Organic Matter and Carbon Sequestration in Agricultural Soils with Dr. Doug Collins and Dr. Andrew McGuire

Crusting at Moses Lake.

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Soil and Water Resources
WSU Center for Sustaining Agriculture and Natural Resources
WSU Agricultural and Food Systems program

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Episode Transcription:


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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In the fall of 2022, students in Agricultural and Food Systems 201: System Skills for Agricultural and Food Systems were given the assignment to speak with someone in WSU Extension to identify a topic of interest and interview a person knowledgeable on that topic for a podcast episode. We have selected two of the podcast episodes share with you. This podcast episode was created by Eric Johnson, Bailey Sherley, Joe Plagerman, Codi Thomas, and Jaron Schneider. I think you will enjoy it.

Jaron Schneider: Hello, this podcast has been developed by Washington State University students in partnership with WSU Extension as part of our Agricultural and Food Systems class. My name is Jaron Schneider and our team includes Codi Thomas, Bailey Sherley, Eric Johnson and Joe Plagerman. Our guests today are Dr. Doug Collins and Dr. Andy McGuire. They are Extension specialists and soil scientists with WSU Center for Sustaining Agriculture and Natural Resources.

Today, we’re going to be discussing how soil health and organic matter affect crop yield and crop quality.

So, first question I kind of have for you guys is what is organic matter, you know? Can you guys kind of define it and what role does it play in soil health?

Dr. Andrew McGuire: Yeah, I can take that one. Organic matter is basically the end product of rot in the soil. So, when plant materials get in the soil–roots and stems and leaves and also dead microbes–and then when they decompose in the soil, you end up eventually with organic matter.

And it’s quite complex. It’s not just one thing. Some of it is highly stabilized. Some of it is less stabilized. And it’s very complex, as I mentioned, chemically. And there’s a lot of discussion about what it actually is. But in agriculture, it serves as a food source for microbes. And you can also think of it as kind of a glue that binds soil particles together into clumps called soil aggregates.

Some of it rots every year so the part that’s not as stable will decompose every year, and that releases nutrients like nitrogen and phosphorus, which are important in crop production.

Jaron Schneider: So, does soil type [have] an effect on organic matter?

Dr. Andrew McGuire: It can, yeah. Small soil particles like silt and clay, they can actually protect organic matter from further decomposition. So finer textured soils are going to have the potential to have higher soil organic matter levels.

Dr. Doug Collins: Yeah, this is Doug. Also add to that that a big part, a big portion of organic matter, about 50 to 55% is carbon. And carbon is brought into the soil, especially in natural environments through photosynthesis. And in a typical natural environment, we see more accumulation of organic matter in heavier textured soils. And that’s due to what Andy was saying about the clay and silt particles being able to protect that organic matter.

But also, those soils will hold water better. So, in a natural environment, soils that are more rich and silt and clay compared to sand will hold water and therefore support more photosynthesis. So over thousands of years or tens of thousands of years, you’ll see more production in those soils and therefore higher native organic matter content.

Jaron Schneider: So, from a crop production standpoint, you know, what is a good level of organic matter? Can you have too much? Can you have too little? What kind of is the sweet spot?

Dr. Doug Collins: So, first of all, the typical agricultural soils, we call those mineral soils–so that means that they’re dominated by mineral particles as a part of the soil. So, in a typical volume of soil, most of what’s there that’s mass is going to be sand, silt, clay particles–kind of weathered rocks. And that might account for about 45 to 50% of a volume of soil. And then you would have some portion of organic matter in the soil. So maybe like 1 to 4% as a general amount of organic matter. And then in the volume, you’re going to have some pore space which is going to be occupied by water or air.

Some soils have a lot more organic matter than that, and if soil has more than 20% organic matter, we call those organic soils. So, you can think of like a peat bog or just environments that are really high in organic matter.

And in urban soils like especially urban agriculture, we do see soils that people are growing in that have 20 to 50% organic matter and even some diversified vegetable farms where they add a lot of amendments, you can see really high organic matters in those soils. And those soils can be quite productive, so it’s hard to say that there’s like a high limit that you want to avoid. But in those lower, more typical mineral soils, we generally think of having higher organic matter is better, right? So, if you’re going from like 1 to 2%, that’s pretty well considered to be a good thing. And we see productive increases with that.

