Episode 6: Soils (in English)

From the Oregon State University's Extension Service you're listening to In the Woods with the Forestry and Natural Resources Program. This podcast aims to share the voices of researchers, land managers, and members of the public interested in telling the story of how woodlands provide more than just trees, they provide interconnectedness that is essential to your daily life. Stick around to discover a new topic related to forests on each episode.

Jason O'Brien (host): Hello everyone! Welcome to another episode of In the Woods from OSU Extension Forestry and Natural Resources. I'm Jason O'Brien, Statewide Program Coordinator for the Oregon Master Naturalist Program and I'll be your host today. In this episode we're going to ground ourselves a bit when it comes to woodlands and forests and discuss the thing that literally everything that grows in the earth and we depend upon and that's soil. While soil isn't something we normally think of when discussing woodlands and forests it is fundamental to what types of trees grow where and the kinds of forests we see on the land. We'll cover a wide range of topics from different types of soils and how they're formed to soil disturbances to what woodland land owners and managers should know about the importance of soils. So grab your shovel and let's dig into this topic!

Jason: My guest today is Dr. Tom DeLuca, the Cheryl Ramberg-Ford and Allyn C. Ford Dean of the Oregon State University College of Forestry. Tom holds a Doctorate from Iowa State University, a Master's degree from Montana State University, and a Bachelor's degree from the University of Wisconsin-Madison all in soil science, which is why we've asked him to be our guest today on In the Woods. Tom welcome and thank you for being here today. I know you're very busy getting settled into your new position as dean of our college.

Tom DeLuca: Oh well thanks it's it's really wonderful to be joining you today and i'm excited about our discussion. And yes it's a bit busy right now but it it's all good.

