AoR 74, Why Does Soil Organic Matter Matter?, Doug Collins & Andy McGuire, Part 1

Soil organic matter is considered an indicator of soil health, there are numerous methods promoted for building it, SOM is credited with the potential to alleviate greenhouse gas increase, but Andy & Doug say the way we've understood SOM is perhaps an artifact of the old ways of measuring it. The complexity of what we call 'organic matter' is significant, challenging, and prompts a re-thinking of both soil health measurement and soil-building practices. This episode is not quite "Why Everything You Thought You Knew About Soil Organic Matter is Wrong", but advances in soil biology and chemistry will certainly challenge your long-held notions about this topic. Be sure to come back for Part 2 of this interview with Doug Collins and Andy McGuire, both of them agronomists and professional soils enthusiasts.


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>> Welcome to The Art of Range, a podcast focused on rangelands and the people who manage them. I'm your host, Tip Hudson, range and livestock specialist with Washington State University Extension. The goal of this podcast is education and conservation through conversation. Find us online at

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This is the first in a two part interview with Andy McGuire and Doug Collins on soil organic matter. To make sure you don't miss the second episode, subscribe to the podcast feed in the podcasting app of your choice if you have not already. For those listening on the website, instead of a mobile device, you should consider listening on a podcasting app for two important reasons. One, because each new episode will automatically show up on your device every two weeks and two, because you can download the episode so that you can listen when you're not connected to WIFI or even the SEO signal. To make sure you're downloading content so the audio is available offline, go to the show in podcasts on Apple device and tap the three dots in the top right corner of the screen, then touch the Settings button. When the Settings menu is displayed, look toward the bottom of the screen to make sure Automatic Downloads is turned on. In Stitcher, touch the cog just below and to the right of the logo to turn on downloads. Again, you can get to the show in the podcasting app through the website or you can search for The Art of Range, inside the app. That first word, The, is part of the official podcast name so it's important to include it in some searches. Before we move on to the interview, I need you, yes, you, to complete a short survey on the usefulness of the podcast to you and some information about who you are. This will help greatly in securing funding for continuing the podcast. This new listener survey is linked at Go to the episode's page and scroll to the bottom where it says, 'We want your input.' I can't tell you how valuable this information is. For those who feel like they've responded very recently to a survey, this one is different but similar to what you may have completed already and it will still be really useful if you take 60 seconds to respond. And finally, please put your email address into the Subscribe button at the bottom of the Homepage. This will allow me to communicate more directly with you and occasionally distribute materials related to the podcast.

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My guest today on The Art of Range are Andy McGuire and Doug Collins. Andy started with WSU as an agronomist a few years before I did. I didn't start as an agronomist and that'll become obvious pretty soon. And he's done a lot of work on cropping systems in commercial agriculture. Doug is a soils guy with WSU who has been with WSU not quite as long and also works in this, or at this intersection between soil science and food production. I want us to talk about soil carbon and how we and various sectors of agriculture manipulate and measure soil carbon. And after reading some of what the two of you have written on the topic of organic matter, I am humbled. I normally go into an interview with about a B plus understanding of the topic that we're discussing. But in this case, I feel like I'm at about a D plus or a C minus and in my world, those grades don't even count. So as you know better than I do, we could discuss this for days rather than an hour and I think we may just have to wait in and see where we get. So the two of you assume that the listening audience knows about as much as I do with which feels like not much right now. So welcome and why don't each of you give a brief introduction on what your background is, educational background, your life experience that lead you to the current role that you're in with WSU. We could flip a coin but you can't see me. Andy, take it away.

>> Okay. Thanks, Tip. My background is actually in ag engineering, that was my undergraduate degree so I tell people, I'm a recovering engineer. And then I got a graduate degree and master's degree in agronomy after that and so agronomy kind of combines soil science and crop production. And here in Washington, I work in the irrigated Columbia Basin. And with the renewed or increased interest in soil health, I've been working with growers on their soils here. And in terms of how do we improve them, mainly through the use of cover crops and specifically mustard green manures. And then also with reduced tillage systems, so no till and strip till. And whenever you get into soil health, you start discussing soil organic matter and so that's how I've gotten into it.

