The Ins and Outs of Soil Organic Matter, Compaction, and Water Infiltration with Dr. Gabe LaHue

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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. Gabe LaHue. Gabe is an assistant professor of soil science based at Washington State University’s Mount Vernon Northwestern Washington Research and Extension Center. His research and extension program focuses on soil-plant-water relations, soil physical health, soil fertility, and water nutrient interactions. He is particularly interested in how we can manage soils to improve infiltration and water availability for plant growth.

He teaches undergraduate and graduate coursework in introductory soil science, soil fertility management, and organic production. Gabe earned his B.S. degree in plant science from Cornell University, his M.S. in international agricultural development from UC Davis, and his Ph.D. in soils and biogeochemistry from UC Davis.

Hello, Gabe.

Dr. Gabe LaHue: Hello, Drew. Great to be here.

Drew Lyon: Yeah, it’s nice to be visiting with you about your work with soils and water. Water is a pretty important limiting factor here on the east side of the Cascades–not quite so much on the west, but I think what you have to talk about will have significance or interest in our growers here in dryland wheat country.

So, I wonder if you could start us off by telling us how building soil organic matter influences water infiltration?

Dr. Gabe LaHue: Absolutely. And I’ll just say, you know, as you mentioned, my research program is really at this kind of nexus of soil and water management. And even though I am based in northwestern Washington, we have a surprising amount of water issues here that we deal with both in terms of too much in the winter but then not enough in the summer. So, I spend a lot of my time thinking about how do we manage water and how can we use our soils to manage water, which, as you mentioned, is really important when you’re thinking about dryland production. And we often hear about soil organic matter as one of these kind of foundational things that you can build because it influences so many important soil properties, whether that’s nutrient cycling, for example. But in our case, I’m really interested in how this influences water movement in the soils and how soils store water.

So, you know, the first thing that has to happen for us to be able to get our soils to store water is we have to be able to get the water into this soil profile. And so especially in, you know, an area like the Palouse where there’s so many rolling hills, having that better infiltration ability is really, really important for making sure that we get that down in the profile and not just have it running off and lost to our field, potentially carrying, you know, some of our soil with it.

So, soil organic matter is something that is really important for increasing infiltration because of how it affects soil structure and also because of its inherent nature just in the fact that this organic matter does have a lot of porosity in it. So, it kind of by itself, it can help impart some structure to our finer textured soil, so especially any soils with a little more clay or even these, you know, beautiful silt loam soils that you have on the Palouse. And so, it’s really important.

We’ve seen that–we’ve done some work in dryland wheat systems–even though I’m based in northwestern Washington, we do really try and work statewide. And, we’ve done some work in low rainfall dryland wheat systems and we’ve seen that building soil organic matter–in this particular study was through biosolids application–but that definitely had an effect of increasing infiltration. So, building soil organic matter, I think we have a pretty good foundational understanding of how important that can be to improving infiltration into soils.

Drew Lyon: Okay. So, it increases infiltration. How does soil organic matter influence the ability of the soil to store water for plant growth?

Dr. Gabe LaHue: Good question. So, we mentioned the first thing you have to do is really to get that water into the soil. But then, really, there can be–once you get the water through the soil surface, one of two things can happen. Either that water kind of continues draining down into the soil profile further and further until it’s beyond the depth that it can be accessed by roots. Obviously, in a lot of our systems, you know, those wheat roots can grow pretty deep in search of that water so it has to drain pretty far down there to be lost to the plants. But that can happen just draining due to the force of gravity. The other thing that can happen is that we, you know, the soils can kind of hold on to that water indefinitely until it’s either evaporated or used by the plants through transpiration.

So, there’s been a lot of work on this, both globally and nationally. And in general, you know, there’s some disagreement about the exact–how much exactly soil organic matter increases the ability of the soils to hold water. You know, some studies have put that at a half a percent increase in water content per increase in soil organic matter. Another one put it at 1.5%. But there’s pretty good agreement that soil organic matter increases plant available water, which is kind of our word for the ability of soils to store water for plant growth.