Jason: That's good to hear. Yeah busy is probably good these days. Um so i think you know when we think of forests and woodlands we immediately jump to trees but today we're going to start literally from the ground level and our topic today is soil so and as i mentioned in your in your introduction you're a soil scientist so can you tell listeners what a soil scientist is and how that relates to forests? Tom: Yeah absolutely. So of course a soil scientist is somebody that is dedicated to understanding the functional aspects of soils as they relate to uh plants and plant growth and so you know the soils is really uh initially an or historically an agronomic science. It was started as a science to understand how to better propagate crops uh but of course um it's such a fundamental and important part of ecosystem and ecosystem function that uh you know uh soil science really covers the gamut of uh um all ecosystems and uh and soil scientists address the world from the bottom up literally understanding the microbial community that drives the processes in soils and making nutrients available for plant roots and plants are absolutely dependent upon the soil for that nutrient and water uptake and we can talk a little bit more about that aspect. But soil scientists are devoted to understanding how a plant soil community interact and how the soil provides for the uh plant community and all that is affiliated with it.
Jason: I see. So regardless of the type of plant community whether it's agronomic or a wild ecosystem soils a critical part of that.
Tom: Yeah, absolutely. Jason: Uh, I had a professor in college who told me that dirt is what's under your fingernails and soil is what's underneath your feet so maybe what we can do is give listeners a sense of uh what soil is so can you define soil for us?
Tom: Yeah that's great I appreciate what your your professor said because I agree completely that our tendency to call soil dirt is a mistake because it implies that it's something that you don't want where obviously soil is uh is absolutely necessary for our survival as a species and and um and as as i said earlier such a fundamental part of an ecosystem but uh. So soil literally is that interface between our physical and biological worlds. It's it is that transition from geologic sort of physical uh uh geologic uh material that's void of life to the vibrant above ground community of diversity of species, plant and uh microbe and uh you know mammalian and uh and etc. And so uh soil is in itself actually a living body and you can look at it in that regard from you know you could look at soils as a uh with the sand and silt functioning as almost like a skeletal material in soil and then the clay and the organic matter function as or the colloidal materials they function as the connective tissue and then the water and dissolved solutes function almost like a lifeblood within the soil and the microbial community functions as the respiratory and digestive uh component of that living body and that living body literally like if you took a a teaspoon of soil which uh is roughly a gram of soil uh and within that single gram of soil there's a billion bacterial cells in a gram of healthy uh agricultural soil or healthy forest soil have a billion bacterial cells and also right in there with those bacteria are about three kilometers of fungal hyphae so those are the small threads of vegetative growth of fungi that are living in that same gram and also in their tens of thousands of protozoans and uh as well as a uh host of nematodes and microarthropods and it really is this dynamic, thriving uh living body and uh it lives in symbiotic or obligate association with the plant community. The the plant community is 100 dependent upon a thriving uh soil uh ecosystem or living body and the soil is dependent upon the plant community to feed that soil with carbon uh and uh that's delivered from the you know photosynthesis within the plant community. So it's like a an obligate symbiosis and uh is an amazingly diverse and complex ecosystem we only have a limited understanding of the total diversity of species that inhabit the soil but we we do have a pretty good sense of the uh functional uh components of the soil or the functional diversity of soil in and the activities that go on in soil that that drive nutrient cycling or drive uh really fundamental processes like nitrogen fixation or phosphorus solubilization and how that dictates nutrient availability and uptake for the plant community. It's really a beautiful and intricate uh uh system the uh the uh plants also you know in providing that carbon to the to the soil environment the microbial community processes that carbon and converts it to this long-term uh soil organic matter component that helps build what we call tilth within the soil or sort of that soft sense of feel that you get when you hold a healthy uh handful of soil and it breaks apart nicely because of the organic matter in it and um and that increases the water holding capacity of the soil and and creates an environment in which roots can easily penetrate and grow. It's it's if there was more time to talk about we could spend the rest of the day just talking about that relationship between plants and and uh soil and and that sort of symbiosis as I call it.
Jason: Well that's fascinating Tom I think that would be a great topic for another podcast and um I really really like that analogy made between um like a living organism and their systems that keep an organism alive and what a soil is. I'd never quite heard it that way so that's that's very helpful and thank you for that. Um fascinating! Uh thinking back to my one and only soils class in college I know there are different soil types depending upon where someone lives around the country whether you're in a mountain ecosystem or a grassland ecosystem or or uh or a forested ecosystem so can you tell us how those soil types differ between different plant communities so like a for example a grassland uh ecosystem versus the forest ecosystem?
Tom: Yeah absolutely. Um that's a great question it's also a really big question. It's another one we could spend the day talking about but um soils are as you indicated directly related to that plant community and sort of environment in which they're they're formed and so soils are really a mosaic on the landscape. They're not constant from you know you can't go out and just uh you know dig up soil and have it be the same all across the landscape that's it really varies greatly. And you know Hans Jenny uh developed this really elegant model. He's a famous soil scientist from the uh early 1900's that developed this beautiful and elegant model for soil development that said soils are a product of climate, topography, geological parent material, biotic activity, and time. And if you vary those things as you move across the landscape you'll see how that influences the soil development. So just as you said, Jason, the uh a grassland environment will have a very different soil. So if you transition from the uh grasslands out in the Willamette Valley up into the savannah up into the uh fir forests you'll have a really dramatic change in the soil morphology as