Think Different

There are areas where gypsum is not the answer. To find areas where it might have a fit, consider the following:

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A centuries old amendment for tight soils may help resolve phosphorus (P) pollution while also serving as a low-cost source of sulfur and improving soil health.

Ironically, the same flue gas desulfurization (FDG) systems in power plants, which have helped reduce the sulfur formerly deposited by acid rain especially east of the Mississippi, is a popular source of pure calcium sulfate dihydrate or FDG gypsum. Both are side effects of the 1990 amendments to the Clean Air Act.

Joe Nester, Ohio based independent crop consultant, suggests the irony goes even deeper, laying the groundwork for solutions to P degradation of Lake Erie and other bodies of water.

"Over the past 10 years, as the Clean Air amendments took effect, we have seen the pH of rain in the Lake Erie basin rise from 4.2 in 2006 to 6.3 this past spring," says Nester. "That is almost 100 fold less acid. At a low pH, the phosphorous binds easily with other elements in the soil. At pH 6.5 it is not binding. The good news is that it is more available to the plant. The bad news is it is more soluble and likely to move."

Gypsum cuts P in tile water

Nester is one of multiple cooperators in a study headed by Dr. Warren Dick, Ohio State University. Dick sought to sequester soluble P in fields and keep it from moving into water systems where it could create algae blooms in Lake Erie and elsewhere. The project centered around sampling water from tile outlets in 10 fields draining areas treated with gypsum or left untreated.

"We took samples (250 in all) from the outlets with every rain event, and over a three-year period, there was a 35 to 50 percent reduction in phosphorous (concentration in the water) coming out of gypsum treated areas," says Nester. "The calcium in the gypsum created a light bond with the phosphorous to retain it for crop use."

Good news for the environment translates into good news for growers as retention of more soluble P translates into less P inputs needed to maintain yields.

Soil tests fail on P?

For Nester, this finding explained why growers he worked with achieved great yields with lower P application rates, even as soil test levels fell. He suggests the higher pH in the rain was making bound P available, including that not identified in conventional soil testing. Nester points out that available P, as identified through soil tests, may be only six to 10% of all P in the soil. He argues that what is used by the plant and exported from the field in grain doesn't necessarily have to be replaced. Less available forms of P become more available, especially with higher pH rainfalls.

Less soil P needed

"The soil tests we use haven't changed, but field conditions with the acid levels in the rain have changed," says Nester. "As a result, we are getting more mileage out of the phosphorous in the soil, but the challenge is that it is more slippery and available to move out of the field."

Nester recommends a range of tools to his growers to sequester soluble P, including gypsum, filter strips, cover crops and no-till. He amends the industry's recommendation of following the 4Rs of Right Source, Right Rate, Right Time and Right Place to include Rain Risk. "Don't apply phosphorous when the risk of rain is high," he says.

Change P guidelines?

Even greater changes in P rates may be coming. Nester suggests that guidelines of 40 to 50 ppm falling to 25 to 30 ppm may need to be recalibrated. He also suggests that using water with a pH of local rain in soil testing may be advisable. He reran soil samples using water at the 4.2 and 5.2 pH (equivalent to rain pH 10 years ago and three to five years ago) as an extractant. Every sample at the higher pH was significantly higher in available P.

"We are working with a more soluble form of P than we were 10 or 20 years ago," says Nester. "We are seeing a huge change in soil chemistry at the soil surface."

Gypsum boosts yield, soil health

Rich Mort, who farms near Pendleton, Ind., is interested in potentially cutting back on P applications; however, replacing the sulfur missing from those lower pH rains is his immediate concern.

He has applied gypsum on his corn ground since 2008, purchasing a spreader and becoming a dealer for Gypsoil brand gypsum when he couldn't find a local source or supplier. It wasn't until 2014 that he really appreciated the benefits accruing.

"I had a 21-acre field next to a 22-acre field of my dad's," recalls Mort. "I variable rate applied P and K and used gypsum at a ton per acre while my dad used no gypsum and a flat rate of P and K. We both planted the same hybrid with the same starter and rates on the same day. Both emerged uniformly, but mine stayed green while his got really yellow."

Tissue samples told the story. "His was super deficient in sulfur and mine had abundance," says Mort. "We sidedressed both fields with 28 percent and his got green, but when we went to harvest, there was a 46 bushel per acre difference between the two fields. With sulfur deficiency, if you can see it, it's already too late."

To add insult to injury, Mort's input costs with the gypsum and variable rate P and K were $19 per acre less than his father's. After that experience and harvesting 71 bushels per acre beans (nine bushels higher than any other field) from a field treated with gypsum, Mort reports that he and his father are now putting 1,000 lbs. of gypsum on every acre, whether going into soybeans or corn.

Fix high magnesium soils

Southeastern Minnesota farmer Jay Solum turned to gypsum for its sulfur, but also to deal with high magnesium soils. He believes it increased soil oxygen to fuel soil microbes and speed breakdown of crop residue, as well as respond to sulfur deficiencies. He prefers no-till, although he uses light tillage of residue in corn-on-corn acres. When he first applied gypsum, it was accompanied by tillage to speed dispersion and impact. 

"We were struggling with no-till on our high magnesium soils," says Solum. "The tight structure meant poor drainage and cold soils, and the crops struggled to get out of the ground in the spring, producing shorter, less healthy plants. We saw noticeable improvement in crop health the first year, although some of that was the tillage releasing carbon."

Solum applied elemental sulfur in 2010 and tried mined gypsum in spring 2011 before moving to Gypsoil's FGD gypsum for its finer particle size and increased surface area. Gypsoil founder Ron Chamberlain explains that positively charged calcium and negatively charged sulfate move rapidly through the soil with some of the sulfate binding to positively charged magnesium. When bound, the two become Epsom salts, which move even deeper, outside the rooting zone.

Reduced runoff, increased OM

"The calcium replaces the magnesium and with its ability to flocculate, helps bring clay particles together," says Chamberlain. "This enhances the movement of water followed by air, which invigorates the biological system, which in turn produces glomulines and further improves soil aggregation." 

As soils improved, and with the addition of cover crops, Solum has returned to no-till on most of his acres. "Gypsum improved the soil profile and soil tilth, as the base saturation levels of magnesium fell and calcium levels increased," says Solum. "Organic levels have gone from 3.1 to 4 and CEC values are increasing, as well as water infiltration."

This past year, Solum hosted an NRCS water runoff and infiltration test on his farm. "They compared soil from fields with multiple applications of gypsum and two years of cover crops, 20 years of no-till, a hayfield and long term tillage," says Solum. "The gypsum/cover crop soil had no runoff, and the water that ran through came out crystal clear. The no-till alone had runoff and came through dirty, while the hayfield had lots of runoff. The tilled soil had less runoff, but the water came through mucky. It really showed the benefits of healthy soil."