The Low Down On Dirt

“Tryon Creek’s Most Precious Resource”

By Bruce Rottink, Volunteer Nature Guide & Retired Research Forester


People come to Tryon Creek State Natural Area to see the beautiful trilliums, the colorful pileated woodpeckers and the towering Douglas-fir. Yet I’ve never met anyone who said they came to see Tryon Creek’s most precious resource – the dirt!

Dirt, it seems, is treated like, well, dirt!

Even using the fancy name “soil” doesn’t seem to help. But the truth is that if there were no dirt, there would be no trilliums, no woodpeckers, no Douglas-fir, and no human beings either.


What’s so special about soil?

Tryon Creek’s dirt performs many vital functions. These functions include:

  • storing water for plant use between rainfalls

  • supplying vital nutrients like phosphorous, iron, and nitrogen to the plants

  • serving as an anchoring place for the trees and other plants of the forest


Still not convinced?

One of the reasons we don’t really appreciate dirt is that, unlike pileated woodpeckers and trilliums, dirt seems to be literally everywhere. When we look at the ground, it’s easy to imagine that the dirt continues on down below the surface almost forever. Nothing could be further from the truth!

Fierce Fragipan

As far as the plants are concerned, Tryon Creek SNA, and much of the area surrounding the park, has only 2 to 3 feet of useable soil (just imagine, we’d only be about waist deep in dirt if we sunk into the soil). The reason is that at that depth, there is a nearly impenetrable layer called the fragipan. The fragipan here at Tryon Creek is a dense clay layer that neither water, plant roots nor even air can easily penetrate. It’s as if we started with a shopping mall’s paved parking lot, spread a couple of feet of dirt on top of it, and decided to grow a forest there!

A Closer Look

Every now and then, nature gives us a great opportunity to see how this thin layer of dirt supports the forest. Last September, a bigleaf maple (Acer macrophyllum) tree at the edge of Tryon Creek’s main parking lot blew down. When the maple blew down, its roots pulled up the entire layer of soil that sat on top of the fragipan. It looks like a pancake standing on edge, as you can see in the first photo. This pancake of soil is only 24-1/2 inches thick.

Fragipan "flapjack"

The blowdown

You can easily see in the next photo how impermeable the fragipan layer is by the fact that 3 days after the storm there is still a pool of water sitting on top of the fragipan. The water has not soaked in. Notice also that no roots are sticking out of the bottom of the soil pancake.

Fragipan flipped flapjack

Puddle stomper spot


Looking directly at the bottom of the soil pancake that the roots lifted up, it is easy to see how the roots were growing. As you would expect, the roots grew down into the ground until they hit the fragipan layer. Then, they turned sidewise and grew horizontally just on top of the fragipan layer. You can see the horizontally growing roots in the photo below (taken 6 months later when rain had rinsed off the roots).


A fragipan facial


The fragipan layer has a very high clay content, which is why it is so impermeable. Looking at a piece of the fragipan, you can see it is mottled, or has spots of different colors, often indicating the state of iron in the soil. The red color is oxidized iron (“rust”) while the gray is an area of non-oxidized (“reduced”) iron. The gray color is an indication of extreme oxygen deficiency in the soil. This mottling is typical, and diagnostic, of soils which are saturated for long periods, and thus contain little or no oxygen.


A piece of fragipan pie

A piece of fragipan pie


The soils at Tryon Creek SNA go beyond just mottling, and in fact contain numerous very dark to black iron-manganese nodules, as seen below. These nodules are extremely hard, and are another clear indicator of soils which are wet for extended periods of time.


Black iron-manganese

Black iron-manganese


Fragipan always wins

Every tree that has been blown down in the park reveals how thin the layer of useable soil is. While Tryon Creek’s trees may start life by having a taproot, it doesn’t last for long. The fragipan sees to that. As a result of these shallow root systems, the trees at Tryon Creek and nearby areas, depend upon the neighboring trees to protect them from high winds. This mutual aid process works well until some of the trees are removed, leaving the remaining trees, especially on the edge of the cut area, vulnerable to strong winds. Thus, cutting out a cluster of trees to build a house in the middle of a forest leaves the trees immediately surrounding the house extra vulnerable to being blown down.

Heart & Soil

As thin as the soil layer may be, it is essential to life as we know it. The next time you’re out hiking at Tryon Creek on a wet day, at the end of the hike pause for just a moment and give thanks for that stuff you have to scrape off your shoes. It’s the most important part of the park!






Posted on April 13, 2014, in Plants & Wildlife, Trees and tagged , , . Bookmark the permalink. 4 Comments.

  1. Thanks to Bruce and Stephanie Wagner, I’ve learned to really appreciate all that stuff my dog Charlie carries into our house.

  2. Bruce, Before Tryon Creek was logged out in the late 1800’s and the first half of the 1900’s, did the soil used to be deeper? Has human activity damaged the top soil? Can we do anything to improve the natural conditions?

    What about the micro-organisms in the soil? Can mycorrhizal fungi and bacteria break down the fragipan and/or grow more topsoil?

