Friday, July 30, 2021

The quiet nutrient

Don’t ignore sulfur deficiencies, which have become more common as atmospheric emissions grow cleaner.

• By Larry Oldham •

Normal plants (left) and S-deficient plants (right) — photo courtesy North Carolina State University

Sulfur is essential for growing plants. It is a component of two of the amino acids that make up proteins. According to The Fertilizer Institute, 200 bushels of corn per acre remove 16 pounds, and 60 bushels of soybeans per acre remove 11 pounds S per acre. From North Carolina State: cotton seed and lint remove about 5 pounds S per acre.

Practically all S used by plants is absorbed as the sulfate ion (SO42) from soil. Most S already present in the soil or added by manures such as poultry litter is in organic forms that become plant available when mineralized to the form used by plants.

Rates of these microbally mediated conversions depend on temperature and moisture in the soil

For many years, soil S was supplemented by atmospheric deposition – it literally fell out of the sky. However, with improved smokestack scrubbing technology and widespread use of automobile catalytic converters following implementation of the Clean Air Act, S deposition decreased over the decades.

Prone to leaching

Another similarity with nitrate nitrogen is because the plant-available sulfate ion is negatively charged, it is prone to leaching downward through the profile with water movement. Leaching loss of either ion is more likely in coarse-textured, sandy soils.
Sulfur also was once co-applied through some pesticides and phosphorus fertilizers of which it was a component. Over time, these sources either have been refined to less or no S content or are no longer used.

Soil testing for phosphorus and potassium provides insight into plant-available levels of each in the soil and provides the basis for relevant recommendations. With S, however, similarities with nitrate (e. g. mineralization from organic matter, leaching vulnerability) also hamper developing soil tests that can provide interpretable results in our warm, humid southern climate.

Note: Public laboratories in Tennessee, South Carolina and North Carolina provide some S soil testing services. The Mississippi State University Extension Soil Testing Laboratory provides estimated soil S if organic matter is requested. The estimation of soil S based on organic matter is based on past research from studies completed in Mississippi.

Symptoms and solutions

Determining whether S deficiency exists depends on observation and using what you know. Based on the issues above, sulfur deficiencies will occur first on sandy, low CEC soils and/or low organic matter soils that may be higher in the landscape, such as a ridge.

Young leaves that are pale green to yellow in color are a symptom of S deficiency. Older leaves on the same plant may be darker as S does not relocate within growing plants.

S-deficient plants will be spindly and small with slow growth and delayed fruiting. Early season deficiencies do not always carry forward to reduced yields because soil organic S, the chief source of “natural” S, mineralizes more rapidly to the plant-available sulfate form as it warms up.

Deficiency can be addressed early in the growing season with a fertilizer containing readily available S, i.e. sulfate, but note the location and adjust nutrient planning for the next crop.

Several sources of fertilizer sulfur are available (see https://bit.ly/2LXrW9T). Elemental S must be oxidized to the sulfate form before it is available to plants, which takes several months.

Readily available (in the plant nutrition sense of available) S sources include ammonium sulfate, potassium sulfate, gypsum, and zinc sulfate. The fertilizer industry has developed various nitrogen-phosphorus-sulfur products that are being evaluated by Extension across the country.

Larry Oldham is a Mississippi State University Extension soil fertility and nutrient management specialist. He may be reached at larry.oldham@msstate.edu

Crop nutrition with S is challenged by:

• Amount of S in soil depends on soil organic matter levels.

• Less land application of S containing soil amendments, including manures.

• Less atmospheric deposition is occurring.

• More leaching potential in coarse textured, sandy soils.

• Increasing crop removal through yield increases.

• Inconsistent soil test results on which to base recommendations.

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