What’s the dirt on Fayette County’s urban soil?

by Max Puchalsky

LEXINGTON, Ky — Beneath the surface lies silt, sand, clay, or loam…just don’t call it dirt. Soil is a living ecosystem that impacts the entire biosphere above the ground. It teems with microscopic organisms and supports the growth of countless plants and trees, playing a crucial role in sustaining life on Earth.

October 6–15 marks Lexington’s 6th annual Tree Week, a week-long celebration of local trees and greenspaces presented by the University of Kentucky’s Urban Forest Initiative. While trees are the star of the show, soil made an important appearance as well during a walk and workshop at The Arboretum, State Botanical Garden of Kentucky on Tuesday courtesy of the UK Natural Resources and Environmental Science Program. The event was co-facilitated by Jessica Slade, Native Plants Collection Manager and Curator at The Arboretum, and Hannah Angel, Academic Coordinator for the Natural Resources and Environmental Science program at the University of Kentucky.

On the walk, Slade and Angel explained the basic elements of soil, demonstrated soil sampling techniques for testing purposes, and provided soil management and planting recommendations for the health and longevity of trees.

WHAT IS A SOIL?

Soil is a mix of organic matter, minerals, air, and water. The ideal soil for tree growth is roughly 50% permeable and 50% solid, with the permeable space filled with equal parts air and water. According to Angel, “soil biology is what makes a soil special and provides all the ecosystem functions” and soil organisms such as worms, beetles, and arthropods are “the natural tillers of the earth,” transforming soil and defining what separates the Earth’s soil from say, that of Mars.

The acronym “CLORPT” (Climate, Organisms, Relief, Parent material, and Time) can be a helpful way to remember the factors which impact soil formation. Weather characteristics such as temperature and rainfall, the animals, plants, bacteria, and fungi living in the soil, topographical factors such as elevation, slope, and cardinal direction, the kinds of rocks or transported sediments the soil formed in, and the age of the soil are all important factors which influence a soil’s structure, color, texture, pH, and mineral composition.

DON’T GUESS, TEST

Angel, who holds a Ph.D. in Crop & Soil Environmental Science, explained how above-ground symptoms are often caused by below-ground issues. This is where soil testing comes into play. Angel first demonstrated how to examine a soil profile by looking at its horizons, or layers. The main soil horizons are the A-horizon, or topsoil, and the B-horizon, or subsoil layer. The density of each horizon is easier to determine when the soil is moist. Peds, or aggregates, are another important feature of soil mapping and classification. Peds are the unit of structure or “clumping” based on the organic composition of the soil. Different materials lead to different soil structures and colors as the soil sinks and swells over time. A slake test can determine a soil’s structure-aggregate stability by placing peds in water and observing how long they take to break apart. More organic material in the soil means more stability, which results in less slaking and therefore less overall erosion.

Next, the facilitators, along with the help of several volunteers, gathered soil samples for lab testing. Angel instructed participants on how to gather 10 cores from an area with similar soil, mix the cores together, fill a sample bag, and send it to any local County Extension office for testing, which costs $7 per sample. Test results will offer basic data such as pH, phosphate, potassium, and zinc levels as well as specific recommendations based on intended use (trees, vegetables, grass, etc.).

PLANT NATIVE

Slade emphasized how selecting the right tree for one’s soil can be just as important as testing. The Bluegrass region, for example, is rich in phosphorous and has a higher pH than Eastern Kentucky and Appalachia. Native, as opposed to invasive, species are well matched to Kentucky’s soil and are therefore more likely to thrive. A pamphlet provided to workshop participants lists a variety of native alternatives to invasive trees, shrubs, groundcover, grasses, and herbaceous plants commonly used in landscaping and gardening.

Emulating natural phenomena is another way to improve soil quality. For example, allowing leaf litter to accumulate and compost in place can reproduce the conditions of a woodland area. Similar practices include allowing plants to die back into the soil, planting a cover crop, and adding compost tea or other organic amendments to the soil. 

SOIL MANAGEMENT FOR HEALTHY TREES

Angel believes, “it’s important to understand our soil so that we can care for it and create an ideal environment for our trees, which will ultimately take care of us.” When planting a tree, dig a hole twice as wide as the tree’s root ball (but not any deeper) so that its roots have room to grow into uncompacted soil with good airspace. Give it plenty of water and use woodchips to form a circle or ring at the base of the tree. This helps nutrients return to the soil and also helps prevent soil compaction caused by, for instance, driving too close to the base of the tree. Also, consider investing in tools like a broadfork, or depending on the size of the planting area, renting an aerator or tiller. However, be careful not to over-till, as this can negatively impact soil aggregate stability. The goal is to break up the hardpan, just beneath the uppermost topsoil layer, and then introduce organic matter such as compost before planting.

CLIMATE RESILIENCE 

Soil not only tells the story of a landscape, but is crucial for the long-term survival and thriving of aboveground communities in the face of the climate crisis. Angel described how, “as rainfall events are more extreme and we have more droughts, we just want to create a more resilient system that can support healthy microbial community.” Slade agreed, a well cared for soil can “create a resilient urban ecosystem that doesn’t all just die when we get these extreme changes in climate and weather patterns and pests.”

In a world grappling with the challenges of climate collapse, fostering healthy soil ecosystems is not just a matter of environmental stewardship but a fundamental building block for a resilient and regenerative future for all living beings that depend on this precious resource.

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Top photo by Max Puchalsky