you go from one to the other. Grassland soils tend to have these really very much organic-enriched surface horizons called A horizons and um that they rarely have any litter on the surface that litter gets incorporated into the A horizon and uh in a grassland you can have an A horizon that which is an organic matter enriched mineral soil horizon it can be anywhere from eight inches to literally uh uh you know 24 or even 36 inches thick depending on the environment in which it's formed. So it's it's organic and rich to depth and um and that's what most people think of when they think of a soil they think of that organic enriched black earth that we like to relate to a healthy agricultural soil and because of that we call that type of soil in the U.S taxonomic system a Mollisol. And and we really have a Mollisol-centric society. Everybody thinks of when they think of an uh ag soil or think of a good garden soil they think of a uh a Mollisol. But the fact is the vast majority of our soils around the world are forest soils and forest soils are quite a bit different they we tend to have a thick litter layer on the surface and we don't have that incorporation of this organic matter into the mineral soil surface instead you have a layer of humus and litter on top of the mineral soil with lower concentrations of organic matter in the surface mineral soil. And then you have different uh horizons beneath that are dependent on the maturity of the soil and the age uh and the amount of water translocating through the system on an annual basis. So um you know as i said the Mollisol is the dominant uh prairie soil in contrast uh there's you know there's a total of 12 orders in the taxonomic system the the U.S soil taxonomic system and uh in forest soils we commonly have Alfisols, Inceptisols, Spodosols, Ultisols, Oxisols, Andisols, and Entisols. So that gives you a sense that the vast majority of the world's soils really fall within those forested environs as opposed to you know the prairie systems which tend to be made up of Mollisols and occasionally Aridisols if it's a dry uh grassland and short grass prairie environment. So I mean we like I said we could talk about that for a long time. Uh safe to say that it's soils are really a mosaic on the landscape and the vegetation has a huge influence on the nature of the soils that uh exist in that locale. Jason: Well that's that's interesting. So let's just stay on that just for a second so what accounts for the the um the lack of incorporation of that organic leaf litter into the soil in a forested ecosystem as compared to a grassland ecosystem. It sounds like you said that duff layer, that vegetative layer doesn't stick around in a grass when very long. Tom: Yeah so you got you the couple things you have to keep in mind. Number one, in a prairie ecosystem you have annual turnover of the vegetative biomass both below ground and above ground. Even if it's a even though it's a perennial grassland that high turnover of organic matter is happening year in and year out and the uh and you have this really dense fibric mass of roots in the uh in the uh prairie ecosystem. So you um you just are incorporating already have in the mineral soil a large amount of turnover of organic matter in the root mass associated with that system. Now contrast that with the forest soil and you've got these woody perennial roots that don't turn over regularly and you do have understory plants with the more high turnover and you have litter fall that is you know leaf litter and needle fall litter that uh contribute to that surface but you had tend to have acidic conditions in the um in the litter types that are in forests and um and that inhibits the types of organisms that mix that organic matter down into the soil. Whereas in the grassland environments we have pH soil pHs you know on the close to neutral or above and you have all these mixing organisms like beetle grubs and worms and and they do a really effective job of incorporating any litter material down into the mineral soil whereas in the forest environment you have less of that mixing community and you have a vast majority of the biomass in the ecosystem tied up in the woody biomass on an annual basis and not turning over regularly. The you know in a grassland environment the by far and away the majority of organic matter or carbon is below ground in a forest soil you have similar amounts of carbon above ground and below ground even though more is below ground than above ground. Most people think of forests they think "oh let's store carbon in the forest canopy" but there's actually more below ground in the in the uh the soils in terms of just total carbon but. I hope that answered your question.
Jason: Absolutely. Yeah I I've often wondered why that is why we don't see the turnover like we do in grasslands compared to forests so thank you for clarifying that. Um so in your definition of soil you mentioned geology. So I don't know how much exploring you've been able to do since you've uh arrived in Oregon but you know when you look across the landscape geology is abundantly apparent with our mountains and and uh lava flows and and you name it right so when we start with that geological foundation on the landscape how does that determine the soils and then from that point forward how does that affect what kinds of forests and woodlands grow uh based on that geological history?
Tom: Yeah so that's a that's a good question and we do have a real diversity of parent geologic parent materials here in Oregon. Uh we're fortunate that way if you're interested in soils you have a lot of diversity to consider and look at. We have uh you know a lot of volcanic activity like you said and so we have um we have basaltic flows associated with the volcanic volcanism we also have volcanic ash cap soils um which are called Andic or Andisols or Andic soils. We have um we have uh soft uh marine sediment based soils on the coastal uh in the coastal range. We have uh um outwash from glacial Lake Missoula that's here in the Willamette Valley. We have you know uh we just have a great diversity of um uh of and then of course the Eastern uh the uh Wallowas and the Eastern uh uh mountain ranges that we have uh get away from the volcanic materials and into um uh uplift from the um you know Rocky Mountain Orogeny so we have a mix of granitics and and argillites and that type of thing that are. So anyhow so the the geologic material has a really significant effect on soil development but like I said earlier the soils are a product of the combination of you know geologic material, the biotic material, the topography, the so so um the uh there are some examples of uh soils that are grow really unique plant communities associated with the unique geologic materials so those serpentine soils that we have in places here in Oregon will create really unique plant community composition and unique soil characteristics associated with that because the the uh the vegetative community is directly and really dramatically affected by the geology. Whereas others when you contrast the um uh the residuum and colluvium and the basaltic materials of the uh of the Cascade Range with the soft sedimentary uh

