Resources and show notes:
Hartemink, A.E., Barrow, N.J. Soil pH – nutrient relationships: the diagram. Plant Soil (2023).
Slessarev, E., Lin, Y., Bingham, N. et al. Water balance creates a threshold in soil pH at the global scale. Nature 540, 567–569 (2016).
Daniel C. Schlatter, Kendall Kahl, Bryan Carlson, David R. Huggins, Timothy Paulitz, Soil acidification modifies soil depth-microbiome relationships in a no-till wheat cropping system, Soil Biology and Biochemistry, Volume 149, 2020, 107939.
Dr. Melissa Letourneau can be reached by email at firstname.lastname@example.org.
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Drew Lyon: Hello, welcome to the WSU Wheat Beat podcast. I’m your host, Drew Lyon, and I want to thank you for joining me as we explore the world of small grains production and research at Washington State University. In each episode, I speak with researchers from WSU and the USDA-ARS to provide you with insights into the latest research on wheat and barley production.
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My guest today is Dr. Melissa Letourneau. Melissa is a research soil scientist with the USDA-ARS Northwest’s Sustainable Agroecosystems Research Unit. She holds a Ph.D. in soil science from Washington State University, a B.S. in geology from Indiana University, and a B.S. in computer science from Oregon State University. As a Ph.D. and postdoc, she researched root-microbe-mineral interactions, soil-borne fungal diseases, and soil microbiomes in the root zone of wheat.
She started her current position in July 2021 and aims to integrate multiscale biophysical, chemical, and ecological data to enhance nutrient and water use efficiencies and soil health throughout the Columbia Plateau. Her current research focus is soil acidity. Hello, Melissa.
Dr. Melissa Letourneau: Hello, Drew.
Drew Lyon: So soil acidity, I’ve heard quite a bit about it lately, but it’s on this funky logarithmic scale and maybe not real understandable to a lot of people. Why should people care about soil acidity?
Dr. Melissa Letourneau: That’s a great question and one that doesn’t get addressed a lot, I think. The main reason that we really should care about this is because crops and their associated microbiomes are adapted to a certain pH range, a certain soil pH range.
And so if you’re unfamiliar with the concept of pH, it’s a measure of hydrogen ion concentrations in a solution. So you don’t even really need to remember that much. All it means is low pH means a high hydrogen ion concentration–which means high acidity, acidic conditions– whereas a high pH is a lower hydrogen ion concentration and therefore kind of alkaline conditions.
So the native grasslands in North America, including those around here in the Inland Pacific Northwest, typically have the soil solution around seven, which is a neutral pH. And that can vary by, you know, half a unit or maybe even a whole unit from time to time, so there’s a little bit of a range that’s reasonable. But this is optimal–this is the optimal range for most of our crop plants.
And so what happens when you start to move outside that optimal pH range is that the chemistry of the soil starts to change in such a way that the plants and their associated microbiomes aren’t adapted to get their nutrients from that soil anymore.
And so there’s a classic diagram that shows kind of rough changes in nutrient bioavailability and toxin bioavailability associated with pH. And there’s actually a recent, really recent publication that describes the history of this diagram. It’s pretty conceptual and it hasn’t been parameterized or tested with a variety of soil conditions, so there’s really a lot of basic research that still needs to be done to understand how soil pH really impacts nutrient cycling, nutrient uptake.
One area that we that most of us are pretty familiar with at this point is you do see an increase in the bioavailability of aluminum, which can cause toxicities for the plants. And the most common symptom of this that we see is stunted root growth. And so that’s automatically going to impact water uptake, nutrient uptake by the plant because you just don’t have as much root surface to manage those things.
And then beyond that, there are also processes that occur in the root zone. There are a lot of chemical and physical and biological gradients that are set up between the bulk soil and the root zone that might be disrupted by a change in pH. And then as far as the microbiomes go, we know that the microbial community composition is altered by pH and there are some publications by people from Tim Paulitz’s group and from Tara Sullivan’s group that have looked at this.
