AoR 75: Why Does Soil Organic Matter Matter?, Doug Collins & Andy McGuire, Part 2

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. If you haven't heard part 1 of this interview with Doug Collins and Andy McGuire, go to episode 74.


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>> Welcome to The Art of Range, a podcast focused on range lands 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 the education and conservation through conversation. Find us online at

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This is the second of our two-part series with Andy McGuire and Doug Collins, on soil organic matter. Go to to find the first episode. It seems like one of the things that is commonly generalized is methods for building soil organic matter, particularly in crops, soils. Is trying to build soil organic matter a good goal in agricultural soils? And if so, how do we go about that?

>> I think it's a good goal. Again, you know, I mean a lot of the, keeping in mind that a lot of the benefits come from the mineralization of the organic matter. So, you know, if we're talking about an agricultural setting where we're dependent on those ecosystem services, then we need to, you know, we have to have some energy for the soil, and that's going to come from the soil organic matter. There's also you know, aggregates are encouraged by some of the organic matters, so that's good for your soil texture. Its water holding capacity, organic matter can help encourage that, so there are a lot of benefits. We do know that tillage has had a big impact on organic matter. You know, the, it's been known for a long time that tilling your soil increases the fertility, and that happens by mineralizing the carbon, which is to say your kind of burning off the carbon, and in the process, you're releasing a bunch of plant nutrients.

>> And is that the stuff that was previously bound up in soil aggregates and was therefore unexamined?

>> Yeah, it's in the aggregates, it's part of the organic matter compounds, so it's, you know, it is chemically bound to the carbon, so it's not available to plants, you know. Basically plants, I mean, there's some evidence that plants can take up organic compounds, but for the most part, they're getting most of their nitrogen, you know, from a mineral nitrogen form, like nitrate or ammonia. And they, when we till, when we when we see them like microbial decomposition, then we get the mineral forms of nitrogen phosphorus made available that plants can take up. So, tillage is, in a way, burning up some of that stored carbon, and then encouraging more productivity. So, but the question was, you know, how do we store it? So, the alternative to that is, or how do we build it up? We'll, can we moderate the tillage to allow for organic matter that will help, you know, maintain aggregates, water holding capacity, some stored food for microbes. So, I think reducing tillage is one way to do that. And then, the other big thing is just adding organic matter, we can do that through cover crops, we can do that through manure, compost, all these organic matter compounds are just adding it to the system.

>> And one thing to think about in terms of building soil organic matter, and we mentioned the fertility that comes from organic matter decomposition, is that if you're trying to do both, rely on organic, the decomposition for, of organic matter for soil fertility, those nutrients that are made available, and a certain amount of the, a certain percentage of the organic matter in your soil is going to be decomposed every year, even if you don't have any tillage. So, you get those nutrients, but to build organic matter actually, you have, you're actually putting those, putting nutrients back into that large pool. And so, if you're trying to grow a crop at the same time, you need nutrients for your crop and nutrients to build organic matter.

>> Right.

>> And so, then the question is, where are those nutrients coming from? And Doug mentioned a few sources, but in the context of grazing and pastures, you know, you think of nitrogen from legumes. Well, that nitrogen, if you're trying to build organic matter, some of it has to go to the new organic matter and some of it has to go to the grass, which you probably have a legume grass mix in that case. But it's kind of a balance there, in terms, you just, it's not a free source of fertility, because if you're going to rely on it for crop production, and it, to restore that organic matter, you know, even just to maintain it, requires a certain amount of nutrients every year.

>> Right.

>> The rate of increase has to exceed the rate of mobilization.

>> Exactly.

>> Yeah, one question related to that. In the article that you wrote a little while ago, Andy, I think on the bio ag blog about adversity healthy soils, being able to reduce but not replace fertilizer. You mentioned that even smaller, moderate amounts of synthetic nitrogen fertilizer, they stop the biological processes that make nitrogen available to plants in soils. It sounds like that could be useful if, like in a cropping system in particular, if the objective is to begin building up soil organic matter, while still providing for plant growth. Will that be an accurate application of that concept?

>> Well, Tip, I think you're mixing two different processes there. So, you've got the nitrogen effect on nitrogen, biological nitrogen fixation. So, you can either have free living microbes in the soil that fix nitrogen, so they're taking it out of the air and making it into an available form.

>> Right.

