A partnership between the Woodland Trust and the National Trust

The History of Woodland Soil

Dr. Tim Harrod brought Fingle Woods right down to earth in last week’s walk-and-talk lecture. As an expert soil scientist who specialises in the Dartmoor area he threw some light on how the soils of the Teign gorge were formed and how, through many years of human activity, we have the landscape of today. At the first stop on the walk, Tim described the general base rock of the area; a hardened mudstone that had been baked by the nearby igneous Dartmoor granite.

Tim Harrod describes the bed rock of the Teign gorge

Tim Harrod describes the bed rock of the Teign gorge

Taking us back over 70 million years, once the existing sea bed had dried, he described how the surface water began to form the gorge, eroding the surface over time. The east-west channel was created by a raging torrent, the earliest stage of the river Teign. Then, quickly moving on to “recent” geological time (around 10,000 years ago) he described the movement of sludgy rock and soil mixed with melt water of the ice age that slid down hill and began to build the soil structure that exists today.

Tim explains the appearance of horizons in a “brown earth” soil

Tim explains the appearance of horizons in a “brown earth” soil

At the second stop Tim had exposed an earth bank beside the track that showed the horizons of soil, typical of Fingle Woods. At the top, the “A” horizon, or the top soil is where the organic enrichment has darkened the soil. Beneath that, the “B” horizon showed the bright colour of iron where it has been weathered for many years. The “C” horizon, or unweathered parent rock sits below that, unaltered for thousands of years. An interesting story emerged as he explained how, during the mid-20th century great areas of the top soil were scraped away. This soil was sold as a horticultural product and left a permanent mark; a thinner layer of enriched woodland soil.

grey soil

A sample of grey soil from wet woodland

At a nearby area of wet woodland, Tim demonstrated how waterlogging could prevent the reaction between iron in the soil and oxygen in the air. The “B” horizon here was metallic grey rather than rusty red. The few steps it took to walk below the spring line was enough to find a very different woodland character, influenced by this wet ground.

A few hundred metres to the west, another cross section of soil opened a different window into the history of the valley. The top soil had not been removed from this part of the woods, leaving a deeper “A” horizon, but many years of oak coppicing has removed nutrients from the soil, raising the acidity (pH 3.8). The rain water leaching through the acidic soil has removed the iron from the horizon immediately below the top soil. This “elluviated” iron from the “E” horizon has enriched the “B” horizon below.

The grey “E” horizon visible beneath the dark top soil

The grey “E” horizon visible beneath the dark top soil

This turned out to be the whole story of Fingle woods in two metres of soil and rock. Through possibly thousands of years, even before Wooston Iron Age hillfort was constructed, the layers of soil have locked up the diary of events. Now, as revealed by Tim Harrod, the tale can be told from the bedrock of this remarkable valley to the grand old trees standing in the woods. The degraded soil from hundreds of years of oak coppicing on an industrial scale is now home to a thick carpet of heather and bilberry. These plants are a sign of the low nutrient soil and are over stood by oaks of a regular size, struggling to regrow and replenish the soil. Now the Woodland Trust and National Trust are guardians of the Teign gorge, the soil may begin to recover but it’s going to take some time – geological time!
The science of the soil is a fascinating demonstration of the connection between Fingle’s history and today’s woodland ecology.

by Matt Parkins

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