What role does rain acidity play in phosphorus runoff?

Why wasn’t Lake Erie green with algae 10 years ago?  20 years ago?

Are the farmers in the WLEB really doing a poorer job of managing nutrients than they did back then?  I have to answer no to that question.  I was involved in the retail fertilizer business 20+ years ago, and there was very little soil testing.  No GPS or VRT.  Very little conservation practices, no controlled traffic and auto swath shutoffs.  Rates were big and frozen and snow covered ground were business as usual.  We were told that P does not move.

 

So what has changed, what has made the improvement in nutrient management practices not keep pace with water quality? I review 10,000+ soil tests annually personally, and my group does twice that many. We have watched sulfur levels in soils fall from 30 to 40 ppm 10 years ago to 5 to 8 ppm today- a drastic change in the soil chemistry because of the huge impact sulfur has on soil pH, and now we see sulfur at a low enough levels that we see it effecting yields.  This is due to vastly reduced atmospheric deposition.

 

If you look at the PowerPoint, the first slide shows the trend line of sulfur over the last 10 years in soil tests.  It has gone from 30+ ppm to below 10 ppm on the average.  (Spikes in the graph are from fields where additional sulfur was applied.)  Next you will  see the maps of atmospheric sulfur deposition over time.  You can also see how the pH of rainwater has climbed from 4.1 to over 5 by 2014- the trend line has really changed the last 5 years as the air gets cleaner.  SO4= content has dropped from 3.5 ppm to less than 1 ppm in 2013.

 

When we sample soil, we probe 7” deep and strive to keep that “layer” at 6.2 to 6.8 pH for maximum nutrient availability.  However, it is well documented that the surface layer is a lower pH, and on most soils in the WLEB, the deeper you go, the higher the pH.  So we may have an “average” pH of 6.2, but that could be 5.5 at the surface and 6.9 seven inches deep. Acidifying fertilizers and roots exchanging H+ keep that surface at a lower pH, and obviously acid rain had an impact on that, especially the surface layer.

 

The chart in the PowerPoint shows the availability (solubility to plants) of various essential elements.  Notice P becomes fully available at 6.5, and very unavailable at 6.0, a pretty narrow window.  So let’s assume that our surface layer, maybe the top ½”, was hanging around 5.8 pH five years ago, which would be a very safe assumption.  This is also the layer that I would consider most at risk for P movement during a rain event.  Stratification is definitely there, but not a real issue for crop production because the crop needs that available P early in the season, when roots are small and located near the surface.  Now- every time it rains, we flood the surface with 5.1 pH liquid instead of 4.1 pH.  That surface pH has HAD to rise.  So maybe we are at 6.3 now, not 5.8.  A HUGE difference in the way P behaves.  At 5.8 all kinds of cations, like iron+3 and aluminum+3, bond with the anion P making it very insoluble, and at 6.3 that ceases, leaving phosphorus free to move with water.

 

During the last 5 years, farmers have done a much better job of keeping up with lime needs of their fields than they used to.  Lime was ignored in the past, and many had the thinking to just add more P and K instead of investing in lime. It was an easier job to manage.  But when fertilizer tripled in price 5 or 6 years ago, farmers saw the light to invest in lime instead, so their fertilizer dollars were more efficient.  Good evidence of this in the WLEB is that the Toledo and Ft Wayne water treatment plants produce somewhere north of 400,000 tons of high calcium lime per year.  10 years ago I met with the Ft. Wayne plant manager, and he was concerned because they were accumulating lime at a rate that their 450 acre treatment plant was filling up.  They were exploring buying more land to store it.  The last 3 years, that plant has run completely out of lime by mid-September, something they never fathomed 10 year ago.  All due to farmers paying more attention and working towards soil quality and optimum pH.  This has had to have some impact also of raising that surface pH to a level that could facilitate P movement.

 

I think we have been trying to blame certain individual practices, like no-till, vertical tillage, tile installation, bigger planters, etc.  4Rs definitely play a role, and there are management practices that need to be improved upon for everyone.  BUT- I would offer that the change in soil chemistry from the change in pH of our rain has changed the speed, ease, and volume of off-site DRP more than any one practice.  It is broad, it covers every acre of the watershed.  It happens every time it rains, even in growing crops when tillage and fertilizer applications have been forgotten about. It has been hidden, unnoticed, but common sense says it has to be a major player. This also would explain algal blooms in watersheds that have no agriculture.

 

We need to have research focus on this theory, and either prove or dis-prove it.  If I am right, we can then explore BMPs that minimize this risk. I ran this theory by several researchers at Ohio State in the past 2 months, and they agree that this has been overlooked and could have an impact.  Led by research professor Jon Witter,  they have received a small grant from the Lake Erie Commission to do laboratory studies on soils with different pH rainwater, and measure dissolved reactive phosphorus in the runoff.  Jon reported to me a couple weeks ago that the preliminary data from their lab experiments looks significant. They have applied for a couple larger grants, but are in a waiting period on those.  If you have any questions or if something needs a better explanation, please contact me.

 

Joe Nester

Nester Ag, LLC

www.nesterag.com

 

Jim Hoorman, Associate Professor at OSU Extension, provided THIS LINK explaining pH and phosphorus availability, noting: "P is most available from pH 6-7. If the soil pH gets too high (above pH 7), calcium will tie up the P (Ca-P)."