colluvium and residue along the coastal range and look at the the differences in the vegetative community it and the soils as a result of it are not quite that dramatic as the you know when you have something unique like a serpentine based uh soil and a you know Port Orford Cedar growing in the serpentine soils in the plant unique plant community associated with it.
But geology has a really significant effect on soil development and um and what grows and and but it's one of those one of the five forming factors and and uh and its influence is um is uh hard to tease apart from the climatic influences and biotic influences that go with that. Jason: Okay so what I think I understand you saying is that based on geological history uh the more uniform that geological history the maybe more um and more sameness across the landscape based on that geological history that the maybe the more uniform the plant communities are uh the diversity of different trees as opposed to something like a serpentine in Southern Oregon where there's a lot of jumbled up different kinds of geological parent material that that and and different uh types of minerals in those soils that mix things up more and give that uh plant community a chance to diversify a little bit more greatly or is that a misunderstanding of...

Tom: No I think I think that's that's pretty accurate I mean what I was trying to say is that the geologic material has an important effect on the plant community and and some places you can see it really clearly. I'll draw from a example from Montana that we used to take students to uh regularly where there was literal there was literally a block fault that resulted in two parent geologic parent materials being right next to each other separated by you know a meter of distance. You can step across it. On one side was uh an argillite residuum and on the other side side was a limestone residuum, and. In other words the geologic uh parent material the strata was literally, the soils were literally forming directly in place on the on the rock. Whereas normally when we talk about parent materials we're talking about some type of reworking and deposition of the geologic material as a lacustrine deposit a lake deposit or a alluvial deposit a river deposit or a glacial lacustrine. Where here we're talking about residuum we're talking about geologic parent material and now we can tell the actual effect of just the geology on that plant community because the climate is constant, the topography is constant, the everything is the same except for the geology and what effect does it have. Well the limestone of course is alkaline. It has a pH around eight whereas the argillite was quite acidic and the limestone is also fractured at depth it fractures in vertical fractures so in the springtime when the snowpack is melting the water just flows right out of the system. Whereas in the argillite it's this fine textured decomposition at depth and it holds that water beautifully against the you know melting of the spring rain uh spring melting of snow. So the plant community was day and night. There was an open stand of what looked like low elevation douglas fir with a with a very open with a grassland type uh Mollisol A horizon and right next door to it and the argillite was a you know classic Alfisol with a you know Bt horizon, no A horizon, and then a big thick litter layer. And literally just stepping from one... But finding those really obvious changes from one parent material to another hard to come by actually. And there and so and two, teasing out the effects of geology versus climate versus biotic material is is difficult so the serpentine soils I brought up is an example where the geology has such a dramatic effect on the plant community composition that you can really see it when you get into the serpentine versus into non-serpentine materials in the immediate vicinity. Jason: Okay. Alright. I get that. Yeah so what you're saying is really it's a complex story that... Tom: Yes, it is a complex story. [Jason laughs] Geology is just one piece of the puzzle.
Jason: Right right. So on that same point you kind of started talking about uh our weather, the climate, the processes that drive soil formation and I think I think it's obvious that here in the in the West part western part of Oregon and the coast and west of the Cascades it rains a lot. We know that.
So how does precipitation affect soils. And maybe we don't need to just stick with rain but in places where it snows and there's snow pack so how does precipitation affect soils.

Tom: Yeah that's a that's a really good question because you know precipitation has a huge effect on soil development as you can imagine. You know the the amount of precipitation dictates how much biomass can grow at the site just in terms of water is such an important ingredient for community composition and growth. And um and water also has a huge effect on how much development there is in the soil profile over time because water is uh is the you know the ingredient of um of uh decomposition and uh and development within the soil. We often talk in soils about uh acquired versus inherited characteristics. And inherited characteristics are what are what exist the day soil started to form in that geologic material. So let's say we start with a

flood that deposits a bunch of sediment and in that sediment soil starts forming that day.
And what whatever was inherited from that geologic parent material is the inherited characteristic and over time what we acquire is the change in that soil over time. So we acquire a bunch of organic matter that gets incorporated into the surface soil from the growth and development of plant community. And then over time water translocating through the soil transports uh colloidal materials and minerals downward in the soil profile and that long-term translocation is a really fundamental process in soil development. So as you as you start to move colloids downward they get depleted in the upper horizons and they get concentrated in the lower horizons at the depth where that effective precipitation takes place. In other words where water moves in the profile on a regular basis where water moves those colloids on a regular basis. So we end up with you know

silicate clays being leached out of the A horizon and deposited in the B horizon. Or we get them leached out of what we call the E horizon, a zone of alluviation and deposited in the B horizon, a zone of illuviation. And the um uh and so you can take a range of soils with desert soils with very little precipitation, they have the least amount of translocation of materials downward in the profile and they're very they end up being relatively immature soils as a result of it. They they tend to be uh alkaline they tend to retain their silicate minerals and whereas soils that have been leached and leached and leached by thousands of years of of water moving through the profile have those silicate minerals being either translocated downward or literally decomposed and re-mineralized into lower and lower silicate containing mineral forms over time forming what we call uh you know very mature soils like an Ultisol or an Oxisol which we would find in the subtropics and tropics of the the world. And then here in this part of the country we might find you know silicate clays being deposited in the B horizon and forming an Alfisol
or having uh humus and uh sesquioxides transported into the B horizon forming a Spodosol uh which is a would be common in sandy acidic uh forest soil environment. So that rainfall and that precipitation is a huge driver of soil development uh and as you go from uh the dry side of the Cascades up over the to the top of the wet and back down you you see huge differences in soil development as a result of the you know sort of effective precipitation uh in those respective locale locations.
Jason: I see so that weathering due to precipitation is moving minerals from the uh from the upper uh layers of soil down through the soil profile [Tom: Yeah] and accumulating them in the lower horizons [Tom: That's right] of that soil and leaching them out of that upper layer and depositing them uh where that water ends up in the soil profile. [Tom: Yeah] Is that a good summary of what...

Tom: Yes that is that's a good yes summary of what i was saying that it ends up with a in uh an increased concentration at depth uh so you have an acquired increased concentration of the colloidal materials at depth and uh the soil uh that we you know is characteristic of uh soils of this region would be to have a you know Bt horizon or a potentially a Bs horizon that's a product of that translocation and accumulation. So that's where I get back to that acquired versus inherited characteristic [Jason: mm-hmm] the inherited was a clay content that was uniform from top to bottom, the acquired characteristic is that it's now lower in the top and concentrated down at depth in the soil and uh and there's you know plant community benefits from having that stronger structure and stronger clay content at depth in the soil but we can save that for a discussion at a later time.