    • Bruce, Before Tryon Creek was logged out in the late 1800’s and the first half of the 1900’s, did the soil used to be deeper?
      Honest answer: Well, Amazon still hasn’t delivered my time machine (Dang!), so I’ll have to say, “I don’t know for sure!” However, my best speculation is “yes and no.” When much of Tryon Creek area was logged, the technology involved dragging the heavy logs over the ground to a processing or collection point. This resulted in some parts of the soil surface being very disturbed (think “digging shallow little ditches with compacted bottoms”).
      Let me take one small example. As noted in the Naturalist Note posted January 18th, back in the day, there was a sawmill in the canyon near Obie’s/Beaver Bridges. In all likelihood, some logs were dragged right down the hillsides to the sawmill resulting in a ditch straight down the side of the canyon to the creek. If true, these “ditches” doubtless resulted in some soil being washed from the hillside into Tryon Creek. On the other hand, where the land was relatively flat, like near Trillium Trail and Old Main Trail near the Nature Center, I would anticipate very little soil erosion (but some compaction), regardless of the soil disturbance.
      Also, modern studies indicate that road-building during a logging operation is a major contributor to erosion. This is primarily because road building compacts the soil, resulting in less water infiltration and more surface runoff, resulting in more erosion. We know there were some roads associated with the early logging of Tryon Creek.
      Has human activity damaged the top soil? Yes! (Warning: This is not necessarily a “feel-good” answer.) Let’s bring this issue into the current day. Every trail at Tryon Creek SNA has-damaged/is-damaging the top soil. Ever wonder why there are big puddles on some of the trails on rainy days? It’s because when the trails were built, and when we walk on them, we are compressing the soil, and reducing the water infiltration rate to approximately zero. This results in more surface runoff, which, depending upon the local topography, might result in some soil erosion.
      A second effect of the trail building is more subtle. We humans easily observe water running over the top of the ground. However, much water moves below ground from the hilltops to valleys and streams. This is termed “subsurface flow.” The trails that we build compact the soil, and essentially result in the production of a “dam” for the subsurface water. On even a gentle slope, these trail-dams result in the subsurface flow being backed-up, and the subsurface water rises up above ground level. Now the choice is to let it flow over the top of the trail, or put in a culvert under the trail. A good example is the culvert on Old Main, not too far from the Nature Center. An interesting example of how this has affected the forest can be seen on the Old Main Trail just before it reaches Obie’s Bridge. On the uphill side of the trail there is a lot of jewelweed. On the downhill side of the trail, there just a little jewelweed. Jewelweed is a plant that LOVES wet spots. One interpretation of this difference between the uphill and downhill sides of the trail, is that the trail is acting like a dam, and water is accumulating on the uphill side of the trail, thus creating ideal jewelweed habitat. Whether you consider the change in water levels “harm” is up to you, but it is at least a change that effects the plants.
      Can we do anything to improve the natural conditions? What about the micro-organisms in the soil? Can mycorrhizal fungi and bacteria break down the fragipan and/or grow more topsoil?
      The fragipan is composed primarily of clay particles, the smallest mineral particles found in soil (the other two being silt and sand). Being small, and shaped approximately like a pancake, they can pack tightly together, which results in them not being very open to the passage of air or water. A great photo of clay particles can be seen at .
      The bad news about using mycorrhizal fungi to break up the fragipan is that they are aerobic organisms (meaning they require oxygen to live.) As indicated earlier, the fragipan is typically a low- to no-oxygen place. At least some bacteria could survive the anaerobic (oxygen-free) environment, but the clay particles are mineral in nature, not organic. As such, they are considerably less susceptible to breakdown by microorganisms. I leave you with two quotes from scientific works that seemed to pretty much sum it up.
      With regards to the processes that result in the natural breakdown of the fragipan, one article summed them up by saying:
      “These weathering effects are the result of hundreds or thousands of years with no practical significance.”
      -(Reference: A. D. Karathanasis, L. W. Murdock, C. J. Matocha, J. Grove, and Y. L. Thompson, “Fragipan Horizon Fragmentation in Slaking Experiments with Amendment Materials and Ryegrass Root Tissue Extracts,” The Scientific World Journal, vol. 2014, Article ID 276892, 13 pages, 2014. doi:10.1155/2014/276892)
      Another article concluded:
      “The mechanisms for fragipan degradation are poorly understood” [Author’s note: The mantra of scientists is “Our current state of knowledge only represents the point at which we’ve decided to stop asking questions!”]
      – (Reference: James G. Bockheim. Soil Geography of the USA – A diagnostic-horizon approach. Book published by Springer-Verlag, 2014.
      As to “growing new topsoil” rest assured, that nature is working at this, night and day, all year. That said, it is a very slow process, involving the slow decay of organic matter at or near the surface, and the weathering of mineral material. From a human perspective, this is abysmally slow.
      I appreciate your interest, and your questions.

  3. Are there tools/technologies that can inexpensively assess how deep the top soil is and what the nature of the fragipan is?

    My home is near TCSNA. I have planted a few Western Red Cedars and Hemlocks. If the soil is shallow, the fragipan is dense and the slope fairly steep … I worry that these trees may be especially vulnerable to storm blow down at maturity.

    Short of using heavy equipment, can one dig deep enough with pick and shovel to punch through the fragipan layer in this area of SW Portland/LO? Getting ready to plant some street trees.

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