Honeywell Sulf-N® Ammonium Sulfate

Changing Sulfur Needs

North American Soil Fertility Study Illustrates Changing Sulfur Needs

The recent release of The Fertility of North American Soils, 2010 by the International Plant Nutrition Institute (IPNI) included some striking news about sulfur – a jump of more than 300 percent in the percentage of soil samples that tested low for the nutrient. Coupled with growing evidence of crop response to added sulfur across the Corn Belt, the IPNI report is sure to spark discussion on the role of sulfur in optimizing crop production.

Among the 2.5 million soil samples in the study, 13 percent tested below the low sulfur benchmark (3 ppm calcium phosphate extractable S or 6 ppm Mehlich 3 S), compared to just 4 percent in IPNI’s 2005 study.

The top state for low sulfur levels was Wisconsin, with 82 percent of the samples falling below the low sulfur threshold. The East Coast, from New Jersey down to North Carolina, also featured relatively high numbers of low-sulfur results. That’s not a big surprise, according to Mercedes Gearhart, agronomist with Honeywell – the sandy, low-organic-matter soils along the coast typically receive applications of sulfur fertilizer to compensate for low natural levels of the nutrient.
 
Striking News

But the big news in IPNI’s results is that many of the states with high frequencies of below-threshold sulfur levels are in the western Corn Belt – Minnesota, Iowa, Missouri and Illinois – and the Great Plains, including Nebraska, Kansas and Oklahoma.

"What is especially interesting is that agronomists in many of those states, which have always been considered to have adequate levels of sulfur in their soils, are seeing yield responses to sulfur applications", Gearhart says. "What these results may indicate is that sulfur will be more important to growers in the next several years in places where many of them have never thought much about sulfur in the past".
 
Why the Drop?

Several factors could be influencing the increase in low-sulfur readings, says Gearhart. Smokestack emissions of sulfur dioxide from factories and power plants dropped 40 percent between 1990 and 2006, she notes, due to emissions scrubbing required by the Clean Air Act. Those windborne clouds of sulfur deposited a considerable amount of the nutrient on fields in a broad swath of the country.

"That's probably a bigger deal in the Northeast and the eastern Corn Belt, which were in the 'flight path' of industrial emissions from the Rust Belt", Gearhart notes. "Farther west, the drop may be mostly related to higher rates of sulfur removal by better genetics and higher yielding farming practices. Those 200-bushel corn crops and big cuttings of alfalfa can remove a significant amount of sulfur. If we don’t pay attention to sulfur's balance sheet and add sulfur sources like ammonium sulfate when needed, sulfur availability can easily become a most limiting yield factor".

Even where soils contain relatively healthy native sulfur reserves, many growers and researchers are seeing crop responses to applications of the nutrient in fall or spring.

"Every one percent of organic matter in the soil contains two to five pounds of usable sulfur per acre, but that resource isn't mineralized and available until soil temperatures reach 60 degrees Fahrenheit", Gearhart explains. "So the current trends of early planting and reduced tillage, both of which put young crops in cool soils, can create a situation where sulfur isn't available to the plant when the crop first needs it. Where a source of soluble sulfur (sulfate) makes the nutrient immediately available to the roots, we can see a significant improvement in performance and yield".
 
Significant Consideration

Ammonium sulfate applications can also acidify the rhizosphere, the area immediately around crop roots, Gearhart adds. That can provide both a chemical and biological boost, making nutrients such as zinc and phosphorus more available. As a result, sulfur can often help combat zinc deficiency in the soils where the nutrient is present but under-utilized, she says.

The IPNI report is a great reminder of the importance of soil testing and careful consideration of fertilizer programs, Gearhart says. It’s also a reminder that sulfur shouldn’t be taken for granted, even in areas long considered to have plenty of the nutrient in the soil. In addition to soil test results, tissue test data, type of tillage, form and timing of application, and yield potential considerations can all help you find the best fertilizer formulation for your fields, she suggests.