Farmers understand that Lake Erie turns green in the summer and that part of the blame is rightfully being directed at agriculture due to issues related to nutrient management, specifically phosphorus. What is less understood is why this is happening.
In a time period where on-farm phosphorus application levels have decreased substantially and recommended conservation practices have increased in the agricultural landscape, the troubling harmful algal blooms again started showing up in the Western Basin of Lake Erie after many thought the water quality issues had been corrected decades earlier.
Even more confusing are the smaller lakes in more remote parts of the state and country where algal blooms are showing up in lakes surrounded with little to no agriculture to blame on the issue. There are numerous theories as to why this could be happening. One of them was a topic in a study published in the Journal of Soil and Water Conservation last summer: “A possible trade-off between clean air and clean water.”
Douglas R. Smith with the U.S. Department of Agriculture Agricultural Research Service Grassland Soil and Water Research Laboratory in Texas was the lead author. The study looked at possible connections between the phosphorus mobility in the soil and the well-documented change in the pH of rainfall since the implementation of the amendments to the Clean Air Act in the early 90s to address concerns with acid rain.
“I think it is certainly a fascinating topic. There have been quite a few publications on the potential causes of the Lake Erie issue,” Smith said. “I do not think it is possible for acid rain recovery to be solely responsible for the eutrophication issues. I have been a co-author on several papers on the issue, and think it is the culmination of many factors. I cannot say for sure that rain chemistry is one of the contributors, but I cannot rule it out either. If it is not a contributor, it is certainly an interesting coincidence. I hope there will be an additional paper coming out in the next year or so on this issue.”
For some, the publication of the article made sense in helping to explain why the harmful algal blooms are showing up, particularly in non-agricultural areas. Agronomist Joe Nester, owner of Nester Ag, LLC in Williams County, has been talking about this possibility for several years.
“That study should open the door for more discussion and research on it. I’ve been working with nutrient management for 40 plus years, most of it in the Western Lake Erie Basin. When we started having problems with phosphorus loading, even with all of the improvements I have seen over that 40-year period, I struggled with how we were going backwards. I noticed the huge drop in sulfur in the soil like everyone else had. When the soils get low in pH we tie up phosphorus and have a tough time getting it in the plant. It just makes sense to me,” Nester said. “In my opinion, the change in the pH of the rainfall from about 4.2 to roughly 6.2 today — that is a 100-fold change in acidity —is liberating P that used to be tied up in the soil. That is a pretty drastic change in soil chemistry in my opinion. When the Clean Air Act amendments were passed, pH levels of the rain started to rise. By the end of the 20-year implementation period for that you can see a real jump. The biggest increase was in 2008 and that is the first year Lake Erie turned green.”
In addition, Nester is finding less need for phosphorus applications in many fields.
“In the last 10 years I don’t think I’ve seen phosphorus deficiencies anywhere, even in some zones we manage where we should be showing a P deficiency and we aren’t,” he said. “The mileage that we are getting from P applications led me to believe it even more. As agronomists we have cut back on the phosphorus as opposed to what we put on 10 or 15 years ago and we are raising better crops. I am convinced that there is something else happening in the soil.”
So the bad news, Nester said, is that phosphorus seems to be more soluble and mobile in the soil which makes it easier to lose, but that also could be the good news for agriculture because it means potentially lower nutrient costs.
“For agriculture that really is good news but we have to learn the techniques and management systems to keep it within the field boundaries instead of moving with the water. We mange tens of thousands of acres of nutrients a year and I’m convinced that P is a more mobile unit than it used to be. A farmer should really be evaluating the right rate by soil type and then try to draw down phosphorus levels to see when it starts reducing yield. I haven’t seen it and frankly I have been surprised on some 15 to 20 ppm soil tests with nothing applied that can still produce some pretty awesome yields of corn and soybeans,” Nester said. “We did some of our own work using water as an extractant for comparison and each time the higher pH water extractant showed more soluble P than the lower pH water extractant. I definitely think there is something there. I don’t know what else would explain it. When you talk with farmers about this being a real possibility the light bulb pops on and they say, ‘Well we can work with that.’ Otherwise we see them say, ‘We spent tens of thousands of dollars on technology, we’ve reduced P rates, increased yields, employed cover crops and minimum-till, and we’re going backwards? It is not me.’ But if the environment has changed, it makes sense.”
Though it does make sense to some agronomists, others are skeptical that the pH of the rainfall could influence the mobility of phosphorus in the soil so dramatically. Harold Watters, Ohio State University Extension agronomist, thinks the increased mobility of phosphorus has more to do with the 4Rs.
“Phosphorus can be tied up by calcium at higher pH levels, but P can also be tied up by iron and aluminum at lower pH levels. As we have cut back on the acid we have in the rain, yes we have had a slight shift in the soil pH levels. Is that enough to really solve the problem or cause the problem with dissolved reactive phosphorus in Lake Erie? My guess is not,” Watters said. “We have a pool of calcium under us. It is our bedrock below us and we have basic pH levels on the western side of the state. In the eastern side of the state we have acid pH levels. We have plenty of calcium and we have plenty of aluminum and iron as well. So I don’t see gypsum, calcium, iron, or aluminum, or bringing back acid rain as the magic bullet to solve the problems for us. We’re going to have to work on best management practices as a group. That means incorporating phosphorus into the soil and not putting it on the surface.”
Watters thinks the concentration of nutrients near the soil’s surface due to application practices and increased conservation tillage has much more impact on increasing dissolved reactive phosphorus loss.
“We have bigger farms today and more minimum tillage. I recently asked 10 to 15 farmers and some Extension educators about tillage tools being used for incorporation of nutrients and what is their primary tillage tool. In some cases I was told that 50% of the acres were using vertical tillage for incorporating nutrients and also primary tillage. The lowest number I had for vertical tillage tools was 20%,” Watters said. “When you talk about vertical tillage, that may only be an inch or two deep or some stirring. You are not really incorporating nutrients that well and contributing to the stratification of nutrients in the top. Guys are also telling me they are going fast and that tells me they are not incorporating nutrients all that well. We have to change some things. We need to rethink why we are using those tools — those are not for incorporating fertilizer. I think farm size and reduction of tillage is a big part of this.”
The timing of nutrient application is also a significant contributor to the problem, Watters said.
“I talked recently with a fairly average farmer in Ohio with 1,500 or 1,600 acres. When he pulls out of the field harvesting soybeans he calls his co-op to come spread fertilizer in his fields. He doesn’t know when the fertilizer goes on and he’s not going to do any tillage until the next spring immediately before planting. He stirs it up and then in a week or so later plants corn. I am seeing more of that. And if they are doing tillage, it is vertical tillage,” Watters said. “That risk exposure from the time of application until the spring tillage is when there is the potential for the phosphorus to go away. I feel more confident in this kind of concern than the lack of acid rain.”
The article “A possible trade-off between clean air and clean water” was published in the July/August Issue of The Journal of Soil and Water Conservation and is available online.