On the really high end, ironically, soils that are really high in organic matter don’t aggregate very well. They can kind of just create like a compressed mass. So, they may not have as much air available to plant roots at those really, really high levels.

Jaron Schneider: So as an agricultural producer of like a large-scale farm, is it even cost effective to attempt to manage organic matter to a finite level of trying to increase it by a percentage or so?

Dr. Andrew McGuire: It really depends on what sources of plant material you have. If you are near a livestock production operation where you have lots of manure available and you don’t have to transport it very far–because it’s heavy, because it’s wet–then you have a source there of organic material that can go on your soil, and you can increase it fairly economically that way.

The same thing goes for if you’ve got a good source of compost or in a lot of cases it’s manure that’s composted that you have. If you don’t have those kinds of sources where you’re actually taking plant biomass that’s grown on other land and concentrating it on your own fields, then it’s a little bit more difficult because you have to basically rely on the photosynthesis–the plants or the crops that are growing on the land–to supply that organic matter of the soil.

And there are things you can do. You can reduce your amount of tillage because tillage adds air to the soil and basically increases the amount of decomposition going on–so you can lose organic matter that way. So, if you can reduce tillage, you have the potential to increase your organic matter level and you can also increase your crop productivity–so irrigation, keeping the right amount of nutrients, those can all increase crop production, which is going to increase your biomass and the return of that plant material to the soil.

Jaron Schneider: So, there’s a lot of talk about cover crops. Do cover crops kind of play a role in that? Is there a way to boost organic matter or is it not one of the focuses of that?

Dr. Doug Collins: Andy was talking about bringing in compost and manure. Cover crops are another way to add organic matter to soil. So just as a definition, a cover crop, I like to call it kind of like a gift to the soil. So, this is a crop that you’re growing not to take to market. You’re really, as a farmer growing a cover crop just to increase soil health generally.

Cover crops provide a lot of different functions. One of the functions is that it can increase soil organic matter. So, if that was your primary goal, you would want to go for a cover crop that was going to produce really high biomass. Another thing that cover crops can do, some cover crops are legumes and they can add nitrogen–they can biologically fix nitrogen from the atmosphere and add that to the soil. So, a lot of times with cover crops, you’ve got competing objectives, but in general they’re a good way to add organic matter.

Jaron Schneider: So, from a, you know, kind of switching gears a little bit to soil health, like you mentioned, we talk a lot about soil health–you know, a lot of the horticulture classes we’re in. But what about actually applying all that talk? How do we do that? What are some things that should be considered to a large-scale producer or even a small-scale producer when coming up with a soil management plan for health or organic matter, whatever it is, that’s the focus of that plan?

Dr. Andrew McGuire: I kind of look at it in two different ways. The first way is you look out and say, “what problems do you have with your soil? And first and foremost, is it prone to erosion either by wind or water?” Because you can’t really improve your soil’s health if it’s also eroding because erosion takes the best part of your soil first.

It’s taking the topsoil, which is mainly your fine particles that are going to blow away. And that organic matter—they are small particles–they blow away or wash away with erosion. So first of all, you got to control erosion and then ask what other problems you might find that are traced to your soil.

So, water infiltration, water drainage, do you have crusting where your aggregates break down. You get that surface crusting that actually reduces infiltration, reduces or impedes emergence of crop seedlings. And then the other problem is soil-borne disease. And although soil health doesn’t guarantee that you won’t have disease problems, it can also help because it creates a better environment for your crop growth.

So, if you look at all those problems and you don’t have any of them, then you can look at actually improving your organic matter levels and that’s going to help your soil become less prone to those problems in the future.

Jaron Schneider: So, what’s the incentive to convince a farmer to go more of an organic matter approach–a natural approach–versus just synthetic fertilizer that’s cheap, easy, accessible? You know, how do you convince a farmer or a producer to go the sustainable route?

