Soil specialist Alain Plante of the School of Arts & Sciences explains what soil is, how it works, and why its health matters far beyond plant growth.
3 min. read
Penn Vet’s New Bolton Center.
(Image: Elliot Bullen)
Plants, in the form of bushes, trees, crops, and grasses, play a critical role in people’s health and well-being, providing sustenance and forming the basis for green spaces like parks, gardens, forests, and even lawns where people can relax and connect with nature.
But what about the ground beneath? Soil is one of the most important natural resources on Earth. It not only helps plants grow, but it also regulates water quality, acts as a habitat for countless organisms, recycles nutrients and organic wastes, and shapes atmospheric conditions like temperature and humidity.
To gain a better understanding of and a deeper appreciation for soil, Penn Today spoke with soil scientist Alain Plante, a professor in the Department of Earth and Environmental Science in the School of Arts & Sciences. Plante studies carbon cycling and storage in soils sampled from places such as Ghana and Liberia and closer to home from the banks of the Schuylkill and Susquehanna Rivers.
Soil is a natural body in the landscape formed over time through the interaction of minerals and organic matter, air, water, and living organisms. It's the surface layer of the Earth that supports all life and is the natural medium for the growth of plants. I usually make the distinction by saying that soil is a living (though not alive itself) and functioning part of an ecosystem, while dirt is soil that has been disconnected from its origin and function.
One important note is that soils are diverse. They vary spatially with climate, parent material or source rock, vegetation, topography, etc. In an acre, you might have a handful of different soil types that have different observable properties and need to be managed differently.
What makes soil good for growing plants or grass?
Soil quality for growing plants is a combination of physical and chemical properties. A soil needs good structure or tilth for plant growth—a combination of pores that allow both water and air to be retained and for roots to growth through. A soil also needs good fertility—a supply of nutrients and optimal pH, usually near-neutral.
Where does carbon fit in?
Carbon is the primary element of life. It enters soil originally from plant materials, gets processed through decomposition by soil fauna, fungi and bacteria, and a small fraction of this accumulates over time as soil organic matter. Soils exchange around 10 times more CO2—a greenhouse gas—with the atmosphere than fossil fuel emissions and so have a large impact on climate. The key is that CO2 in and out of soils is about equal but is sensitive to soil management.
What do pesticides and fertilizers do and are there broader implications for their use?
Fertilizers are used to boost the fertility of soils and plant productivity. Soils have a limited and sometimes relatively small pool of plant nutrients (e.g., nitrogen, phosphorous, potassium). Agriculture is designed as a process that extracts nutrients from soil to the plant for export [to eat]. The soil pool of nutrients needs to be replenished by fertilizers, whether synthetic mineral or organic in the form of manure.
Pesticides are products used to control pests, such as weeds, insects, fungus, etc. They boost plant productivity by reducing competition. The problem with both fertilizers and pesticides is that they can be overused and result in pollution or other unintended effects.
How do human activities like farming, construction, or deforestation change how soil works?
Agriculture is, by design, an extractive process. Soils need to be managed to close the nutrient loop, keeping the essential microbial community alive and healthy. This is the principle behind regenerative agriculture—that a healthy soil is one with a thriving microbial life.
The other major threat to soil is erosion. When a soil is left bare and with no vegetation cover, it is susceptible to erosion by wind or water, thus depleting the soil resource. Deforestation changes the plant cover and as a result, the carbon and nutrient balances. Construction often disconnects soil from the landscape by moving it or sealing it under built structures. This disconnection prevents the soil from performing its various ecosystem functions.
Why should people care about what’s happening in soil? What’s at stake if soils change?
‘Soils sustain life’ is a former slogan from the Soil Science Society of America. Soils provide a vast range of ecosystem services including and beyond food production. Without soil, there would be no life and we wouldn't survive—it’s that simple.
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