Landscape Confidential: Spodosols

A spodosol has a gray horizon from which acids have leached nutrients, depositing it in the reddish horizon below. Photo by Audrey Clark

A spodosol has a gray horizon from which acids have leached nutrients, depositing it in the reddish horizon below. Photo by Audrey Clark

Editor’s note: Landscape Confidential is an occasional column by Audrey Clark, a curatorial assistant at the Pringle Herbarium at the University of Vermont.

The forest floor is shady, cool and carpeted with needles. You dig a hole eight inches wide and a foot deep. You peer inside.

Twigs, leaves, roots and dark earth occupy the first three inches. Below that is an ashen gray stripe of soil, about two inches thick. Under the bleached stripe, a brilliant rusty red layer of packed sandy silt, and below that a darker sandy layer that extends to the bottom of your hole and beyond.

From this cross-section you can tell what kind of trees have grown in this spot for the last few thousand years. You can learn whether farmers plowed or worms wriggled here. You can guess whether the soil is underlain by bedrock, glacial till or sand. You can even tell how acidic the soil is, and whether it gets more or less acidic as you descend into the earth.

This soil you have uncovered is a spodosol. Spodosols are rare in the world, but are actually very common in Vermont. In fact, our official state soil is a spodosol. Spodosols are so common here, that if you look at a map of them, you have to squint to see that they’re not in the Champlain Valley and not in the Connecticut River Valley. Spodosols are mainly mountain soils, developing on cool mountains where there is plenty of rain and conifer trees. As in, most of Vermont.

Here’s how a spodosol forms. The glaciers scrape away plants and earth, then melt away, leaving a region of bare rock. Over the millennia, bits of dirt collect on the rocks and tundra forms, then, as the climate warms, the dirt deepens and boreal forest moves north, then temperate forest. Because we’re in the mountains, and those mountains are still cool, in some places there are mostly conifers — trees like hemlock, spruce and fir.

These trees drop their fibrous needles onto the forest floor where water and microorganisms break them down and release the acid. Conifers tolerate acidic soil more than many other trees, so it’s a feedback loop that works in favor of the evergreen trees.

This slow decomposition in a moist environment is key. With each raindrop, acid washes from this thick layer of organic material down into the soil, so that the soil forms a gradient of high acidity at the surface to less acidity lower down. The pH of your soil pit will likely range from 4.5 (very strongly acid) at the surface to 5.5 eight inches down (medium acid). (Note that pH is a logarithmic measure of acidity where a low pH indicates high acidity and a high pH indicates low acidity.)

Rebecca Bourgault is a Ph.D. candidate at the University of Vermont, where she studies soil chemistry. She says the organic acids released by decaying leaves are key to the formation of spodosols. Organic acids bind easily with metals like aluminum and iron. When they do so, they form chunky molecules called spodic complexes. These complexes are washed deeper into the soil, thereby moving metals downward.

The interaction between organic acids and these metals depends on how acidic the soil is to start with. As these spodic complexes move down through the soil, they move from a strongly acid environment into a medium acid environment, a change significant enough that the spodic complexes move out of the water and attach to soil particles. There, as the soil dries out, the metals are oxidized, which simply means oxygen is added (though it can be more complicated). Oxidize iron and you get rust. The result is a bleached gray layer from which colorful metals have been pulled and a dark brown or reddish layer where those metals have been deposited. In fact, the term “spodosol” comes from the Greek for “ash,” which refers to the ashen gray layer.

The spodosols in Vermont are over 10,000 years old. It can take that long for water moving through the soil to sort the atoms into something resembling Dethier’s colorful layer cake.

You might find a darker or browner layer under the rusty red one in a spodosol. David Dethier, a geologist at Williams College in Massachusetts, says that’s because most of the organic acids drop out of solution when the pH reaches around 5.2, so that even though water may continue on down through the soil, the acids and the colorful iron they carry do not.

Dethier says it’s important that the material beneath a soil is well drained if a spodosol is to form. Spodosol formation depends on the water percolating through the soil. If there’s standing water in the soil, organic acids won’t move downward carrying metals. That’s why spodosols form best on sandy soils.

Dethier appreciates spodosols for their looks. “They’re pretty to look at, chocolate frosting above, vanilla for the leached horizon, then a mixture of chocolate and, I suppose, orange.”

The spodosols in Vermont are over 10,000 years old. It can take that long for water moving through the soil to sort the atoms into something resembling Dethier’s colorful layer cake.

It doesn’t take much to mess up that nice layer cake. Add earthworms, which in Vermont are not native, and their movements through the soil break up the distinct horizons and move material back up through the soil horizons. Use a plow on these mountains, and what took 10,000 years to take shape is gone in a day.

“You can basically destroy a spodosol by plowing it and adding lime,” said Caroline Alves, a soil scientist with the Natural Resource Conservation Service. But here’s the redeeming thing about Vermont’s spodosols: they’re resilient. Alves said if you add any kind of fertilizer, the soil has the capacity to hold onto those nutrients and release them slowly, something that enables Vermont to continue being a decent place for agriculture, even after centuries of cultivation.

If you’re looking for a spodosol, you might not know you’ve found one unless you test the acidity. Farmers, burrowing animals, and even falling trees can destroy a spodosol’s vibrant layers. The place to look, says Alves, is under a conifer tree, especially under a hemlock.

So grab a shovel, dig a hole, and see if you can figure out what happened here in the last 10,000 years.

Audrey Clark

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  • Joanna Cummings

    Thank you for the natural history lesson. I studied agronomy many years ago and appreciated the refresher.

    It would be interesting to know the elevation range for this soil type. Also what effect logging has on disturbing the soil and how that impacts the health of subsequent tree growth.

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