>> Or you can have it done in lagoons, which most people are going to be familiar with. Now, nitrogen application of synthetic fertilizer will tend to reduce both of those processes. The other process that we were talking about is the decomposition, decomposition of organic matter, and I don't think there's any consistent evidence that nitrogen is going to, well there's quite a bit of debate about it, whether it in the long run.

>> Gotcha.

>> It enhances decomposition, or does it slow it down? I think where, you know, 10 years ago, people were saying that nitrogen fertilizer additions actually enhance the decomposition and so speed it up, but it turns out, they weren't looking at the entire soil profiles. So, if you go down to the deep soils, what you have is that a lot of organic compounds become, they get dissolved in the water and actually move down in the soil profile and get stored a lot deeper than we used to think. And so, what they found is that nitrogen fertilizer, because it enhances plant growth, and if you look at the whole soil profile, it actually enhances storage of organic matter, but you may not see that at the soil surface. And for most, you know, land managers, unless you're looking at carbon sequestration, and I tend to separate carbon sequestration from land management, if you're into land management, you may not increase your surface organic matter by use of nitrogen fertilizer.

>> Yeah.

>> But overall, in the deep, you're going to have more carbon sequestered in that deep soil profile. Does that makes sense?

>> It does. Yeah, I think that, I think that was part of my question. There's this tie between, call it the carbon cycle and nitrogen cycling, you know, where, theoretically anyway, part of the available nitrogen is coming from decomposition soil organic matter, and if that's being substituted by, you know, an off-farm application nitrogen, does that then allow soil organic matter to build up, or does it increase the rate at which it's being made available to plants, and therefore, you know, is disallowing soil organic matter from building up.

>> Right.

>> And it sounds like some of that's still sort of unknown.

>> Yeah, they're still doing a lot more measurements. In the past we haven't measured, you know, much below a foot, sometimes we get down to three foot, but they're talking going down to six-foot, 10 foot even, to get some of these dissolved organic carbon molecules measured. Again, that's more of a concern for carbon sequestration, in terms of climate change, and not so much in terms of Land Management, because it's really the surface organic matter that's going to affect your plant growth, your infiltration, your aggregation, those kinds of things.

>> And then, in a more, say natural setting, like a range land ecosystem, where you have nearly zero off farm input of nutrients, what you're getting from plant growth really is based on what's being made available annually, through the natural soil processes that are making nitrogen available, right?

>> Yeah, unless you have a lot of lightning. I saw a paper out the other day that, this is from the Midwest, again, talking about different contexts, where they had, I think it was Minnesota, they had a lot of lightning one year, and they could actually measure the difference in terms of the nitrogen inputs to a field, because the lightning somehow fixes nitrogen in the air, and it gets deposited through the rainfall. We don't have that problem here.

>> Yeah, I'm seeing an interesting field trial in your future.

>> Yeah, we just need a lightning created.

>> Hundreds of lightning rods.

>> I volunteer Andy to stand out there.

>> Out in the field.

>> The big copper rod.

>> Yeah.

>> So, what one of the areas of controversy has to do with cover crops, and you've written a little bit about this, that the cover crops have value, but maybe not having 37 different species of plants in a single cover crop. And then, I guess the other question in that that's not so much relevant to range lands, but you know, a lot of, a lot of ranchers that are listening will be managing both native range lands and irrigated pasture lands and irrigated hay ground, and maybe involved in, you know, some of these, many parts of the West were trying to reintegrate livestock back into cropping systems for some of the benefits that have already been mentioned, such as converting some of that crop residue into manure, that may more readily break down and contribute to available soil nitrogen. What are the benefits of, how much of that is occurring right at the soil surface? To what extent is it useful to turn that under? And what are some of the limits of using cover cropping to build soil organic matter in places where it's low?

>> Well, I think, yeah, there have been some reports that have shown if we just use cover crops you can see that, and I kind of like the way Andy was separating, thinking about things in terms of land management versus sequestration, because some research has shown that using cover crops alone, the there might be a translocation actually, of carbon to the surface. So, you see carbon increasing at the surface, but not necessarily at depth, and using cover crops with manure, and I'm not sure in this case, I think they used compost, but I'm not sure how that would, what that would look like with an animal's system at that, you know, it'd probably be similar. Then, we saw more carbon at depth. So, if you look at the whole profile, the manure, or compost integrated system with cover crops showed more carbon buildup, more sequestration in the top, you know, meter, whereas if one had just looked at the surface, and a lot of times when we soil sample for agriculture, we're sampling it, you know, six inches or maybe a foot, and so we're, you know, a lot of decisions and conclusions have been made from that kind of analysis.