On our own work—so, we’ve done work with a couple different long term research sites, which provide this tremendous resource because we can kind of have differences in soil organic matter that are built up due to different management practices over time. And we saw that we did have more of an impact of building soil organic matter in a finer textured soil than we did in a coarse textured soil. But in general, we still do see that building soil organic matter is important for storing water for plant growth.

Drew Lyon: Okay, I know I’ve often wondered when you say move from a traditional tillage system to a no-till system, you do increase the organic matter, but it tends to be pretty much limited to at the surface, at least in annual cropping systems. You don’t really get the buildup of organic matter at deeper depths so maybe its ability to hold water doesn’t increase a great deal, but that organic matter at the surface really helps with the infiltration, is that the real benefit? Or are we actually increasing organic matter deeper and helping to store more water?

Dr. Gabe LaHue: Yeah. So, I mean, I know there’s been a lot of controversy in, kind of, in research fields about that–you know, how much is, for example, no till building soil carbon versus stratifying soil carbon. But I think either way by having that residue cover, that’s certainly going to help with the infiltration and building some of that soil structure and increased water holding capacity in the top part of the soil. But down deeper, you know, if the organic matter isn’t being built down deeper then it’s not necessarily going to have that same effect on increasing the water holding capacity.

I should mention that the, you know, the ability of the soil to store water, really what it depends on is what we call the pore size distribution. So, how many large pores do you have in the soil? How many small pores do you have in the soil? And so, that’s something that regardless of how it affects organic matter in the soil, something like no till is really going to have an effect on that pore size distribution independent of its effect on soil organic matter.

Drew Lyon: And that effect is to what?

Dr. Gabe LaHue: So, for no till, honestly, I’m not as familiar with some of the research on direct impacts of no till on soil water holding capacity. I’ve seen a lot more of what’s been done on infiltration. for example. But in general, if you create some of these larger pores that drain more easily that can have the effect of kind of moving that water down deeper in the soil profile. Again, with our wheat systems, because they’re so rooted, you know, even in a no-till system, it’s not like you’re necessarily creating these big pores that might move it all the way out of the soil profile. But you certainly might be redistributing some of that water to lower depths if you have more of these large pores in the soil surface.

Drew Lyon: Okay. So, organic matter influences infiltration and water holding capacity of soils. How does soil compaction influence water availability for plant growth?

Dr. Gabe LaHue: Yeah. That’s something my program has really been interested in trying to start to tackle. I know that Dr. Haly Neely out of Pullman has also been doing some really good work on this subject, and there’s a lot of interest in soil compaction right now because it is one of these problems, these threats to soil health that has probably been kind of historically, maybe, underappreciated and under quantified.

You can see when you look at a field, if there’s an issue with soil erosion, you can see that muddy water coming off of it. You can see the reduction in soil depth over time. You know, you can see the exposure of the subsoil layers.

But, soil compaction can be a little bit more hidden. I know for me, in a lot of the systems where my program works that’s been something producers have really identified as a problem. And so, we’ve been trying to understand how soil compaction starts to influence water availability for plant growth.

The first thing we know is by definition soil compaction reduces the pore space in the soil, right? So, it’s kind of shrinking the amount of space in the soil that’s available for either air or water. So, our kind of maximum water content we can store definitely decreases with soil compaction.

The complicated thing is, and there’s not honestly as good of agreement on this in terms of how soil compaction affects what we call plant available water, but what happens is that when you compact the soil, some of these large pores that might typically drain fairly quickly after a rain event can turn into smaller pores that may not drain through the effect of gravity. So, you can kind of have this artificial increase in the amount of water that’s being stored in the soil, at least temporarily. But on the flip side, that also means that space that’s storing water isn’t storing air, so that’s where you can really get issues with things like water logging, for example, and lack of oxygen for good root growth. That can also affect some soilborne diseases. So, there’s certainly these kind of even though it might have more water at least in the short term, there’s all these other kind of potentially negative implications that that’s having when you have soil compaction.

