What is the Vadose Zone?

Scientists call the region between the soil surface and the groundwater table the vadose zone. It also is often called the unsaturated zone. This zone comprises soil and underlying sediments or bedrock that lie above the groundwater table.

The vadose zone contains “the living skin” of our planet, that is, the outermost layer of Earth that contains the myriads of soil organisms and is the foundation of agriculture. This variety of microbial life is possible because of the simultaneous presence of water, air, and a solid substrate. This makes for an ideal habitat for microbial organisms.

All agricultural activities depend on the vadose zone, in particular the uppermost layer of the vadose zone: the soil. Here, a peanut plant is sprouting during the growing season in South Carolina. Credit: Christianah Oladoye

How does it support ecosystem services?

The vadose zone provides critical ecosystem services such as carbon sequestration, temperature regulation, flood control, water filtration, groundwater protection, and food production. Besides the earth’s crust, the vadose zone is the largest pool of carbon on our planet, followed by the oceans, biomass, and the atmosphere. The vadose zone therefore plays a critical role in regulating the global carbon cycle, and storage of carbon in the vadose zone can combat global warming.

The vadose zone can regulate temperatures, thereby preventing the occurrence of extreme temperatures. Unique physical characteristics of the soil allow heat to be conducted away from the soil surface and stored in the vadose zone. Similarly, water can ready seep into the vadose zone. There it can be stored and released again. The vadose zone acts like a sponge, soaking up rainwater before rain can turn into floodwater. It stores and provides water to the plants that can later be used by plants during drought periods.

Another important function of the vadose zone is filtering water seeping through the vadose zone. Rainwater usually contains various pollutants that are picked up from the atmosphere or from urban runoff. These pollutants are then held up or removed in the vadose zone as the water migrates downward to the groundwater. The vadose zone thus acts as a filter that protects groundwater and drinking water sources from becoming contaminated.

Scientists call the region between the soil surface and the groundwater table the vadose zone. The vadose zone contains “the living skin” of our planet and many activities – like food production and water movement – take place in the vadose zone. Credit: Markus Flury/SSSA staff 

All agricultural activities depend on the vadose zone, in particular the uppermost layer of the vadose zone: the soil. Soil is the foundation of life, it is the living medium of myriads of organisms that life in the soil and of all other organisms living on top of the soil. It is the medium that sustains human populations by providing not only food, but also all the other ecosystem functions mentioned above.

The vadose zone is critical for life on our planet. Much has still to be learned about the vadose zone, and we are yet to understand all the complex interactions that take place between physical, chemical, and biological processes occurring under our feet.

Answered by Markus Flury, Washington State University

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