Dr. Andrew McGuire: I’ll give you my take. Doug can follow up afterwards.

I worked with a group of farmers here in the Columbia Basin probably five or six years ago, and one of them really said something that really struck me. We asked him this basic question “why are you worried about organic matter and soil health?” And he said that since he’d been working or using practices that increase soil health so cover crops, reducing tillage, compost, those kind of things–he found that everything else just seemed to work better.

And so he viewed it as a risk management practice. So, his overall operation, it was part of reducing the risk of all these other operations that had to do with the soil. So, planting and harvesting–this is a potato grower, so harvesting is a big deal. If you have good soil structure, it’s easier to harvest, it’s easier to irrigate. So that’s kind of how I think of it now as risk management.

Dr. Doug Collins: Yeah, it’s a good question. I guess kind of the economics of soil health and there’s probably not as many short-term benefits. So, I think if you look at a traditional approach of just ignoring your soil health and adding everything that the plant needs to the soil for one year, that might pencil out better.

But over the long term, you know, like Andy was saying, you might get a dry year or something. And if you’ve got a little more organic matter that could help with that, you’re going to have more nutrient holding capacity. So, it’s probably more buffered from nutrient loss or drought–different things unexpected.

Another component in the market could be differentiating your farm. So, if you did get certified organic, then as part of that certification, there would be demonstration that producer’s adhering to some soil management practices, trying to improve their soil and that could pay off, you know, in terms of a better price in the market for organic.

It’s also possible that we can see in the future some other elements that might pay off in terms of carbon sequestration associated with management practices–increasing organic matter, growing cover crops.

Jaron Schneider: So, my next question, you know, kind of going off of that. What about [growers] with perennial crops like orchards or vineyards? They’re a little bit more limited on what they can do to manage soil health? What can they do, I guess, to improve their soil health?

Dr. Doug Collins: They have kind of a big advantage. I would say like one of the major disrupters to soil ecosystems, especially the animals that live in the soil, you know, we’ve got macroinvertebrates and nematodes and protozoa and bacteria and fungi–there’s this whole complex food web in the soil. And one of the most destructive things to that food web is tillage. So, we definitely in perennial systems, you’re more liable to see indicators of a more stable ecosystem–often more diversity, more fungal life, macroinvertebrates, those things.

So that is a big benefit. And then organic matter can be added to the surface of perennial soils and the benefits will be seen also in those crops. So, there are still things that can be done. In some instances, if you needed to change pH of the soil or some major nutrient changes or you want to incorporate organic matter, a lot of that is best to do before planting a perennial crop.

So it’s definitely good to do some pretty serious soil analysis and you know, at a moment before you’re going to go into perennial to see what you can do more easily before planting a perennial. But in general, perennial soils I think, have some advantage because they don’t have that annual disturbance that’s typical in more annual cropping systems. Not all, but most.

Dr. Andrew McGuire: Yeah, just one thing. Doug and I had a colleague that’s now retired, David Granatstein, and he did a lot of work in tree fruit and shared that keeping a mulch under the trees rather than bare ground would really improve the soil quality, soil health there quite a bit. And then one of the other benefits of having perennials is you have deep roots systems and so you actually put carbon through those roots, you put it much deeper than you can with annual crops. And there’s some advantages of that–it stays around longer. So that’s again, another advantage of perennial crops.

Jaron Schneider: So, you briefly talked about carbon sequestering. So since obviously organic matter is mainly carbon, you think it would be beneficial to farming if farmers were essentially paid to store carbon? Would it help soil health? Would it be a beneficial policy, I guess?

Dr. Doug Collins: I would be in favor of that in a lot of instances.

You know, I think like if we’re talking about, as I said at the beginning, kind of differentiating between a typical mineral soil, you know, with 1 to 4% organic matter versus an urban soil or any soil that’s been really highly amended where you’re getting very high organic matter levels–I think in those soils there is a potential to sequester carbon through management practices that increase the amount of carbon added to those soils. It’s not a straightforward thing. There’s a lot of modeling that’s involved in this. So, you know, if you add this amount of carbon, how much is going to be lost just through decomposition and then it goes back into the atmosphere. How much can actually be stored?

