before planting and during the growing season. This approach can improve plant health and ultimately yield, while also helping growers reach their profitability goals.

Sources

  1. University of Georgia Extension, “Cotton Production Guide,” 2018.
  2. Gaylon Morgan, Hunter Frame, Danny Fromme, Darrin Dodds, Keith Edmisten, Mike Jones, Randy Norton, Andrea Jones, Bill Robertson, Randy Boman, Tyson Raper, Katie Lewis, Dennis Delaney and Austin Hagen, “Impact of Soil Applied Potassium on Cotton Yield, Quality, and Plant Growth across the Cotton Belt,” 2016.