Chapter 2: Soil Stores Fresh Water & Trees Move It

Soil has structure, which means it has plenty of gaps between the dirt (particles of sand, silt, and clay) to hold air and water. It is helpful to think of soil as a sponge. Much like a sponge, it can hold water when there is rain and air when there is none. But how are these gaps—the soil structure—formed?

In short, life is what creates ideal soil structure. Bacteria, fungi, protozoa, nematodes, springtails, mites, ants, earthworms, and all other members of the soil ecosystem eat, poop, reproduce, and die. To do this they wriggle, crawl, and tunnel their way through dirt. Some are predators that eat others, and some are prey that hide from predators. Some eat organic matter, which is the dead remains of anything that was once living. Dead leaves are a good example. Some secrete slimes, glycoproteins, and other sticky substances. These and other activities produce soil structure in three primary ways:

1. Life moving in the soil—wriggling, crawling, and tunneling—produces gaps between particles of dirt.
2. Poop, dead bodies, and other organic matter form spacers between particles of dirt, producing gaps.
3. The sticky materials bind dirt together in clumps, which generates more gaps. We call these clumps aggregates.

If we could pull a cube of soil from the ground and closely examine it, we would find that roughly half of that cube is empty space created by these various activities. These gaps fill with water when it rains. During 1 inch of rainfall, approximately 27,000 gallons of water falls per acre. Soil can absorb nearly all of this rain. As water percolates into the soil, bacteria and fungi filter contaminants, pollutants, and heavy metals. Eventually clean, fresh water settles in giant storage areas underground called aquifers. These storage areas provide crucial hydration for terrestrial life.

Water’s journey does not end there. When a tree has access to plenty of water stored in the soil and is basking under the warmth of the Sun, it sucks water through its roots, up the trunk, and into its leaves, where that water is used for photosynthesis. Well, some of it is used for photosynthesis. Most of it evaporates from the leaf into the atmosphere. This movement of water through the tree is called transpiration.

Transpiration generates a substantial amount of water movement. For example, a mature oak tree can transport over 40,000 gallons of water from soil to sky per year! By comparison, a typical swimming pool holds around 20,000 gallons of water. That is a lot of water moving from the soil into the atmosphere. As forests perform transpiration, they generate an upward air movement that leaves an area of low pressure underneath in its wake. Air from nearby—some of which is filled with moisture from neighboring bodies of water—rushes in to fill this low-pressure area. The trees then send volatile organic compounds (VOCs) into the sky which interact with the moisture to generate rain. When the rain falls, it waters the trees, refills the soil, and the process repeats. As this pattern continues, the rain moves further inland so long as there are sufficient trees present to continue facilitating this process. This helps illustrate some of what I’m attempting to describe:

Let’s summarize, since this is a complicated process: trees help turn dirt into soil, which then acts as a sponge to hold water. Trees suck water from the sponge and release much of it into the sky, thanks to photosynthesis and transpiration. This release draws in moisture from adjacent bodies of water. When that moisture arrives, trees use it to help create rain—and this process then repeats, moving further inland.

Trees and soil are vital parts of Earth’s irrigation system. They help make fresh water readily available to the life that needs it. Collectively, plants, soil, and water create the foundation for all other terrestrial life to flourish.