The amount that can be stored is, we know related to, for example, clay content–so soils with higher clay are able to store in sort of a longer-term fashion–a little more carbon management practices have a big effect. So, carbon can be protected in the soil by a close connection with clay particles. It can also be protected in aggregates and tillage is something that breaks up those aggregates.

So, carbon addition is important. But then also what’s happening in terms of management following the carbon addition and what is the soil texture and some of the other needed parameters of the soil. But I think it’s important for everybody to recognize the importance of soils and how soils are a part of the carbon cycle and how they’re related to greenhouse gases.

They can be both a source or a sink of carbon. And I think the more people that are aware of that and just turn around and sort of point the finger at farmers and say, “oh, well, you should put more carbon in your soil.” Well, you know, as we mentioned with cover crops, for example, that’s a crop that a farmer grows and doesn’t get paid for–so I think that if the societal pressure is always just towards producing the cheapest possible crop that is not necessarily going to happen in parallel with the best management practices.

So, if we can somehow find a balance between those two often competing pressures, I think that would be helpful rather than just leaving farmers to kind of hold the bag on that.

Dr. Andrew McGuire: I would just add that in terms of soil carbon sequestration is that it addresses the climate change part of it, obviously. But if you start looking at soil health and we tend to get focused on soil organic matter and how much is stored in the soil, but there’s new search and there’s actually some old research that shows that we really get some of at least some of the benefits from the flow of carbon through the soil–so using up organic matter, that decomposition of organic matter. I mentioned the nutrient release before–so there’s this kind of a balance–we need to store it, but we also need to use it. And I think when we focus on just sequestration, which is the storage part, we miss that whole part of it about we have to actually have some decomposition going on to get some of these benefits.

Jaron Schneider: So, my next question to you guys, what do you guys think the future of soil health looks like? What would you guys want it to look like? What’s the best practice? If there really is one answer to that.

Dr. Andrew McGuire: There isn’t.

Dr. Doug Collins: I like Andy’s answer to that. Assessing where you are and then what can you do to improve it if that’s needed. Again, stressing the importance of keeping soil where it is. You know, we can talk all day about what’s the best soil health measurement, but if you’re losing soil to erosion, either from wind or water, then that’s the first place to start.

I think another important concept is that, you know, you need to do soil health testing or soil testing more than once. So doing a test and then if you’re altering management practices, you know, doing another test a couple of years later to see how things are changing, and paying attention to the functions that you want your soil to achieve and making sure that your management practices are in line with those functions.

So, the only function that–we’re not just asking farmers to just store carbon in the soil, we’re asking them also to produce food or animals or livestock food from that soil. So there has to be some decomposition of organic matter in order for that to happen. That’s necessary. We want that to happen. So, trying to figure out what’s the function or the functions of the healthy soil and then managing towards those functions. And so, carbon sequestration may be one of those, but it’s not the only one.

Dr. Andrew McGuire: I agree with the focus on function, we need to have that. We also need to remember that it’s all very local. What the soil health tests that come from the Midwest or back East may tell us is going to be different than, you know, on the eastern side of the Cascades. Here where we have a very arid climate, and therefore, we have very low native organic matter levels versus the west side, where you’re going to have higher organic matter levels just because you have higher plant productivity over there, because you have more rainfall.

And so, in each of those locations, I think the growers need to ask themselves are their soils improving or are they degrading? And start there.

Jaron Schneider: Well, thank you, Andy and Doug, for taking time out of your busy schedules to talk to us. So, thank you for that.

Dr. Doug Collins: You’re welcome. My pleasure.

Dr. Andrew McGuire: Yeah, you’re welcome.

Drew Lyon: I hope you enjoyed this podcast episode produced by AFS 201 students Eric Johnson, Bailey Sherley, Joe Plagerman, Codi Thomas, and Jaron Schneider, and thank you to their guests, Doug Collins and Andy McGuire, WSU Extension specialists with the Center for Sustaining Agriculture and Natural Resources.


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 — ( You can find us online at 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.