And this can impact actually carbon cycling potentially because the CO2 that’s respired by soil microbes can actually be incorporated into carbonate minerals under the right conditions. But a low pH typically isn’t very favorable for that, or acidic soil conditions typically are favorable for that. So there are just a number of processes that could theoretically be impacted by soil acidity that we just don’t know–we don’t know a whole lot about those interactions yet. So there’s a lot of room for additional research to pin some of that down.
But the bottom line is soil acidity over time is going to degrade your plant health and it’s going to degrade the plant nutrition. And this can result in plant disease, poor nutrient-use efficiency, and ultimately it can impact the soil health because the plants aren’t healthy, the microbiome isn’t healthy. You’re not going to have healthy soil under those conditions. So that’s why we should care about soil pH, soil acidity.
Drew Lyon: [It] affects a lot of things, some real knock-on effects, it sounds like, of soil acidity. So you mentioned soil acidity–as your hydrogen ion concentration increases, your soil acidity goes down or pH goes down, your soil acidity goes up. But what causes soil acidity? What causes that to happen?
Dr. Melissa Letourneau: So there are a couple major processes that drive this, and one of them is actually just natural soil weathering. So what happens over time to soil profile as rain washes through it, it leaches base cation. So that’s calcium, magnesium, and potassium. It leaches these cations from surface layers of the soil to deeper layers of the soil.
And that seems kind of counterintuitive. Why would that have an impact on soil acidity? What happens through that process is the soil matrix, the mineral matrix changes. You see a decrease in what we refer to as cation exchange capacity. And so the soil matrix isn’t able to hold on to hydrogen ions as well. So those hydrogen ions are more likely to go into the soil solution and cause soil acidity problems. So that’s one that’s one major issue.
Organic matter can also play a role here because organic matter has a high cation exchange capacity. So if you degrade organic matter, then you can lose that ability, the soil matrix isn’t going to hold on to hydrogen ions as well either. And they’re going to go into solution and you’re going to see a more acidic soil.
And I should point out here, when we talk about soil pH, often we are talking about the pH of the soil solution, acidity of the soil solution. And the reason that’s important is because that’s what the plant sees when it’s taking up nutrients, when it’s interacting with the soil. It’s that solution that it’s dealing with for the most part.
So another consequence of this change in the mineral matrix is you start to release aluminum and iron into the soil. These things become more exposed. And so these can be released in the soil and this can get really ugly because you start to see a positive feedback. What happens with iron and aluminum is they’re capable of driving these hydrolysis or water-splitting reactions, which also increases the concentration of hydrogen ions in solution.
And so you can get to a point where you start to fall off a sort of cliff in terms of soil pH. And we see this in a lot of tropical soils—if they don’t have enough organic matter to buffer against that sort of natural acidity that arises as a result of natural weathering processes and then they can become very acidic. So that’s common in tropical soils, and this is also why we tend to see more acidity problems in the higher precipitation zone around here, you know, far eastern Washington, northern Idaho. So it’s like how do we combat something that’s a natural process?
There’s a there’s a silver lining because this can all be counteracted by plant transpiration. Plants are capable of recycling those base cations back to surface soil layers in some situations. And in fact, there’s a paper by Eric Slessarev from 2016 and a number of collaborators that found that 40% of global soil acidity can actually be accounted for just by comparing mean annual precipitation with potential evapotranspiration. So that balance is really, really important in terms of maintaining a healthy soil pH.
One interesting thing, if you actually do take a look at that paper, which I think is going to be posted with this transcript, you can see there’s a map that shows where some areas are more acidic than expected and some more alkaline. And in our area, you see that there’s more acidity than was expected from just that simple comparison.
So there are a couple potential reasons for that. It may be that our soil weathering here is accelerated because we get high precipitation in the wintertime when we don’t have as much evapotranspiration. So when that when you get that asynchrony of precipitation and plant transpiration, that could really leave the soils more susceptible to rapid weathering in the wintertime, you know, especially if there’s nothing growing at all.
And then of course, we’re also removing some base cations when we harvest plant biomass. So if we’re not retaining enough residue, you know that can also be a source of acidification. But probably the bigger driver in this region and other agricultural areas is really going to be the transformation of ammonia-based fertilizers that are left behind in the soil to nitrate.