[acoustic guitar - Musical Interlude]

Jason: Alright, so I'm going to shift gears a little bit away from precipitation and talk about something that's uh that comes as no surprise. We've had some uh devastating wildfires uh here in the west and um those wildfires uh appear to be getting larger and hotter over time and uh and so let's talk about fire and how it affects soils [Tom: Yeah]
and maybe divide that into short-term versus long-term effects.
Tom: Yeah that's great thank you. Um it's an area of particular interest to me I've
spent a lot of my career in terms of research looking at long-term effects of fire and sort of long-term effects of the lack of fire uh in some cases on ecosystems and ecosystem function. And, um, you know when we think about fire and we think about um the you know the legacies of fire what gets what's it do to the soil there there's a few things in particular but before I jump into that let me just say that you know fire is a natural part of ecosystems. All biomass has the potential to burn. It's just a matter of how frequently and our forests on the east side of the state have evolved with a fire return interval that could be measured in tens of years you know anywhere from 20 to 35 or 40-year fire return intervals at the low elevation dry forest types such as, you know, ponderosa pine uh Douglas-fir forests of the east part of the state. On the west side of the Cascades we have a what we call a high severity low fire frequency type regime characterized by a fire return interval that we'd measure
in hundreds of years so literally 150 to 400 years here on the west side of the Cascades. So it burns really infrequently but it it does burn and when it burns it burns big and we had one of those years this year. What made this year different is that we now have a lot of people living up in the mountains and it was tragic the impacts of the fire were significant and tragic and um and uh but from a community, from a forest community and forest ecosystem perspective it's a natural part of that cycle to burn occasionally. And what's it do to soil? Well it's uh there are several effects. Number one you have uh you have a bunch of dead trees and shrubs and grasses and they contribute a pulse of organic matter to the soil because all of a sudden this plant root community that was alive is now dead and the microbes attack it and they start breaking it down almost all immediately. Um the the the other aspect of it is that you you heated the soil surface often depending on the fire type. If you had a crown fire um without any uh fire on the forest floor you can actually have very low impact to the soil but most often you have fires on the floor as well and those fires depending on heating duration and temperature have variable impact on soil.
The a soil is actually a surprisingly good insulator and a very poor conductor of heat. So if you have temperatures of 600 degrees at the surface which would be a common surface temperature achieved during a wildfire and that fire was burning in that location for an hour at 600 degrees at that surface temperature the temperature down at 20 centimeters deep might still be only 40 degrees celsius. [Jason: Wow!] So the soil is really a good insulator against that heat transfer because it's so porous and filled with air pockets and doesn't conduct heat particularly well. If you have something like you know a bunch of logs on the uh surface and the and the the fire can just sit in place and smolder and burn for a long period of time at high temperature now you can have a significant amount of heat transfer into the soil and you know you see that when a slash pile burns you go out and it's just white ash sitting on the ground and and the soil is uh you see the organic matter is gone from the soil and that's a high severity fire and that has a big impact on the soil. Where the um where the uh when you see a bunch of black ash which is char charcoal and uh ash um then uh the fire severities was not as great and the heat transfer into the soil was less. Ash and charcoal are two of the big legacies after fire as well in addition to the dead plants uh the ash is a fully decomposed organic matter and it, the white ash, is a fully decomposed organic matter or excuse me combusted organic matter with the uh the concentration of oxides of the most common nutrients that are left behind. Nitrogen and carbon get evolved partially during the burning process but potassium, magnesium, calcium, sodium, those all get left behind. They they volatilize it a thousand degrees uh celsius so they're not going to get burned off in most fires. So if you recognize those those are all alkaline metals so uh you actually wind up with a bit of a a liming effect of ash because once you wet that ash up with the first rains that come after a rain after a fire the ash gets converted the alkaline oxides in that ash get converted to hydroxides and uh and so you have a liming effect of the magnesium and calcium hydroxide that's left behind in that ash. The black ash, or what we call charcoal, is this really wonderful material that people have very little knowledge about its benefit in soil. It's only recently that people have started to talk about the value of charcoal in soil. Um and charcoal is a uh carbon rich material that where the the condensation reactions from the burning process under uh low oxygen conditions leaves behind almost like a graphene like structure to the this organic matter so it it ends up being really resistant to decomposition. So charcoal will have a longevity in the soil measured in hundreds to even thousands of years [Jason: Hmmm] as opposed to the wood which will decompose in you know tens to uh you know 20 10 to 20 years or something depending on the size of the wood particles that are or if it's small particles of course it decomposes much quicker. So charcoal has this longevity in soil and it has all these beneficial effects for the soil. It functions like a porous water high water holding capacity material, it absorbs nutrients, it absorbs organic compounds, you know we use charcoal as activated carbon we use it [Jason: right] for filtering water, putting odor eaters in our shoes, so it has all these effects on the soil environment by absorbing these organic compounds and it can absorb compounds that are inhibitory to other microbes or to other plants. So it's a only a limited understanding right now of all the things that the charcoal does in the soil but uh it's it is a valuable material that is a benefit that gets left behind after a fire if it can stay on site. If it you have steep slopes and high rains it can erode away and be you know deposited in creeks and rivers and ultimately uh just serves as sort of long-term carbon storage in that regard. If I could just take a quick uh um diversion I'll just mention that you know the indigenous people of the uh uh the Amazon Basin uh uh knew there was all these benefits to charcoal and they they uh piled up their residues and uh garbage and bones and and uh would burn it in place to create these uh charcoal-enriched
soils that today that are called terra preta soils and today those are the highest biodiversity and highest productivity soils of any in the amazon basin and it is a one of the few places where humans have a really positive effect on biodiversity as opposed to a negative effect on biodiversity. But they they um