Then the other big thing that I should mention is that we do know that compaction drastically reduces infiltration, right? And so again, if we’re not getting that water into the soil in the first place, that’s an area where that’s just kind of a loss from the start. And whether it drains out of the bottom of the profile or doesn’t is kind of a moot point if it’s not getting in there in the first place.

Drew Lyon: Okay. So, how does soil compaction and soil organic matter kind of interact to influence how water gets stored? So, in other words, is soil organic matter impacting soil compaction or soil compaction impacting soil organic matter?

Dr. Gabe LaHue: Yeah. And so, that’s really kind of the where we’re trying to take our research program in terms of figuring out these dynamics, right? Because we know that soil organic matter has a direct influence on water movement and the ability of the soil profile to store water. There’s been a lot of work done that suggests that, or I would say a lot of conventional wisdom, that suggests that soil organic matter is really important for protecting against soil compaction. You know, you can look at NRCS factsheets, articles in magazines like Crop & Soils, magazines that really kind of highlight the benefits of soil organic matter in terms of reducing soil compaction. But I think the reality is there’s a lot of nuance and complication there.

In our own work, we haven’t necessarily seen that you have a reduction in compaction with soil organic matter. You know, soil organic matter is fairly compressible, right? So, you can get some initial compression. You might also get some more what we call rebound or elasticity, right? Where that kind of soil might recover from compaction better. But again, in our own research, we haven’t really seen that this soil organic matter from long-term sites hasn’t necessarily reduced the susceptibility of our soils to compaction, though it also hasn’t increased it.

But, you know, one thing that is important to mention is regardless of whether it’s affecting the susceptibility of the soils to compaction, overall when you have more organic matter in the soils, that’s decreasing the density of the soil, that’s kind of increasing the porous base too. So, you’re kind of starting when you have more organic matter, even if you’re still getting soil compaction, you’re starting from a better place. And so, you’re still potentially supporting the health of the plants in spite of compaction, even if you’re not necessarily reducing the overall compaction.

And so, because you’re really not reducing the overall compaction, we’re also not seeing that organic matter necessarily affects the ability of the compacted soils to store water, right? We’re seeing kind of the direct effect of organic matter on the ability of soils to store water, but we’re not necessarily seeing an effect on compaction in our research.

Drew Lyon: Okay. So, there is an interplay between the two, but it’s not a straightforward sort of thing.

Dr. Gabe LaHue: Yeah, exactly. I mean, I wish I had a simpler story to tell. But I do think that, you know, we expected at the outset just because there is so much kind of conventional wisdom about, you know, organic matter really being this thing that can help reduce soil susceptibility to compaction that we’d kind of see this interactive effect with how it affects plant available water, water for plant growth. And that isn’t something we’ve necessarily seen. So, it really does seem to be that, you know, this conventional wisdom around organic matter helping with compaction from what we’ve seen in our own program may be less of a direct effect on compaction and really more of an indirect effect by supporting plant growth through, say, having more plant available water just because of the effect of the organic matter.

Drew Lyon: Okay. So, probably true that more organic matter generally is pretty good for most of our soils and for plant growth, but it may not be necessarily directly affecting compaction and the issues around that.

Dr. Gabe LaHue: Yeah, I think that’s an important point. I mean, I think, like I said at the beginning, organic matter has so many different benefits to building organic matter. And we like we said, we do see that direct effect of organic matter improving water availability for plant growth. But I think we just, you know, we want to kind of make sure that we’re telling a nuanced story here where sometimes there’s this kind of paradigm of, you know, soil organic matter is going to save the world or, you know, it’s going to help all these soil functions–and I think the reality is it kind of depends on the soil function you’re looking at and there may be a bit of a more complex story here. But certainly, I think still we have seen definitely that building soil organic matter can help with infiltration, can help with water storage, so it’s definitely still net positive, and it can help plants deal with the negative effects of soil compaction more.

Drew Lyon: Okay. So, how does your program fit into all this research?