So when that happens, there are a bunch of protons left behind, essentially. The nitrate leaches away and you leave a bunch of protons or hydrogen ions behind. And so that again decreases soil pH, increases soil acidity. And this process is called nitrification, and it’s driven by soil microbes. So the rate and the timing of that process is going to be dependent on things like temperature, moisture, and then, of course, who’s present in that microbial community. So that can be quite variable as well.
But these are really the main drivers of soil acidity in this region, the soil weathering and that nitrification process.
Drew Lyon: Okay. I tend to hear more about soil acidification in no till systems. How does tillage impact soil acidity?
Dr. Melissa Letourneau: So tillage may or may not have a huge impact on the actual overall concentration of hydrogen ions in the soil, the overall acidity. What tillage does is it mixes those surface soil layers.
We’ve been we’ve actually been observing this process over the past 20 years at the Cook Agronomy Farm north of Pullman. On the west side of the farm, we have a field that’s been fairly heavily tilled and then on the east side, we have a field that’s direct seeded. And this is for the past– we’ve been monitoring this since 1999 when that sort of large experiment was set up.
And so what we see between the two sides of the field, as we see in the tilled side, we have a sort of thick surface layer of soil that’s moderately acidified. And we would expect that to continue to acidify over time. But that the point is the acidity goes kind of deeper into the soil profile, and that can be problematic because that’s very difficult to mitigate when it when it gets deep into the profile like that.
So what happened in the no till side is we see more of a stratification of soil acidity. So based on what I was saying about nitrification, you know, it’s not unexpected that we see a concentration of soil acidity in the fertilizer band. So that tends to be very highly acidified, maybe much more so than the tilled side.
But then the interesting thing is, as you get deeper into the soil, we see an increase in soil pH, so a decrease in soil acidity. And we have a lot of digging to do to figure out exactly why that is, but one thing that we know happens in the no till side compared to the till side, we have much better water infiltration.
And so we think what’s happening here is the increased water infiltration is actually causing more leaching of the base cations into deeper soil layers. So in a sense, that kind of sounds promising in terms of remediating subsoil pH, but it’s important to keep in mind there’s still going to be a finite pool of base cations in that soil matrix. And so for not retaining residues that contain base cations, if we’re not replenishing those supplies, then we’re still going to continue to see acidification throughout the profile over time.
So yeah, so that it’s not so much a difference in overall acidity between the tilled and the no till side, it’s more difference in the distribution of it, I guess.
Drew Lyon: Okay. I was wondering about that leaching of base cations because I know no till systems, they do increase infiltration of water. And when we look at pesticide–in my case I work with herbicide–herbicide leaching is actually greater in no till than it is in conventional till because you have those nice soil aggregates and pores taking water in. So that’s interesting that that works with base cations and soil acidity as well.
So we’ve developed this acidity problem. What can we do about it?
Dr. Melissa Letourneau: So, it’s a challenging problem and it’s not something that we’re going to be able to fix overnight. The problem didn’t develop overnight. You know, this is something that’s developed over decades.
So from the from the soil weathering standpoint, one of the things that we can try to do is increase evapotranspiration in our systems, especially in the wintertime. So if we can get more winter rotations, if we can get some deep rooted rotations–things like canola that are really good at mining nutrients from deeper soil layers that might be able to recycle base cations back to surface layers, things like perennials, cover crops–all of these things might help us sort of counteract that soil weathering process.
Another thing I’ve already mentioned a couple of times is just trying to retain as much of that residue and the base cations that are in it as possible, or if some of that’s being removed, trying to replenish that supply with fertilizers–calcium, magnesium, potassium fertilizers–might be an option.
You know, one thing I think we worry about a little bit, especially on the no till side of the Cook Farm, is starting to run into salt problems in some layers. So there’s probably a balance there, but trying to balance what’s being taken away I think is important.
Probably the most important thing that we need to consider is how can we decrease our use of ammonia? How can we limit the amount of excess ammonia in the soil profile? Can we control that with variable rate approaches where we can get lower rates of application? Can we control it with timing, trying to apply it more in sync with plant demand so that there’s not as much left laying around in the soil for nitrification to occur?