those soils haven't been amended for 500 years and they still have the highest productivity of any soils and so charcoal has this really unique role to play in soils and soil development so and uh you know long-term soil organic matter dynamics and forest soils. Jason: So Tom, you mentioned the the effects of fire on the formation of ash and charcoal in forest soils and how that how that adds to uh the soil quality but some of us when we listen to reports or research we often hear comments about the the effects of fire on soil that almost creates a a layer or an impenetrable layer across that soil surface and creates a hydrophobic layer I guess. Can you can you talk a little bit about that or what's [Tom: Yeah] is that is that something that lasts for a long time or [Tom: Yeah] does it go away?
Tom: Yeah, no that's a a great question, and um there's there's some amount of hydrophobicity that can be just naturally generated in soils based on the plant community like eucalypts tend to generate sort of uh that hydrophobic condition just based on the um uh the amount of um waxes in the you know cuticles of the of the leaves and whatnot. But um uh what happens in fires and formation of hydrophobic condition is basically, you know how I said soil's a good insulator, well uh at about you get heat transfer from the fire at the surface downward and right around 280 degrees celsius you distill off these uh organic compounds that then um that then recondense so that you you volatilize those and they recondense in the surface soil just where it's cool enough for those to condense and that's usually right at the surface of the mineral soil and so as that ash layer or excuse me that duff layer burns away you can be left with this hydrophobic layer right at the mineral soil surface and then when the rain hits, as opposed to being absorbed, you know, have an opportunity to soak in and to be absorbed by the uh by the soil aggregates, instead it beads up, the water literally beads up and and can roll off. So if you're on a steep slope that water starts moving downhill and can um initiate rill erosion or you know sheet erosion or even gully erosion. So it can be really significant in some areas you know I um I've seen evidence of hydrophobic soils leading to uh really significant runoff events. Fortunately it's pretty short-lived. It tends to only last about a month or three months as the microbes break down that material in the hydrophobic layer and then um it's also generally not uniform across the uh across the landscape. It tends to be a bit patchy so some places will be really you know perfect conditions for formation and hydrophobicity and other places not. Um, so it it it's pretty variable. And uh the other you know negative impact of fire on soils is just you know as I talked about that symbiosis between plant community and in the soil environment well you've just cut that off, you've just ended it. You no longer have the plant community feeding the soil with photosynthate from root exudation and leaf litter inputs. You've stopped that temporarily for several years so you're starving the soil from that fresh organic matter introduction and that will have a you know negative impact on the soil in the microbial community unless you get good regeneration and regrowth of you know re-sprouting of uh of rhizomes and and germination of seeds in the seed bank and that type of thing. So depending on how hot, how severe that fire was, it can have more or less of a negative impact on the soil and a really hot fire on the surface can have a really pretty significant negative impact on the soil. So those novel conditions take a long time novel conditions created by heavy fuel loading and high temperatures uh can uh create a you know it takes a long time for that soil to recover under those conditions, whereas, under lower severity fire conditions the recovery is faster and the soil you know bounces back much more quickly. So you know managing our landscapes to uh you know reintroduce fire, especially over on the east side, and get fire on the ground in the form of prescribed fire can really help reduce that fuel loading that will otherwise result in a high severity fire that would be kind of unnatural or novel uh conditions that that uh take longer to recover after. Jason: So in a in a high precipitation area like on the west side west side of the Cascades you would you would expect that regeneration might not take as long because there's uh an abundance of rain and and other conditions that would begin that regeneration process or is that a bit of oversimplification of... Tom: Well no I think that's I think that's generally true. The problem is is if you had a really high severity fire because it's, like I said, they're low frequency high severity on the west side of the Cascades so if it was dry and hot and you had really heavy fuel loading from all that water you're talking about creates that much more live fuels and if you you know if you have uh you have the conditions right for a really hot fire uh and on the floor and in the canopy that that can result in uh tough conditions for regeneration unless you're you know back in their replanting and and literally trying to jump start the system. You know the forest service has these uh burned area emergency recovery efforts that they'll put on the ground in places where there's really high severity conditions to just try and protect the soil and try and get things growing sooner than later to to minimize that sort of potential for high runoff erosion events that could occur under those really severe high fire freak high fire severity uh circumstances. But otherwise yeah you're generally right if with more precip you have better chance for rapid regrowth and rapid or more rapid regeneration and