>> Right, so if you have a choice, if we're not considering any of the financial implications, if you have a choice between applying a cover crop, growing a cover crop, and terminating it with tillage verses terminating it with grazing it off, what are the differences in the affect on the soil with those two options?

>> Yeah, I think that's an area that needs more research. You know, I think there's some economic implications, which would be important drivers, right? Can you, I mean, if you can get a benefit from the forage element for the animal, that's good. And then you know, do you let the cover crop grow back, do you, what do you do next? I don't know, I think it's an area, it's an exciting area that I know a lot of farmers are playing around with, and I'm, you know, excited to see what they come up with.

>> Yeah, and I would just add that, just because you're not increasing your soil organic matter with cover crops, does not make them a, you know, a wasted practice, because the first thing that cover crops do is cover the soil, that's why they're called cover crops, and so they minimize erosion, and you can't build soil health while your soils are eroding. And so, they're a step in the right direction, and if you can get grazing off them, that's another benefit. So, just because, you know, I think we've gotten really focused on organic matter, they're definitely helping maintain the organic matter levels, even if they're not increasing them. And you mentioned the cover crop mixtures, which is a very large topic we could talk about, but I'm not against cover crop mixtures, I just think that they've, that the benefits have been exaggerated. With the main exception is, and we've known this for a long, long time, pre-science even, that mixtures of legumes and grasses are beneficial, and that's because of the nitrogen effect from the legumes. So, whenever you can, especially on low nitrogen soils, you're going to see a benefit from that kind of mixture. When you get at 20 species, I really question, and science which back me up, is that you don't see benefits over, you know, if you can find your, and we're talking annual plants here, with annual plants, you're not going to see a lot of large benefit of having that in, because they're in the soil for a very short time generally, because you're trying to fit them between cash crops, even if you're grazing them. You're not going to see a lot of a lot of, a lot more benefit from a, you know, 10 species mixture versus a simple mixture of wheat and hairy vetch, for instance.

>> Yeah, I think you're right that there's.

>> And then one more.

>> Go ahead.

>> One more thing on that is that the best way to build organic matter is not using annuals, but using perennials, and you know, people that are grazing range lands and improved pastures, irrigated pastures even, using perennials, the roots in perennials, and there's research that shows that a large percentage of the organic matter is derived from root biomass and not the above ground biomass, perennials put a lot more carbon in their roots than annuals do, and so therefore, you get a lot more carbon into the soil, and you get it a lot deeper than you do with annuals, just because the roots go deeper.

>> Yeah, if we were thinking in terms of the, you know, ecological processes in a native natural ecosystem, as opposed to agricultural soils that have been homogenized through tillage and other kinds of applications, the benefit of a mixture of plants is that one of them, in any given location you're going to have one or two or three species that do particularly well in that soil niche, and it won't somewhere else. That's a little bit of a different situation with an agricultural soil.

>> Right. And with perennials, you also have the niches in time, in terms of climate. So, you're going to have a different climate in the fall, and so you'll have a plant that, a species that will grow well then, versus a different climate in the hot summer, and so you've got your C4 grasses that do much better then. And so, mixtures and perennial systems that are there all year round make a lot more sense.

>> Yeah, well I have a couple questions that maybe are more relevant to range land situations. You know, we commonly see that range land ecosystems are limited by nutrients, even more than water. I think I've told the story on here maybe once before, about a professor I had in college during my master's degree, who was working with some private timber companies doing, fertilizing forests, and they were trying to develop an economic model that would predict under what staying conditions would it be economical, you know, or would there be a payback from fertilizing a forest? And so, they had, they had these, some large plots, small plots. But in several places, there would be individual trees that had a six-foot radius that was fertilized around the stem of the tree. And you can see if you cord the tree that there was an increase in the size of the growth ring, the annual growth ring after the fertilizer application, which oftentimes was very minimal, say 10 pounds of nitrogen per acre. But what was really visible was the height of the grass inside this little circle that had 10 pounds of nitrogen added. And in particular on many of the Pacific Northwest range lands, some of the dominant perennial bunch grasses that are generally considered to be, you know, say 75% of the total biomass in the plant community are notorious for retaining the previous year's stems and leaves in the air, in, you know, standing up as part of the old plant, they don't, they don't lay down under a snow load or in response to frost. And so, you have all of this plant material that mostly stays elevated, not in contact with the soil surface, and theoretically, you know, is not breaking down and contributing to soil organic matter in those location, and you can see some differences. For example, you know, when places like that undergo wildfire, there is a nutrient flush in response to the wildfire that often generates higher seed productivity and seed viability in a year or two or three after the fire, specifically because of this nutrient flash. You know, given what we're talking about, are there some obvious ways where nitrogen cycling could be enhanced in a way that doesn't damage the plant community? I'm asking you because you can maybe think outside of the conventional range land science boxes.