>> And I got my undergraduate degree in biology. And then I got interested, I don't have a background in agricultures from my family or anything like that. I grew up in Wyoming so there's a lot of ranching there. I've spent a lot of time outdoors and kind of appreciating the natural environment. But got interested in agriculture following my undergraduate and did a master's degree in plant pathology which was great. I think of plant pathology as, as kind of the soil microbiology of agriculture. So I took that degree and did some, worked as a disease diagnostician for a bit. And through that, kind of really through that process, because in, in my master's work, I looked at pathogens in the above ground portion so I didn't focus much on soil at all, but in my work looking at diagnosing plant problems with plants, you know, a lot of times the problem we see above ground is a manifestation of what's going on below ground. And so I started to become more aware of soils and got more interested in that and, and did a PhD in soil science. And through that process, worked on organic vegetables, looking at systems with some integrated animal agriculture, integrated animal and vegetable production, looking at the soil biology so, you know, really relying on that background in plant pathology was helpful, like studying bacteria and fungi in the soil. The change there, I would say, is as a plant pathologist, you're, you know, looking at that 1% of microbes that cause a problem that can be significant but when you're looking more generally at soil biology, you know, 99% of what's in there is actually very beneficial. So that was fun and since then, I wor -- began, yeah, my position at WSU working, still working in organic vegetable production, soil health, I do a lot with composting. And, you know, or have gained an interest in, in organic matter, I'd say from the beginning. What's been interesting recently is trying to keep up with the changes myself, in terms of our understanding of soil organic matter.

>> Yeah, thank you. Organic matter is a pretty hot topic in the world of grazing management. In the, you're in the group of people, I guess, the international, national conversation around what sometimes called regenerative agriculture. It's a big deal at this intersection of grazing and crop management. There's a lot of talk about soil health in traditional range land management, as well. We're halfway through the official international decade on soil health. I think your one was 2015 and that was the UN's International Year of Soils. You know, soil loss is associated with the decline of ancient civilizations. And in all this stuff, organic matter is often held up as the primary indicator of soil health. The CAN, QAnon of agroecosystem management. Building soil organic matter is kind of like the old joke about playing a country song backwards, you get your wife back, your truck back, the boat back, the farm back, send a picture of the boat. So there seems to be this implicit assertion within the regenerative grazing and sustainable agriculture crowd that every soil type has tremendous potential to produce more soil organic matter. You just have to, you just have to work it. Jeff Herrick is a soil scientist with the Jornada Experiment Station that we've had on the podcast a couple times and he would say, there are real physical limits to soil, namely soil texture and soil depth that don't change much. They're, they're pretty static or immutable and that we have some ability to influence, at least, the plant soil interface through human management. But, but there are some real limits on what can be done with that soil. But the big question still seems to be, what is soil organic matter? You know, the, the term, the term feels like it leaves open a really broad range of, you know, of concepts. Organic, I suppose, if I were attempting a dictionary definition of organic, I would say it's any carbon-based compounds that came from living things or the decomposition of living things. I didn't look that up, I was just shooting from the hip. And matters, everything that has mass and takes up space. I remember that from high school science class. But that, that doesn't really help us very much in defining soil organic matter. So, you know, what are some of the misunderstandings that we have about soil organic matter? What is it? What, what did we used to think it was? I'm not sure that's a complete sentence. You know, I understand from some of what you guys have written that our ideas about soil organic matter were limited by the mechanisms that we had for measuring it. So you can pick whoever wants to jump in on that. I think this is a really large topic and you're welcome to come at it however you'd like to.

>> Well, this is Andy, before I hand it off to Doug to answer all that [laughing], I am, what my favorite definition of soil organic matter, and I use this when I think, start to think I know what it is and I'm going to read it for you. It's soil organic matter is a dual meta chemical physical hydrogel structure arranged in hierarchical supramolecular within supramolecular self-assembling architecture. So that's, that's the one I give to growers all the time and [laughs] -- just joking.

>> I think I like mine better.

>> Yeah, so it's a very complex structure and I think we're still trying to figure that out. And Doug knows a lot more about that part of it.

>> I like that, Andy, that's good. I think we can actually pick some of that apart, the supramolecular part, specifically. Hopefully we'll, we'll get to that but I would say, Tip, your definition was pretty spot on. Carbon containing molecules from, you know, living or dead organisms that are in the soil, obviously, that makes them soil so it includes the recognizable organic debris. So if a plant leaf falls into the top of the soil, you know, we call that litter and that, that litter is going to be a big input, I think, for soil organic matter. So that recognizable leaf particle as it starts to get broken down by invertebrates, you know, the shredders that break that apart, that's, that's organic matter, that recognizable stuff, all the way to the biomolecules, the proteins, nucleic acids, organic acids that are going to, you know, be the products of decomposition, that everything, yeah, came from a living or dead organism and, and then it finds its way into the soil and starts to, starts to break down. So I think that, you know, at the, at the high level, that's what we're talking about. You had a lot but Andy's, you know, the definition Andy read just does get to the complexity of the, these byproducts of decomposition and their fate and, you know, how they're interacting with the mineral particles in the soil. They're, they're food for microorganisms and then the microorganisms, themself, become organic matter.

