While the Clean Air Act has been a benefit to our health and the environment, farmers are experiencing some deficiencies in their crops.

It is the primary federal law regulating air emissions – including sulfates – to protect public health and the environment.

But now, researchers with Auburn University’s College of Agriculture, Alabama Agricultural Experiment Station and the Alabama Cooperative Extension System see a lack in sulfur also may be affecting corn yields.

“Sulfur is a nutrient that directly participates in essential amino acids, which, when lacking, will affect some protein formation in the plant,” said Eros Francisco, assistant professor and extension grain crops specialist with the Department of Crop, Soil & Environmental Sciences and researcher with the AAES.

While Alabama is not officially part of the “Corn Belt,” corn remains a major cash crop for state and area farmers, used mostly for fuel and feed. This past year, producers grew about 350,000 acres, with total production estimated at 51 million bushels.

This year, the state’s farmers are expected to plant about 400,000 acres of corn, and yields will be highly impacted by nutrients, primarily nitrogen.

Row crop production in Madison County will also be impacted, Francisco said

“Madison County ranks high in row crops acreage — 100,000 plus acres yearly with soybean (30-50K), corn (25-35K), cotton (30-40K) and wheat (20K-25K),” he said. “These crops have a high demand for sulfur (20-30 pounds of sulfur per acre annually) that is not coming from the atmospheric deposition as it used to, so fertilizer application decisions should consider it.

“The predominant soil type in Madison County is the Limestone Valley, which is a weathered, acidic, low organic-matter type of soil that requires the addition of nutrients, including sulfur, for the achievement of high yields. Sulfur nutrition can easily be assessed by soil or plant tests, and adequate fertilization shouldn’t be neglected.”

Sulfur’s demand is not as high as nitrogen or potassium in corn, with a total uptake of about 30 pounds for a 200-bushel yield and about 16 pounds of grain removal, Francisco said, but it remains an important micronutrient. The main sources of sulfur to crops are soil organic matter and atmospheric deposition.

“With the advent of cleaner fuel regulations over the last decades due to the Clean Air Act, the deposition of sulfur has significantly decreased, causing crops to start lacking sulfur and showing deficiency symptoms, such as light-green and pale-green younger leaves,” he said. “Sulfur deposition accounted for about 10 to 15 pounds of total sulfur a year before the new regulations were introduced but now is less than 2 pounds per year.”

These efforts have reduced both wet deposition (sulfate in rain and snow) and dry deposition (particulates and gas), leading to improvements in ecosystems and reduced soil and water acidification. Sulfur atmospheric deposition in the U.S. has decreased significantly — by more than 80% in some regions. Key factors driving this decline include reduced sulfur dioxide emissions from power plants and the shift away from high-sulfur coal energy generation.

And while this has been good for the environment in general, the Clean Air Act means farmers will need to pay closer attention to sulfur levels in their crops going forward.

“Generally, a sulfur deficiency does not cause huge yield losses, assuming other nutrients are in good shape,” Francisco said. “But when it impacts other nutrient responses, the yields can be greatly reduced.

“Our 2025 data showed a corn yield reduction of 12 and 5 bushels per acre in Prattville and Madison, respectively. Sulfur soil levels will dictate the magnitude of the loss.”

The objective of current research being conducted by Francisco and others is to better understand how the addition of sulfur impacts the corn yield nitrogen response. This multi-state project includes 23 locations in eight states, including Alabama.

The Prattville Agricultural Research Unit and the Tennessee Valley Research and Extension Center are testing five nitrogen rates (0, 85, 135, 170 and 220 pounds of nitrogen per acre) and two sulfur rates (0 and 20 pounds of sulfur per acre). Also, measurements (soil and plant tissue analysis, nitrogen and sulfur uptake and removal, grain yield and test weight) are being taken to evaluate crop performance.

“These results will help educate growers on how important sulfur nutrition is in corn and what adjustments need to be addressed in nitrogen rates to maximize yield and profitability,” Francisco said. Not all the locations have shown corn yield response to sulfur applied. About 50% have shown yield responses varying from 5 to 20 bushels per acre, depending on the nitrogen rate.

“The lower the nitrogen rate, the higher the sulfur impact on yield.”