>> People don't generally, yeah, so an external input is not practical, I guess, right?

>> No, no because you're mostly, yeah, you're thinking of extensive rather than intensive agriculture, you know, where an individual pasture might be 5,000 acres, and it's not cost effective to do any kind of agricultural input to that, like fertilizer.

>> It's an interesting question, because, you know, there's lots of nitrogen fixers out there, I mentioned the free-living microbes, the bacteria that can just fix nitrogen and the plants, and so what we find is that most natural ecosystems are actually nitrogen deficient, if that's a word, I don't know.

>> Yeah.

>> There, you get if you add nitrogen.

>> Right.

>> Yeah. And so, you ask yourself, why? Why, with all these nitrogen fixers, why aren't they fixing more nitrogen so that you wouldn't have that deficiency or lack of nitrogen out there in these natural areas? And there's some, there's, it's a complicated question, and there's a complicated answer to it. I don't know if you want me to go into it here. There's another nutrient limitation, I mean, to fix nitrogen you need phosphorus in legumes basically, so it may be a phosphorus limitation. Molybdenum is involved in a lot of nitrogen fixation. There's some energy limitations in terms of, legumes don't do as well in terms of photosynthesis, as non-legumes do because it's an energy intensive process, that fixation of nitrogen takes a big chunk of the energy from legumes, the carbon that they fix goes to those nodules where the rhizobia are fixing the nitrogen. And so, they're less competitive with grasses for instance, or shrubs that are non-nitrogen fixing. So, there's a lot of different reasons why we see that out there, at least, that's what scientists think. And so, trying to change that is a complex question, because you don't know exactly which one of those is limiting. But management, and often with management, especially of these natural range lands I would think, you got to, there's always an unintended consequences of what we try to change out there. And I'm sure you can tell stories about that tip.

>> Sure. I mean, for example, one of the often-observed effects of adding just synthetic nitrogen is that it disproportionately causes an increase in undesirable plants, you know, weedy species, exotic species that are maybe more efficient or are quicker to, you know, to function as a luxury consumer of the nitrogen and it disproportionately increases, plants that maybe we don't want. That's one potential impact, at least in some places.

>> Yeah. And you mentioned that the, after a fire, and that's because, you know, even if you look at the tropical rainforest, people think well, it's got huge productivity, but you actually look at the nutrients there and most of it is tied up in above ground biomass, you know.

>> Right.

>> Large trees and logs lying on the floor. The same thing happens in range land, a lot of the material, a lot of the nutrients are tied up and in that inactive biomass basically, woody plants, you know, stems, that kind of stuff. And so, when you burn it all, all those nutrients become available at once, and hopefully you get plants coming back that'll, you know, save at least the nitrogen from leaching out. But that's the reason why you get that flush. And I can't think of any way to mimic that without burning it.

>> Yeah, there's been there's been some talk about what's sometimes called pyrrhic or [inaudible], the idea that you could mimic some of those effects with grazing, and in terms of nutrient cycling, that is somewhat the case, in terms of the other effects of fire, you know, it's not so much the case. One of the common assumptions, or you know, a rule of thumb that gets thrown around with soil organic matter is that every 1% increase in soil organic matter doubles the water holding capacity of the soil. Does anybody know where that came from? And does it, does that idea hold water? Pun intended.

