A Journey Through the Soil Time Machine

Like most of us, I take soils for granted. But in 2015, the UN International Year of Soils, I am ready to give soils their due.

Soils are like a geological movie. The diverse team of screenwriters includes glaciers, rain, trees, wind, and worms. The narrator for this particularly dirty movie is soil scientist extraordinaire Paul Sanborn, an Associate Professor at the University of Northern British Columbia.

Paul is an affable man who looks the part with his shovel in hand, a bushy white beard, and a red cruiser vest. He will act as my guide as he attempts to change how I view the ground beneath my feet. What I see as dirt, Paul understands as a glimpse into past landscapes. Where else can you travel 600 years back in time in only two hours?

To most, the Great Bear Rainforest evokes images of giant trees growing along fast-flowing rivers, but the islands along the coastal margin—like Calvert Island, where we’ve set off today—provide a different backdrop. Much of Calvert Island’s 334 square kilometers is covered with bogs, stunted trees, and a scattering of sand dunes.

Our first stop is one of these sand dunes, which rises a few metres above the inland edge of West Beach. It is a marvel that any plants can survive on the constantly shifting, nutrient-poor dunes. The “soil” appears to merely be comprised of beach sand.

We follow a thin path back through a fringe of Sitka spruce and alder trees, and the ground cover changes noticeably. About 300 metres inland from the present-day dunes, lies another noticeable mound. “The soil in this old dune is about a hundred years old,” Paul explains.

He digs his spade into the ground, and carefully extracts a 30 x 30 cm square pit. Soil layers are called horizons, and each horizon provides a different element of the story. By looking at the layers, and how they are arranged, Paul and colleagues can start to piece together a geological timeline, or chronosequence. It’s like a storyboard; the scenes are all laid out in front of my eyes.

The top spongy layer, known as the forest floor, is capped by fallen leaf litter. Deeper in the forest floor layer is a dark, rich mixture of decaying organic materials—a teeming matrix of roots and fungi that supports plant growth.

Beneath the forest floor is a grey-coloured layer dubbed the Ae horizon. Here, most of the roots stop and very little organic material is present. It is from this drab layer that this type of soil, a Podzol, gets its name. Podzols, Russian for “under-ash,” are typical in these coniferous forests.

Beneath the Ae horizon is “brown sugar,” which Paul refers to as the B horizon. He assures me the reddish colour stems from iron oxides, and that, “the B horizon would taste awful in your morning coffee.”

There is another screenwriter who influenced many scenes in the geological movie—humans. Shell middens, where ancient First Nations villages discarded their food waste, dot the coastal landscape. Paul compares these midden sites to “a giant compost pile.”

Shell middens can influence the soils around them. Older soils become less productive over time as valuable nutrients are leached out, or locked in a form unusable for plants.

These critical nutrients often cannot be replenished without a major disturbance like a volcanic eruption. As soils age, they also tend to become more acidic, which further facilitates and accelerates weathering.

But around shell middens the pH remains pleasantly neutral, owing to the breakdown of clam shells and animal bones. The decaying organics replenish the soil with vital nutrients like phosphorus and calcium. These positive growing conditions can help archaeologists find midden sites using the lush plant growth as a marker.

Our final stop on the tour is a few hundred metres farther into the forest. Paul wants to take me back 600 years in time. He digs another precise square pit and I peer inside the hole. I am looking at soil that was being formed as the Forbidden City was being finished in Beijing and Joan of Arc was fighting in the Hundred Years’ War. Cue dramatic music.

All the Podzol-ian characters are there—forest floor, Ae horizon, and B horizon—but they have expanded with age. The 600-year soil looks like a 100-year soil that ate too much brown sugar.

“In this moist, temperate climate, there can be an impressive amount of change in soils in a matter of decades or a few centuries,” Paul beams. “We could go back 10,000 years on this island, but you only gave me two hours.”