recovery of the system.
Jason: so fire's one type of disturbance uh and what other disturbances are there that might affect soil.
Tom: Oh my gosh! There's there's so many! You know there's there's things like you know wind throw events in forest environments that tip trees up and take deep buried soil and bring it up to the surface and create this sort of pit and mound topography. You can have

you can have literally just large insect outbreaks or disease outbreaks that kill the overstory and temporarily reduce that you know constant flow of carbon to the below ground from the from the trees. It does result in dead you know increased dead material decomposition in the soil environment but it is a type of disturbance. Humans are a really significant form of disturbance whether it's urban development uh you know uh building back into forested environments with small ranchettes, you know, three acre ranchettes with a you know house uh clear permanent clear-cut you know that's a and uh excavation for a foundation that's a you know that's a permanent disturbance of soil with no uh recovery. Um you know there's a whole host of disturbance types. Um uh small mammals are constantly disturbing soil. They do what we call biopedoturbation. Pedo is

referring to soils and um the turbation is the disturbance and the gophers and you know badgers and you know they're burrowing and mixing and turning over soil and uh so there's lots of natural forms of disturbance that occur and those result in you know rapid uh uh CO2 evolution you know from the decomposition of the resident organic material and um and then uh recovery so those those uh those disturb disturbances in soils are a norm, it's just how frequent and uh and do they facilitate full plant community you know contribution back to the soil environment to maintain that symbiosis that I talked about earlier between the plant community and the soil community. Jason: I see, uh what's that term again with the mammals turning over the soil? That's a fun term I've never heard of that before.
Tom: Yeah, biopedoturbation. [Jason: Biopedo turbidation...say that again] Tom: biopedoturbation [laughs].

Jason: Biopedoturbation. Okay that's that's a fun one.
Tom: That's one to remember! Yeah, knock that out at a trivia

you know pub uh trivia or pub quiz.
Jason: [laughs] Right exactly i just have to practice it a few more
times so it rolls off the tongue. [laughs] So you've uh all along now you've alluded to what makes soil, soil, and a big component of that through line has been soil health. What is a healthy soil, right. So what do tree farmers and folks that have to grow trees for a living, what do they need to know about soil health and how does that impact their livelihoods?
Tom: yeah that's a that's a really good question. You know it's interesting um you know tree farmers in particular, you know, or tree farms, or um you know people that are managing woodlots or just sort of small private acreage, they can manage those in any number of different ways and their activities will have an effect on the soil health and the soil health will have an effect on the productivity of the of the forest. But um you know a healthy forest soil is one that retains its uh that uh tilthy structure, in other words, the uh a root bed that allows for root penetration and and nice soil root contact for water uptake and uh and nutrient uptake and um that is really uh enhanced by uh rich organic matter availability and the surface soil. So anytime you're you're you know thinking about how do I ensure soil health you should be thinking about am I protecting the organic matter inputs and retention in the soil and environment and am I doing things that are are damaging or destroying it. Uh like I said the forests are kind of special in that you disturb them pretty infrequently. You know if even uh even somebody harvesting timber using sort of clear-cut harvest management is only, and and let's say managing on a 60-year rotation, they're only disturbing the soil once every 60 years as opposed to uh a agricultural tillage event that's every every year. And you rarely till the soil in a forest environment. You disturb it with the harvesting activities obviously pretty significantly. So so that's a long term period with very little disturbance. Well people that are managing for you know multi-species um uh and structural complexity you know multi-species multi-age cohorts can manage that system with very little soil disturbance and uh retaining uh the greatest you know health of the the soil possible through uh sort of you know a variable retention harvest or a selection type harvest with minimal disturbance on the soil and and protecting the health of the soil and the health then of course supporting the tree vigor and growth in uh in exchange. So depending on the objectives of the landowner soil health can have a significant effect on the productivity of the of the forest and and uh vice versa.
Jason: Oh well well thank you so much. Yeah because I think I think when we we in Oregon uh are pretty cognizant of our our forest industry and our forest small woodland owners and it's I think it's good to recognize that that soil is that underpinning that affects both those that grow trees for living to those that grow our fruit and vegetable crops as well so i appreciate knowing that that connection there with soil health.

Tom: I really yeah no I appreciate that question i think that you know that the difference a difference between sort of ag and forestry production is that the forest forestry and the view upon forest soils is significantly different because you're dependent on those natural processes to provide for the soil. So in other words natural uh nitrogen fixation by you know species like alder or shrub species like ceanothus or persha that provide nitrogen for the system. And you also have these occasional natural disturbances like fire or wind throw that. So it's really a really different perspective than the agricultural system where it's very input driven and very you know annual tillage operations to achieve an objective. Instead the forest manager has to really be thinking about soils as a holistic part of that system and system function and and they're less in control of the manipulation of the soil and more having to manage the system as a whole providing for the soil through their forest management so the soil provides for the system. So it's a it's just a little different perspective but equally important whether you're an agronomist or a forester.