>> I'm not sure where that number comes from. There has been, you know, people have looked at this question a little bit more closely. So, there're, you know, with water, there's the water content, which you can measure, just how much is there on a weight basis. But that doesn't always tell the whole story, that doesn't tell how much is actually going to be available to plants. And so, to assess that, we need to look at the potential for movement of water. And so, if the water's held on very tightly, you know, in small pores or attachment to a surface, then it's not, you know, the plant, it may be there, but the plants can't get it. So, you know, permanent wilting points come more quickly for sandy soils than they do for clay soils, you know, in terms, well making, think about this. So, the water content could be the same, but there would be more water available in the sandy soil than in the clay soil potentially, because it's, you know, the clay soil holds onto it tighter.

>> Right.

>> So how organic matter affects that is going to be a little bit more complicated than just how much is there, because a lot of it is going to be held quite tightly. So, it may be true what you said, but that doesn't necessarily confer to it, you know, the same kind of relationship for plant growth based on water.

>> Right?

>> Yeah, Tip, I wrote an article on this a long time ago and I don't have it at my fingertips, but that number comes from a paper by Hudson, 1994, I think. And since then, there's been a meta-analysis that has looked at a lot more data than Hudson did, and they found that it's not near that high in general. And as I mentioned before, there's always exceptions with soils. But what organic matter, and the reason that I think soil health really has to do, at least the benefits almost always have to do with water relations, is because of the infiltration. So, if you can get more water going into the soil rather than running off, and if you can prevent some of that water from evaporating, and you really want to limit evaporation and make it go through the plant as transpiration, that's where farmers that are improving their soil health are seeing the benefits come from. And then, some of it is increased water storage, but not near as much as you can see on the internet in some claims. But infiltration is a big deal, especially in a more rainy environment than we have here.

>> Right.

>> So, it enables them to intensify their, either crop rotation or their production system if they're in grazing, when you can get more water in the soil usable to plants.

>> If we switch gears for just a few minutes to talk about carbon sequestration, you know, again and again, this is an idea that's often in the news and I think not that many people have a whole lot of idea of what we're actually talking about. But there's been some stuff published saying that the soil carbon storage in range land or grassland soils is significantly more stable than that in forests, specifically because forests are prone to being burned and almost all the carbon storage is above ground. Whereas in range lands or grasslands you have, you know, according to some estimates, as much as 80% of the total stable carbon in the system, being held below ground and not nearly as prone to being converted into something that is then in the atmosphere say. Any thoughts on that?

>> Yeah, I mean, again, the environment is something critical to the carbon storage, so I think there are some limits from, potentially from clay content and things like that. But you can have carbon stored and then if you come and till that soil, you're going to immediately lose a lot of carbon. So that's definitely a plus in the column of a, you know, permanent rangeland, which I don't think is likely to be tilled, I don't, I mean you could speak to that. But that, and then, I'm not, I'm not sure I could comment on the fire component. I mean, if you're looking, I guess, yeah, we're seeing a lot of forests, that's a lot of carbon that is being burned from that, from that side of things. But I think, you know, looking at the soil management, eliminating tillage in a more permanent or perennial system. From a carbon sequestration point of view, is a positive. I guess in a, in a more managed system, you might be able to incorporate, you know, things like cover crops and maybe add more carbon to the system. But it's hard to say how, you know, how stable it's going to be in those systems.

>> Yeah. And going back to the some of the original soil chemistry that you were talking about, when we speak of carbon sequestration, that's referring to more than just soil organic matter, and the mechanisms that build it up are slower. Am I understanding that right?

>> I mean, I think when we're talking about carbon sequestration, we're just talking about storing carbon in the soil. So, you would have to, you know, do you see, you start with some number and then does that number go up? It's sequestered, it's taken out of the atmosphere. And so yeah, a forest is kind of sequestering a lot of carbon in above ground biomass, but we know those trees are going to die, we know, you know, they're not necessarily stored, that carbon's not stored for 1,000 years, in most cases.

>> Right.

>> But in the soil, you know, that can it be and that, you know, there is even in an untilled pasture system there is still some, you know, like a residence time or some turnover of that carbon. So even that carbon is not necessarily there forever, but it can be.

>> Permanently, yeah.

>> But it can be, it can be a pretty consistent number, I think, if management is consistent

>> And these questions that we have of how long that carbon is going to be there, how do you measure it, how deep do you measure it? And because, you know, even within a field, the organic carbon can vary, you know, tenfold across the field. Can you actually measure that economically and still, you know, have money to pay farmers for that sequestration? Those are all questions that these companies are dealing with, and they're not easy questions to answer.