>> Yeah, I don't know if this is a good starting point or not but in, in one of the factsheets that you put out on this, you described soil organic matter storage and maybe delivery as being potentially conceptualized as water behind a dam where you have, you have this reservoir of, of potential material and then, and then there's a flow of carbon that is being made available, you know, on a, on a annual basis to plants and that represents the flow through the dam. I really like that, that analogy but maybe before we get there, if you were attempting to describe how people thought about soil organic matter, say, in 1950, how would you describe that?

>> Yeah, I mean, there has been a paradigm that, that is definitely beginning to shift. You know, a lot of people are familiar with the term humus. And the connotation of humous is, is that it was stable. So there was a, you know, or this litter, this material falls into the soil, it's broken down, some portion of that becomes part of the humous which is the stable portion of soil and some of it is more active. And the, you know, the idea with humus was that these were new molecules that were formed from byproducts and that these, these molecules were inherently stable. And what's really, inf -- you know, the informing or changing this understanding is the, is the idea that if a microbe can get at it, it's, it's pretty well not stable. So the concept of the inherent or intrinsic stability due to some chemical, you know, element of the, of the molecule is not really true, or especially the idea that there's these large, you know, humus molecules that, or because of their chemical structure, can't be degraded. If a microbe can get at it, it can be degraded. So what, what's becoming more important is thinking about the environment and the location of the organic matter in the soil. So there are some areas within the soil where organic matter can be protected from bacteria and the enzymes that they, they release to try to degrade the organic matter. So aggregates are the, kind of the building blocks of soil structure and inside of those aggregates, you can get organic matter that's pretty well protected, at least until those aggregates are broken apart. And then another place where we can see organic matter persist is if it is able to attach to the surface of clay particles and we get what we call these organo mineral complexes. And the organic matter that's on those clay particles can be very difficult for microbes to get to. So those are, those are the big things. And then, you know, there is, there [laughs], there is this concept of a supramolecule which is not a great word, I mean, humus is a nice word, it kind of just flows off the tongue and we're all familiar with it. But the concept of a supramolecule is, I think, if you, if you really want to or you like that idea of, you know, the stable organic matter, I would try to think of it as a supramolecule. And that's, these are going to be molecules that are derived from breakdown products of litter and other, you know, inputs. And these molecules, when they break down, they're going to have hydrophobic, which is water loving areas, and hydro, I'm sorry, hydrophobic areas which are, avoid water, and then hydrophilic areas which are water loving. And those hydrophobic areas will kind of find each other and get together. And, and then on the outside of those will be the hydrophilic areas with the, you know, the polar charges where they have both positive and negative charges facing the water which is this ultimate solvent of things and it, you know, it has positive and negative charges, too. So the hydrophobic areas come together. And so you get a bunch of little molecules that come together to form these larger molecules. It kind of sounds like we're talking about humus. And they are somewhat stable because microbes can't really, they can't get to that hydrophobic area. They don't really have enzymes that they produce that recognize these big separate molecules and so there is some persistence of those molecules. But again, if you change the environment, if those soils dry out, then the thing that's primarily holding them together goes away. And if wets again, they're not going to form back together exactly the same way. So that is, yeah, I mean, so you've got, you know, the location idea and then the concept of a, of a supramolecule which is smaller molecules that come together and are held together in sort of a stable configuration. But again, if the environment changes, then we can see those, those molecules fall apart and become food, essentially, for, for microbes.

>> Yeah, so going back to the dam analogy, the, the portion of that, that can be fed upon by microbes in the soil is the product of that feeding, what's being, being made available to plants like the flow through the dam?

>> Yeah, I'd say that's a, that's a good way to put it. There is going to be, there's some available organic matter. And one thing, I'm going to take, I'm going to let Andy comment on the dam but just to set him up a little bit there, or the dam analogy, but, you know, one of the things we do get hung up on, you know, trying to store organic matter in soils. But a lot of the benefits that we get from organic matter actually come from the decomposition of the material. So if all the material we put in there didn't decompose, we wouldn't be getting any of the fertility, you know, we wouldn't be feeding the microbes so we need that, we need that kind of flow to happen. And, Andy, maybe you can pick up on the, the dam analogy?

