Crop Nutrition

High-Quality Forage in the Southeast Requires Higher Potassium Levels

eKonomics News Team

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Forage production in the southeastern United States has led to critically low levels of potassium (K), prompting agronomists to call for frequent soil tests in coastal areas with sandy soil and heavy rainfall.

The growing severity of nutrient removal in forages extends through the coastal plain – beginning in Virginia, east of Richmond; south into North Carolina, east of Raleigh; South Carolina, east of Columbia; and across southern Georgia. In Florida, K loss is seen throughout the state.

“A lot of agriculture is in the coastal plain,” says Dr. Mark Alley, an agronomy consultant and Professor Emeritus at Virginia Tech. “Florida is entirely coastal. Bermuda grass used as forage for hay is grown on sandy soil. Rainfall averages as much as 70 inches a year in some areas of Florida, which means the potential for leaching of K from the soil is high.”

Maintaining adequate potassium is necessary for healthy growth of grasses and legumes, such as alfalfa, clover, peanuts and soybeans. To succeed, Alley recommends farmers update nutrition management to aggressively monitor K levels and fertilize throughout the growing season.

Why K Levels Are Dropping

Producers in the Southeast cut back on applications of K in recent years due to escalating fertilizer costs and declining prices for crops and livestock. Just like in the Corn Belt and other farming regions in the United States, curtailing inputs led to soil nutrition levels reaching critically low levels.

According to the industry’s first state-by-state assessment of nutrient removal records, most of the southeastern states show declining K soil test levels.

For example, K balance trends from 1975 to 2015 were down 380, 73, and 180 percent for Virginia, North Carolina, and South Carolina respectively. Due to inadequate data, nutrient balance trends for Florida and Georgia could not be calculated. However, the number of soil samples testing below the critical levels is increasing.

Major Potassium Losses in Every State

A large percentage of soils is becoming depleted of macronutrients at an alarming rate. In the following southeastern states, the percentage of soils below critical levels of K includes:

  • 85%   Georgia
  • 85%   South Carolina
  • 75%   Florida
  • 69%   North Carolina
  • 53%   Virginia

“It’s basic economics,” says Alley. “Farmers look at where to cut costs. When they see they’ve got a medium to high potassium soil test level, farmers think that means they can get by without fertilizing this year. They’ll worry about it next year. When that happens over several years, forage cropping removes more K from the field.”

Compounding the challenge for producers in the Southeast are the sandy soil conditions and heavy rainfall in the coastal plain. Compared to other regions in the United States, soils in the Southeast contain less clay. This causes the soil to retain less K, which is positively charged in the K+ form, and is held by negatively-charged clay on cation exchange sites (cation exchange capacity [CEC]).

To deal with these conditions, soil testing must be performed more frequently. Alley recommends at least once a year, but state extension services and independent crop consultants can provide specific local guidance.

Fertilizer applications also must be carried out more frequently. Farmers have reduced K inputs while continuing to fertilize with nitrogen (N). They did so because K is not mobile in water, but nitrogen leaches at high rates and must be replenished at least once a year. In addition, harvesting forage crops takes all of the biomass of the plants from the field, multiplying the rate of K removal.

“We’re taking off more potassium than we are returning throughout the southeastern states,” says Alley. “In tough economic times, Southeast farmers can’t cut N because it must be applied every year. Potassium, however, is not affected by the wetter soil conditions, so farmers don’t apply these inputs every year. Farmers who have not managed this situation well have hurt their yields and suffered financially as a result.”

Add or Increase Soil Testing and Fertilization

Alley encourages southeastern forage and grazing crop producers to consider multiple applications of K along with N every year. That’s a radical departure from conventional nutrition management approaches, but he argues it’s a necessary change to improve nutrient levels and therefore plant health and yields in the coastal plain.

Forage and grazing crops that can benefit from more K applications include alfalfa, clover and other legumes, which provide high-quality feed. Legumes contain large amounts of protein, calcium and magnesium needed by livestock.

Legumes, however, must compete for soil nutrients with grasses, and their growth is stunted by low K levels. Grass development accelerates when N is applied and the grasses drive out the legumes. The challenge for farmers in the Southeast is to better balance the levels of K and N to promote a mix of grasses and legumes.

To achieve that balance, farmers should regularly test soil for nutrient levels and apply potassium as needed during the growing season. Alley says there are bonuses to adopting these practices. Maintaining high levels of available K in soils can save producers money. Less N is needed and farmers are more likely to produce a higher quality of forage with healthy legumes for grazing.