>> Right. And there's enough interest in trying to answer them, that we've even had things like carbon markets, you know, where a farmer could enroll in a program that is believed to increase soil carbon and you would get paid a certain amount per carbon dioxide equivalent ton of that carbon over a period of time, say a 10-year contract. You know, those programs relied on what was probably at the time, you know, some pretty, I've seen a wide range of estimates for what range land soil in a given location could sequester under, you know, given management regime, and how, how stable that is. So, what would you tell somebody who wanted to measure their soil organic matter and compare it over a time? Should they expect to see that change if they're implementing a change of management, say on range land or pasture, if they, you know, collect a 12-inch soil sample and go down to their local soil testing laboratory and ask them for soil organic matter? Is the number that they get back useful or meaningful? And should they expect that to change over time, and how long a timeframe? Or is there a better way to try to keep track of this?

>> I can take a shot Doug.

>> Yeah, I was going to add something, but I don't want to complicate it too much first, you go ahead.

>> I think it's probably worth doing on a, you know, you mentioned a 5,000-acre field and for, you know, a management unit for range land. You're definitely going to want to do some GPS testing so that you go back to the same point, because of that variability I mentioned in soils, you're just not going to want to do a randomly. So, pick a point or several points across there and go back. You know, for range land, I wouldn't do it any more than every five years, maybe even more in our dry climate. Things are just not going to change very fast, even in irrigated agriculture. I tell people, you know, once every five years, unless you're adding lots of imported organic matter in terms of compost or manure, it's not going to change that fast. The other, the flip side of that is rather than just looking at these numbers, ask yourself what problems you have, and I tell farmers, I said, what are you trying to accomplish by improving that organic matter, or improving soil health in general? What problems are you trying to address? And that's a different way of looking at it, but it also changes the focus a little bit, rather than just looking at a number.

>> I was just going to add, you know, another thing in terms of sequestration that's really important, is the bulk density of the soil, and I don't know how much that changes in a pasture system, but that's just a measurement that says, you know, how much matter is there in a specific volume? And so, in more tilled systems that's a very important thing to consider when you take a soil test as well, because you can have a higher concentration of organic matter, which is essentially what the soil lab is, you know, that's what you're testing when you actually take the sample and send it off, is what's the concentration? But how much on a weight basis is there? You need a bulk density number, and maybe that doesn't change much in pasture, but it can change a lot in tilled systems, or agriculture.

>> That's another complicating factor in measuring carbon for soil sequestration, is that the bulk density can change and does change as you go down in the profile, and so you have to make sure that you account for all those changes.

>> And as a follow up to that, would most soil testing labs be using the same measurement? For example, at the end of your factsheet Doug, where you're talking about some of the recent science behind soil testing and soil organic matter measurements, you know, you list I think six different potential, potentially useful soil health measurements, soil organic matter is one of them. And then we mentioned, you know, in this interview, the permanganate oxilize, or what's that?

>> Permanganate oxidizable POXC.

>> Yeah, I was close, POXC, mineralizable carbon, soil protein, aggregates stability, and soil penetration resistance. Are there places where you can, that are measuring at least the first three of those individually?

>> Yes, those are getting to be more widely available for sure. The POXC and the MINC. And I think, and you know, that's an exciting area just in the whole soil management, is understanding the, that there are these different pools or fractions of organic matter and soil, so you get a little more of us of a story than the total. And you know, sometimes they're highly correlated, and sometimes they're not.

>> Yeah, and interestingly, aggregate stability has been used as a proxy indicator of soil health in range land health evaluations for some time, because it encompasses quite a few of these things, including whatever history of soil disturbance there might have been.

>> Right.

>> You know, but the ability of the aggregate to hold together is held to be a pretty good indicator of site health and minimum soil stability.

>> Yeah, I mean, just to reiterate something Andy said, like what is the, what's the problem that you're addressing and all these measurements of soil health? You know, the first one, we didn't put on our list there, but it's the soil staying where it's supposed to stay.

>> Yeah.

>> And you know, I imagine that can be a problem in range lands, you know, with erosion. I know it can be a problem in a lot of agricultural lands, with wind erosion and water erosion. So that's something where organic matter, yes can help, but also just making sure the soil is covered, that you have plants there, you know, those roots to help hold the soil in place. But yeah, the organic matter can help build aggregates, which might help hold that soil in place, resist wind erosion or water erosion.