>> Yeah, well, just a follow up on that, is that the kind of thing that we would see, say, in a peat bog, in an environment that doesn't have either, you know, maybe doesn't have the temperature regime that supports microbial activity, or you have anaerobic conditions. So you have an accumulation of this carboniferous material that is not getting mineralized, I'm not sure I'm using the right word there. Would that be an example of someplace where you have a large amount of carbon storage but no flow?

>> Yeah, this is Andy again. Yeah, where you have, you know, cold temperatures, you know, for instance, if you're looking at soils in North Dakota versus here, I mean, their soils are frozen a lot more, a lot larger percentage of the year than they are here. And so that organic matter is going to stay around and so they can build higher organic matter levels. So that's the storage part. Or if you're in a bog where the lack of oxygen doesn't allow the organic matter to be decomposed by microorganisms, again, it's going to build up. And so that's where you can get these soils that are 50% organic matter in, or even more in some cases. And what Doug said about the, the flow through the soil, if you follow the carbon flow through the soil, it's actually an energy flow because that's why microbes use it as food to get the energy out of those, those, that fixed carbon from plants because plants, plants are called primary producers because they're the ones that can take sunlight, transfer that energy into storage carbon molecules. The microbes go after that energy by breaking down those molecules. But the question then becomes, you know, do we want to store it or do we want to use it? And the, there's kind of a balance there because we do get some benefits from storage and we do get some benefits from using it. And so I've written about the idea that we should look at carbon flow but I don't think we have the tools there to measure it yet. We have the, we, and that's really why we focus on soil carbon storage because we assume that, you know, a percentage of that stored soil organic matter is going to be used every year.

>> Yeah, maybe to back up one step, too. What do you think about organic matter being used as an indicator of soil health, particularly in agricultural environment where the idea is that if you're managing well, you will maintain some level of soil carbon?

>> I'll take a shot at that first. I think it's probably one of the best measurements we have right now but you can't, it's all relative to your environment so the context matters, you know. What we have here in terms of stored organic matter in the Columbia Basin is never going to be what they have in, you know, Northwest Iowa, where, you know, they've had hundreds of years of prairie that, and perennials to build up that soil and lots of rain that could produce the vegetation. So, you know, all other factors equal what you have in the soil is, is a matter of carbon in versus carbon out. So the more carbon you have coming in, you're going to have the potential to build up higher organic matter levels so their, their potential is much higher on those tall grass prairies in Iowa. They're much higher in North Dakota because of the temperature that we mentioned before, the cold temperatures, they're much lower here and we started much lower because of our rainfall. We just didn't have the potential to produce much vegetation and so therefore, the, the input, you know, that flow into the soil, if you think about the dam, the water flowing behind the dam is much lower. And so we have much lower, you know, organic matter levels here are less than 1% in a lot of cases.

>> Yeah, to extend that thinking to tropical soils, tropical areas notoriously have low fertility soils. In that case, is it because the rate of flow through the dam, again, is high because you have a combination of moisture and temperature that supports nearly maximum microbial activity? So any, any organic matter that gets deposited immediately gets mineralized.

>> That's right. Yeah. And so you have controls on both ends, you have the control or the factor of how much carbon is going in but you also have those controls on the, the decomposition side which are temperature and moisture to begin with but then we can start talking about agriculture and there, the main one is tillage. So you're putting a lot of oxygen in the soil. If it's wet, too, then you get a lot of microbial growth and microbes use the organic matter as food and you get breakdown. So it's the balance between those two things that determines what your, your storage level is, the one that we measure with, soil organic matter, you know, and measurements that you're going to send to the soil lab.

>> And what are, how have soil organic matter measurements changed in the last few decades, Doug?

>> Well, I think, you know, we still, so maybe we should talk about soil carbon. Soil carbon is a part of soil organic matter. But so the soil organic matter is going to include other molecules that carbon is bound to like hydrogen, oxygen, phosphorus, nitrogen, sulfur, potassium, calcium, all those are part of the soil organic matter. So there, you know, some, if you go to a soil lab, you're usually going to get some sort of soil organic matter analysis. But sometimes in research, we will just look at soil carbon to kind of look at that piece more directly. And the total soil organic matter is, you know, still frequently done but there are some new analyses that try to look at, you know, if we take the dam analogy, like how much energy is stored behind the dam, that's, that's, you know, in that available or likely to be, become available pool. And so permanganate oxidizable, carbon is one test, that kind of gets at that and then over, you know, another test is mineralizable carbon and that will predict, you know, how, how much energy will, will come from that soil, you know, over a shorter period of time, like what's going to go through the, through the dam, and in a more quick way. So there are tests like that, that are becoming more frequently used, especially if somebody is trying to get a more in-depth analysis of their soil health. And then maybe direct some management followed, following from the results of those tests. So it's a little more nuanced than just, I want to build my soil organic matter. And then there are other analyses where, and I wouldn't be surprised to see these become more common in the future, would be to look at that, you know, how much carbon is attached to clay particles, for example, how much carbon is just floating around or free in the soil which we call that particulate organic matter. So the other one would be the mineral associated. And the carbon trapped in aggregates is really difficult to analyze so that'll probably stay in the research world for some time.

