In what way is the soil dynamic, rather than a lifeless, static body?

If you think soils are boring and lifeless, think again! They are actually complex systems that support a suite of physical, chemical, and biological activities.

The processes that occur within a particular soil system are dependent upon what the soil is made of. So, soils within different environments will likely have their own unique sets of activities. Desert and tropical soils, for example, do not harbor all the same reactions.

All soils are comprised of a few basic ingredients. These include minerals, organic materials (both living and dead), and pores filled with water and gas. Each component possesses its own unique properties, depending on its chemical and physical composition. When combined and subjected to external forces (such as gravity and climate), soil particles react with one another.

The importance of these soil particle reactions is tremendous! They contribute to the overall health and functionality of soil systems. In the short term, physical, biological, and chemical activities work in tandem to squeeze soil materials together, pull them apart, and transport them to different locations. They contribute to nutrient cycling and to the creation and decomposition of soil organic matter. Over hundreds, thousands, or millions of years, soil processes even contribute to shaping entire landscapes.

ice blister in Alaska
An example of cryoturbation: Ice blister near Mt. Sukakpak, AK. It formed when water became trapped between permafrost and a freezing front from above. As this internal water froze, it expanded upward, pushing the earth up and out into a mound. Credit: M Tiedeman

One particularly interesting physical soil process is called pedoturbation: soil mixing. Though it may be hard to imagine, soils around the world are constantly in motion. Pedoturbation is present in many forms, each of which is identified by the dominant forces that drive the churning process.

Bioturbation is the mixing of soil by biological forces. Burrowing gophers, termites, and ants are a few organisms that move soil around. Trees also contribute to this process by carrying materials trapped in their roots to the soil surface as they fall. Cryoturbation is a form of physical soil mixing that occurs as soil water freezes and thaws. As liquid water freezes, it expands. The newly formed ice then pushes and squeezes large segments of soil together. As ice melts, soils shift back and resettle into vacant pockets.

Soils are also engaged in countless chemical reactions. The extent of soils’ chemical activities is due to the electrostatically-charged surfaces of the mineral and organic components. Like magnets, soil particles attract or repel ions in solution depending on their charge. A negatively charged soil particle, for example, will attract positively charged ions, such as calcium (Ca2+).

The electrostatic nature of soil particles allows soil to loosely hold onto plant nutrients. Nutrients and other ions constantly move between the soil surface and soil water. This frequent movement makes nutrients easily accessible to plant roots. It also serves as a natural buffering system, which helps prevent soil pH from changing rapidly.

Biological activities too, are a constant force within soil bodies. Through their life cycles, animals, plants, and microorganisms contribute beneficial organic materials to soils. As organic materials decompose, they release important nutrients back into the system for plants to uptake. Beyond replenishing nutrients, organic matter facilitates the development of good soil structure and enhances soils’ ability to retain water.

It is true, soils host a diverse array of activities. Though the soil outside your window may seem lifeless and static, it truly is a dynamic ecosystem! The examples above give a mere glimpse at what is happening below our feet. All processes are tightly linked, and influence each other in ways that we are still trying to understand. One thing is clear though: the overall function and productivity of soils are dependent upon these processes. As stewards of our soil resources, we can work to facilitate the presence of beneficial activities. This will ensure the health of our soil systems will remain for generations to come.

Answered by Mary Tiedeman, Florida International University

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