>> Yeah, I think coming back to the, so what question and what problem are you trying to solve? I've run into quite a few ranchers, who have, you know, say, taken over a lease and are managing it differently than the people that ran it for the 100 years before them. And it's pretty common for someone to say, this really looks different than it did 15 years ago when I started managing differently, but they may not have any, you know, any quantifiable, quantitative data to back that up. So, I think there is increasing interest in finding ways to establish, and you know, benchmark values, out of recognition that things do change, and in some cases, can change, you know, relatively quickly, even if some of these measures of soil health aren't the things that change quickly, you know, there are other attributes that the plant will interface that can change relatively quickly, and it's useful to be able to track those over time. The problem is, sometimes you don't know what specific range land health attribute is going to change, neither can you measure everything under the sun. So, the question remains, you know, what are some ways that we can try to get at changes in soil health through, you know, maybe routine monitoring, even if that's not soil testing, but something that gives some indication of how things change over time?

>> Yeah, there may be more direct measurements for what it is that you're interested in, you know, what is the function? And can you measure that directly? Which might, I don't know, correct me if I'm wrong, but I mean, is that primary productivity? You know, just like the above ground biomass, which is, which is going to be affected by a lot of things going on below ground, but.

>> Yes, yeah, net primary production can be a good indicator of health. The problem is that, you know, on most semi-arid range lands, the timing and amount of annual precipitation varies significantly from year to year and did before we were even talking about climate change. You know, that's a, and net primary production follows pretty closely with precipitation. And so, in the same spot, you know, you could have a 100% difference from one year to the next that is only related to precipitation and not to management. Andy, I cut you off I think.

>> Oh, I was just going to say that one of the big differences between natural range land and area, you know, cropped agriculture, is that your management can actually determine what species you have out there in the range land, which is going to have some feedback with the soil, where in cropped agriculture, we determine the species basically, and so that's not a question there. And so, it's easier to focus on the soil when you determine the species, but you, in a range land you have to, you know, both focus on what species you have out there and what's changing there, and what's changing in the soil.

>> Right. The species are usually a function of the soil type and not the way around.

>> And management, I assume.

>> Right. Well, is there anything that I haven't asked that you wished I would have asked, that you think would be useful for people to hear, people that are either ranchers that are directly managing land or natural resource professionals that are potentially overseeing large areas of land that are managed by others?

>> You know, we talk about soil carbon and soil organic matter. Soil carbon, just a rule of thumb, is about half of what soil organic matter is. I mean, it varies, but just a rule of thumb. If you have a soil carbon number, you can double it, and that'll be your soil organic matter number.

>> That's right.

>> Isn't it, Doug? That's what I always use.

>> 53% or something like that?

>> Yeah, half as close.

>> Yeah. One other thing I would mention is, and this is kind of newer thinking, is about the residue. So, you know, again, to build this, to build organic matter, or to maintain organic matter, we need to put residue into the system, and you know, and that can come from the manure, maybe if an animal's grazing, you know, they're taking that above ground biomass and changing it into something else. In a more, in a crop system it might come from cover crops or manures or compost. And one of the things that when people have looked at, you know, how is carbon stored in the soil? For example, this carbon that ends up on clay particles forming these organic mineral complexes, what they found is a lot of that is coming from dead microbes. So, it's like all the carbon is put on the soil is pretty much going to at some point, get eaten by a microbe, and then the bodies of those microbes, it's different carbon that they create and, or different, you know, properties to those molecules, and they are more readily associating with the clay particles where they might be more stable. So, the sort of new thing here is paying attention to the quality of those residues, and good quality residues are defined as things that microbes like to eat, so those are going to be lower carbon to nitrogen ratios, you know, more nitrogen in there. And also, manures and composts are kind of in that, you know, category of more high-quality residue. So that's kind of an interesting new information. I think we used to think that any residue.

>> All [inaudible] is equal.

>> Or even like the more lignin rich, you know, kind of any other kind of residue was more apt to persist in the soil, and it seems to be.

>> Well, I think that's a good final word, Andy, unless you had anything else to add?

>> Nope, that was a good point from Doug.

>> Then I think we'll close out there. Thank you for your time.

>> Thank you, Tip.

>> Yep, thank you, Tip.

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>> 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 range line managers. Please take a moment to fill out a brief survey at This podcast is produced by Connors 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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