>> The, the question for these, some of these new measurements, at least for me is, do they tell us anything more than total soil organic matter? In some cases, it seems like they do, you know, the pomegranate oxidizable carbon, sometimes called active carbon. It's awfully often highly correlated with total soil organic matter but sometimes not. And so my question is, and it seems to change quicker which is maybe one of the advantages of using it instead of total organic matter because it's a smaller pool so it's easier to measure that change. But often, it's, again, highly correlated with total soil organic matter.

>> One of my questions was, what are some common misunderstandings of soil organic matter? You began to hit on some of those but going back to this discussion within regenerative agriculture and the idea that if you manage in certain ways, there's almost no upper limit to how much soil organic matter can be built up. It sounds like, there, there are, there's probably a range of values that are possible in a given, say, geological context. Is that the case?

>> Yeah, I think if you look at natural soils, you know, the, the primary product, productivity is a big deal, right? So what was, what was there before? And so you're not going to have the same primary productivity in a desert that you do in a grassland. And then you can look even further, why are those, why is it more productive? So water, Andy mentioned that's a big one. And then, even within the same climate area, differences in soil texture will lead to differences in organic matter in natural settings. And that probably happens for at least two reasons but I'm not sure which is more important. But though, the water, clay, we know clay can and hold a lot of water. So if you, you know, if you've, if you've got a heavier soil, you find you don't have to irrigate as frequently. And so in a natural environment, those more clay rich soils are holding more water and therefore, encouraging more primary productivity in, you know, natural rain fed systems. And then there's also this concept that the clay, itself, can store more organic matter so that, you know, that, I think there is an upper, I mean, there, there's an upper limit to how much organic matter can be stored on clay. So soils that don't have very much clay can't store as much organic matter in these organomineral complexes and probably, they're going to have less organic matter in aggregates, clay soils just tend to form aggregates better. So texture, texture makes a big deal, a big difference and climate as well.

>> On that point that Doug just made, though, I just read a paper and this brings up the idea that whatever we decide about the soil, there's probably going to be an exception. I just read a paper that just that thought, their idea or hypothesis is that the idea that higher clay soils can store organic matter, more organic matter was based on soils that were not necessarily in the same context which we talked about before, temperature and precipitation being the main factors in that context. And so they found soils that varied from, I think, like 5% clay to 37% clay and found that it wasn't necessarily true that the higher clay soils stored more organic matter. And the reason they had, they found which I found very interesting and hadn't seen before is that in the lower clay soils, they actually found in that mineral associated organic matter, so this, this organic matter that's kind of stuck to clay and small silt particles, it was actually thicker in the soils that had less clay. And so overall, they didn't find big differences between those, those soils with different clay levels. Now, it's just one study in one area but it does give you caution in terms of trying to generalize ideas about soil across big distances and especially across different environments. It's, we should generalize with a lot of, a lot of caution in soils because there are so many different factors.

>> This concludes the first part of my two-part interview with Andy McGuire and Doug Collins. If you have not already, please subscribe to the show in your podcast app of choice to make sure you don't miss the conclusion of my interview with Andy and Doug in the next episode. Thank you for listening to The Art of Range podcast. You can subscribe to and review the show through iTunes or your favorite podcasting app so you never miss an episode, just search for Art of Range. If you have questions or comments for us to address in a future episode, send an email to For articles and links to resources mentioned in the podcast, please see the show notes at Listener feedback is important to the success of our mission, empowering rangeland managers. Please take a moment to fill out a brief survey at This podcast is produced by CANHRAS Communications in the College of Agricultural, Human, and Natural Resource Sciences at Washington State University. The project is supported by the University of Arizona and funded by the Western Center for Risk Management Education through the USDA National Institute of Food and Agriculture.

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

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Mentioned Resources

WSU Extension publication by Doug & Andy: "Understanding and Measuring Organic Matter in Soil"

Recent article at the website BioCycle, referencing Collins' and McGuire's work: Saying Goodbye to a Soil Organic Matter Fundamental

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