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Jason: All of our guests are subjected to a lightning round in this podcast. So we've got three questions we ask each of our our guests so we're just gonna these are rapid fire uh short answer questions and so the first one is, uh, Tom, what is your favorite tree and why? Tom: Uh I'll I'll have to say uh ponderosa pine is my favorite tree. I just I love it. It's uh such a beautiful tree. Uh it has a wonderful aroma to it in mature uh yellow pines uh when the the bark is you know has the big uh splits and it you and uh it has almost a vanilla odor to it it's just really wonderful. It's such a majestic looking tree and it has uh such a capacity to grow under you know pretty pretty rough conditions in terms of water limitation, fire and uh snow loading. So uh that I think ponderosa is probably my favorite, which is tough because I mean there's so many species to love. Whitebark pine is gorgeous and uh uh I you know I Douglas- fir of course is an amazing tree but uh uh and then Scott's pine uh from our my long-term work in in uh Sweden but uh. Sorry I'm not being very lightning about this ponderosa [Jason: That's okay.] it's got to be my favorite species I think. Jason: Excellent. Very good. So, um, on that on our topic of today uh that being soils do you have any books or resources that you can recommend our listeners uh consult to maybe dive in a little deeper uh to the topic of soils? Tom: Yeah so um it well depends. If they're particularly interested in forest soils um there was uh there was a really wonderful book just published um uh in the last year of forest soils book and I'll have to send you the uh the details on it because I won't remember right off the bat. But it was uh Debbie Dumroese and uh Matt Busse, uh edited the the uh it's probably a you know I don't know how many chapters but it was multiple contributions from authors and that's a brand new sort of compendium on forest soils that might be of interest to folks and we contributed a chapter on biological soil health for that book and so that's about as recent as you'll get a book on forest soils. A book that I really love that is uh often overlooked um is a is a book called uh "Out of the Earth" by Daniel Hillel and um it it describes humans relationship to soils as recorded throughout history and uh and through their you know religions and uh cultural uh experiences and it really is a it's really a really interesting and different take on soils that is uh is worth to read and most people won't have heard of that or seen it. So that would be one that I would uh recommend. That was that's older it's probably uh like 1991 or something it's it might I don't know if it's been in a second printing or not but it's a really wonderful book and worth to read. Jason: Ah terrific! Yeah we'll make sure to get those references and links to the to where to get those books if they're still available on our podcast website in our podcast notes. So I appreciate those recommendations. Last and uh certainly not least but we ask everybody what is that one thing you must take with you in a cruiser vest or field kit when you're out there in the woods.

Tom: Well, you mean from a soils perspective or...? [Jason: I guess I'll leave it up to you if you want to want to shake it up a little bit that's fine. Tom: Well there's so many crucial things that should be in that kit or vest um uh that from just uh you know surviving in the woods perspective but I'll uh I'll say that all soil scientists have to have a knife when they go in the field because they you uh you won't be putting a shovel in your cruiser vest but you need a knife to be able to uh rough up the soil surface to take a look at the uh the um uh the structure of the soil and so you always need to have a knife with you as a soil scientist when you're out in the field in addition to of course your spade and you know what not but you got to have a knife. Jason: Great! I'll keep that in mind. I
hadn't thought of that one as a as a top uh soil scientist tool but uh that's the point of these podcasts to learn something new. [Tom: Yeah] Well Well tom it's been a great honor to spend this time with you I appreciate your uh your devoting a little bit of time for our Forestry and Natural Resources Extension Program and I look forward to having more conversations with you in the future. I appreciate your time, thank you very much! Tom: Well, thank you Jason. It was it was really a pleasure and an honor, so thanks a bunch for the opportunity.

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[acoustic guitar] Lauren: Thanks so much for listening! Show notes with links mentioned on each episode are available on our website: blogs.oregonstate.edu/inthewoods podcast. We'd love to hear from you visit the tell us what you think tab on our website to leave us a comment suggest a guest or topic or ask a question that can be featured in a future episode. And also give us your feedback by filling out our survey. In the Woods is produced by Lauren Grand, Carrie Berger, Jacob Putney, Stephen Fitzgerald, and Jason O'Brien, who are all members of the Oregon State University Forestry and Natural Resources Extension team. This podcast is made possible by funding from the Oregon Forest Resources Institute. Music for In the Woods podcast was composed by Jeffrey Hino, and graphic design was created by Christina Friehauf. We hope you enjoyed the episode and we can't wait to talk to you again next month. Until then, what's in your woods?

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