Dr. Gabe LaHue: Yeah. So, I mean, our program, because we do work in a lot of these really compaction-prone systems and because we also work in areas where water is a big issue– whether that’s, you know, in these dryland systems where your yield is really so dictated by water availability or whether that’s in systems where, you know, there might be a potential for irrigation, but we’re dealing with a lot of restrictions on irrigation water use. So, my program has really been trying to figure out how do we manage our soils to increase our water availability for plant growth. And so, we’ve been looking at these kind of interactive effects of soil organic matter and soil compaction. But, and again, leading us to those kind of conclusions about how the soil organic matter may not affect compaction as much, but how maybe the importance of soil organic matter might be even more important in soils that are compacted.

I should also say that one thing we are just beginning to look at, that we haven’t done as much work on, is the interplay with rooting depth here. Because, you know, I mentioned at the beginning that there’s some disagreement about the numbers, but let’s just say, roughly speaking, a 1% increase in soil organic matter increases the soil water content by 1%. Well, it takes a lot of work and a lot of time to achieve a 1% increase in soil organic matter. It may sound small, but that’s not a number to just throw around. That’s a lot of work that goes into that. And so, yes, it’s important–soil organic matter–for building the ability of a soil to store water. But, how compaction and how soil organic matter affect root growth and the rooting depth, you know–if we can augment the root zone that plants can explore to get water by, you know, just 10% or 20%, that would have a huge effect compared to some of these effects that we’re having by building soil organic matter.

So, I think there’s a lot of work to be done to figure out how can we help improve the root zone whether that’s by potentially reducing compaction in some of these systems or by other means, but I think that’s a missing piece that we’re only beginning to look at. And I know some other researchers have been doing some good work on that. Roots are incredibly challenging to study. I have a Ph.D. student who did his master’s work on studying roots, and most people say never again. So, that’s probably one of the areas of this type of research that has been the least explored but one that would be really impactful to explore going forward.

Drew Lyon: Yeah, I know, when I was going through my graduate program, I was advised by a number of people, “Avoid doing seed bank work. All those soil samples and all that cleaning. It’s a lot of work.” There are simpler ways to make a living, but there’s a lot of important information that we need to learn about the soils.

Dr. Gabe LaHue: Absolutely. We’ve been collaborating with some weed scientists who have been doing some of that seed bank work too, and you see how painstaking it is and, you know, some of the things I think that we’ve historically not done as much of because the methods aren’t to the standard they need to be, you know, at the same time, we can’t just ignore them.

Drew Lyon: Exactly. And things are often more complicated than we at first think as well, which sounds like a story of your research as well.

Dr. Gabe LaHue: Yeah, absolutely. You know, I think, you know, it’s important to be able to embrace the nuance here a little bit. You know, you don’t necessarily want to throw the baby out with the bathwater, you know, just because soil organic matter might not be affecting compaction resistance and recovery as much in our experience, but it’s clearly having a good effect on infiltration and a good effect on water storage. And so, clearly building soil organic matter is still one of the best things we can do for soil health and one of the best things we can do for plant productivity.

Drew Lyon: All right, Gabe. I wonder, if our listeners want to find a little more information on the work you do, is there a website or something they can go to read about it?

Dr. Gabe LaHue: Yeah, absolutely. So, you can look up–my last name’s LaHue, Gabe LaHue–and you can absolutely look up my website and my contact information is also on the website. Happy to point you to the right places. And then this is also work that will be coming out in Soil and Tillage Research and so we’ll be kind of sharing that more widely.

Drew Lyon: Excellent. Gabe, thanks for taking the time to talk to me today about your work in soil organic matter and soil compaction and water infiltration. All very important things for our growers here in the dryland side of the Cascades.

Dr. Gabe LaHue: Absolutely. My pleasure. And thanks for having me on, Drew.

Drew Lyon:

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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 lyon@wsu.edu — (drew.lyon@wsu.edu). You can find us online at smallgrains.wsu.edu and on Facebook and Twitter [X] @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.

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The views, thoughts, and opinions expressed by guests of this podcast are their own and does not imply Washington State University’s endorsement.