Sulfur surprise in soybeans

If soybeans could talk, they’d tell you anti-pollution legislation works.

These laws – first passed in the 1970s – squelched sulfur-containing acid rain fueled by industrial emissions. Applications of fertilizer containing sulfur (S) previously benefited only crops grown on sandy soils, as atmospheric S would leach through them.

No more. "Sulfur responsiveness has occurred particularly more on the corn side, but we have also documented some pretty strong sulfur responses on soybeans," says Shaun Casteel, a Purdue University Extension soybean agronomist. Running parallel to less "free" atmospheric S is increased S consumption by crops due to higher yields gleaned by farmers, he adds.

Purdue studies show S applications spike soybean yields 10 to 15 bushels per acre on sandy or coarse-texture soils with less than 2% organic matter. Applications of S also spur a 4- to 6-bushel-per-acre response on heavier soils with 2.5% to 3% organic matter.

More eye-opening are responses on heavier soils with 3.5% organic matter or higher. Casteel describes these as "situational sulfur deficiencies."

"Those soils have plenty of organic matter that mineralizes and provides both sulfur and nitrogen to crops," says Casteel. "In those soils, we’re seeing double-digit [bushel-per-acre] yield increases with some of these sulfur treatments. The plant response is visually striking in the middle of the season. They’re green as a gourd vs. the highlighter green [of S-deficient plants]."

These yield perks are linked to earlier Midwestern planting of soybeans in late April or early May, says Casteel. Heavier soils high in organic matter tend to be cooler and wetter during this time. This limits mineralization, which squelches sulfur supplied by the soil. Also accentuating S responses is heavy residue keyed by reduced tillage or cover crops, he adds.

"In conventional tillage, you get a little more release [of sulfur] from the organic matter, but you don't get that with no-till and strip-till," says Paul Groneberg, a retired Centrol Crop Consulting agronomist based in west-central Minnesota. "Sulfur is a key element [to add] in those strip-till and no-till situations."

Later planting, though, erases the merits of S applications on these soils.

"These same sulfur treatments applied to soybeans planted during the first week of June have no response," he adds. The warmer soils lead to more mineralization of organic matter, which translates into more soil S, he adds.

Normally, a yield spike of 2 bushels per acre is needed to cover product and application costs. "This can fluctuate depending on market prices and product costs, but it's been a fairly consistent number," Casteel says.

When to apply S is often the larger issue.

"To get the most bang for the buck, you need to apply it in early [soybean growth] stages, from planting to V2 or V3," says Casteel.

Twenty pounds of S applied in the form of ammonium sulfate (AMS) close to planting or during early vegetative growth is a good starting point, he observes. Granular AMS is the most straightforward form, but farmers can apply granular S in other ways, such as a 50-50 mix between granular AMS and elemental sulfur.

Farmers may also mix liquid S formulations with starters like 10-34-0. Applications of ammonium thiosulfate (ATS) and potassium thiosulfate (KTS) in a 2-inch offset dribble on top of the soil surface is another starter option.

The drawback is that not every farmer is set up for such starter applications.

"Most who do so are planting in 30-inch rows," says Casteel. "So, we lose out on some of the potential yield increase with wider rows. That puts us between a rock and a hard place."

Other application options include applying granular AMS with a fertilizer rig to soybeans up to the V3 stage. Soybeans flattened by tires can still bounce back at this stage. After that stage, flattened plants with no extra yield potential result.

Foliar S applications haven't worked as well as granular S applications in Purdue trials, due to phytotoxicity issues. Antagonism can also result when an S form like ATS is teamed with a burndown or preemergence herbicide.

There may be potential, though, when liquid S formulations are teamed with fungicide and/or insecticide applications during reproductive soybean stages.

"There are some intense management synergies that first need to be understood," Casteel says.

Excessive S can be detrimental, particularly on late-planted soybeans. "We saw that in 2019, when 20 pounds [per acre] of sulfur in a concentrated band with starter ATS restricted plant development," says Casteel. "I think the high rate was burning roots and thereby suspending nodulation and nitrogen fixation. We ended up having shorter plants and taking a yield hit, compared with no or lower rates of sulfur."

The appearance of soybeans receiving no sulfur fertilizer (left) reveals a thinner canopy compared with soybeans (right) that received 20 pounds of sulfur per acre.

Unless S can be applied at planting as a starter, broadcasting S requires another field trip. This costs time and money and can delay planting.

"I would rather have growers plant timely when fields are fit, and then come back with the sulfur application," Casteel says.

Yield responses also may vary, even on S-deficient soils. "In 2019, responses were not as high as in other years, even on normally responsive sites," he reports.

Reference strips to which no S is applied and in-season tissue tests can gauge S application effectiveness. This information can help farmers determine which fields are most likely to show an S response in future years.

"These huge responses do not occur on every field and in every situation," Casteel points out. "But these responses are happening with enough frequency that they should be considered, especially for those who are pushing planting dates and higher yields."

Sulfur starter or broadcast fertilizers that also contain nitrogen (N) may also garner an additional yield response due to the inclusion of N.

"We don't see it every year, but there can be a benefit from the nitrogen in a product like AMS [ammonium sulfate] or ATS [ammonium thiosulfate]," says Shaun Casteel, Purdue University Extension soybean agronomist. "You won't have that with calcium sulfate or gypsum. The nitrogen helps stimulate plant growth, especially in cooler and wetter fields. The sulfur is a cofactor for nodulation and nitrogen fixation."

Crops can no longer rely on atmospheric sulfur (S) to meet crop S needs.

"We no longer receive 15 to 20 pounds [per acre] annually of sulfur from atmospheric deposition," says Shawn Casteel, a Purdue University Extension soybean agronomist. "We’re at 5 pounds [per acre] or less across the Midwest."

Here's a hypothetical example from Mosaic about how much sulfur soybeans consume, their natural S sources, and how much S fertilizer is required.

Soybeans