And it’s also possible that using some organic nitrogen sources or biologically fixed nitrogen might help, but that’s going to depend a lot on the nitrogen mineralization rate, which also depends on the soil microbiome. So these are complex processes. We’ll need to look more into how that might be a useful tool to combat soil acidification.
And then probably the most effective and also most expensive approach would be to try to neutralize that soil acidity with lime. So we don’t have a ton of local sources for lime available. I’ve spoken with people that are trying to get some mines going near Orofino. I’ve heard rumblings about sugar beet producers in southern Idaho hoping to establish some kind of distribution for northern Idaho where maybe that waste product from their production system can be actually used to remediate soil health issues in this region. That would be great, but it’s still going to be expensive. And it also, if it’s granular, it can take a long time to react–so it can take a long time to get a return on that investment.
So it may be the sort of thing we can combat this problem incrementally if we can start applying some maintenance lime to offset any excess ammonia that we’re expecting to see with our ammonia applications, that might be worth trying out–if we can add on just a little bit of extra to maybe mitigate the acidified soil over time.
There are some really good WSU Extension publications that are linked from the [WSU] Small Grains site that actually describe liming, lime recommendations. And they also describe some of these other issues like aluminum toxicity.
You know, we have some aluminum and tolerant wheat varieties–that’s only going to get us so far though. If we keep on this track, we will get to a point where we’re not going to be able to grow a whole lot in some of these soils. So it is important to find some way to address this problem.
And so we have set up a lime experiment at Cook East, the no till side of Cook, and we used an application method where we injected liquid lime at four inches to try to target specifically that acidified fertilizer band. So we’re hoping to look at how that impacts the soil acidity.
You know, how effective is that approach? Can we mitigate it? How does that impact soil health over time? Is it worth it? And what are the benefits of doing that? And we also want to look at some more details as far as how this is going to impact actual plant nutrition, maybe some of those rhizosphere processes in the microbiomes.
So we’re hoping to look at really a number of factors to link soil pH and lime to plant and soil health. But, you know, the rates that we’ve applied are pretty high, so it’s probably not practical for anyone to apply that much in a single season, in this case. But we just want to see what what’s the potential if we can get our soil pH back up to where it belongs.
And if you’re curious about this topic, there’s a lot there’s a lot of other research at WSU, University of Idaho, USDA-ARS–almost everyone around here, almost all the researchers in Pullman have looked at soil acidity at some point. So it’s worth just checking out the general research coming out of the groups at WSU, Pullman, and Moscow.
Drew Lyon: Okay. We’ll try to get some of those resources linked on the show notes so people who are interested can dig into that a little bit deeper. Is there someplace they can go to learn about the research you or your unit’s doing on soil pH?
Dr. Melissa Letourneau: Yeah, we so we’re part of the USDA-ARS Long-Term Agroecosystems Research Network. The Cook Agronomy Farm is one of the long-term research sites. There are 18 across the country and so we actually do have a web page–I’ll get that to Drew to post with the transcript.
Drew Lyon: Okay. Excellent. Well, the soil acidity problem seems to be something we can see coming. And we know we have to deal with it, but we need to try to figure out a way to deal with it that is affordable, I guess, and can keep farmers in the in the farming business. So this research is really important because I think we can see if we keep doing what we’ve been doing, we’re going to have a crisis somewhere down the line.
So, keep up the good work and let us know what you learn. Thanks for being my guest today, Melissa.
Dr. Melissa Letourneau: Yeah, thank you for listening in.
Thanks for joining us and listening to the WSU Wheat Beat podcast. If you like what you hear don’t forget to subscribe and leave a review on iTunes or your favorite podcast app. If you have questions or topics you’d like to hear on future episodes, please email me at drew.lyon — that’s email@example.com — (firstname.lastname@example.org). You can find us online at smallgrains.wsu.edu and on Facebook and Twitter @WSUSmallGrains. The WSU Wheat Beat podcast is a production of CAHNRS Communications and the College of Agricultural, Human, and Natural Resource Sciences at Washington State University.
I’m Drew Lyon, we’ll see you next time.
The views, thoughts, and opinions expressed by guests of this podcast are their own and does not imply Washington State University’s endorsement.