Life in Soils

Soil is an integral part of our lives. It grows the food that we eat, houses the plants that we need, and secures the foundation for our lives.

So, what is soil?

It is the skin of the earth and the mother of all life, according to geologist David R. Montgomery. In his book, Dirt, Montgomery reveals the beauty, importance, and necessity of soil. Charles Darwin was one of the first to take note of the dynamic nature of soil. It consists of a system of giving (erosion) and taking (weathering of rocks) based on changes in the environment. Many factors can affect soil, its composition, the cycle in which it participates in, and its level of fertility. These factors include: the presence of various types of flora and fauna; topography; climate; geology of a particular region; and time.

What is the relationship between soil and flora and fauna?

Soil is an inextricable component of the ecosystems of the world. Nutrients from the soil move from plants and animals and back to soil in a never-ending cycle. These nutrients include: nitrogen, phosphorous, calcium, sodium, and cobalt. They return to the soil in the form of decomposing organic matter, which fertilizes and allows the further growth of new plants.

Plants supply soil with the necessary organic matter from decaying dead plants and animals, as well as leaf litter. There are also soil organisms that accelerate the weathering of rock and decomposition of organic matter. These organisms include earthworms, gophers, ants, and a multitude of microorganisms. Roots also play a role in breaking up rocks to enrich the soil with essential minerals.

Are there layers of soil?

The layers of soil are simply categorized as horizons: O horizon, A horizon, B horizon, and C horizon. The topmost layer of soil is the O horizon, which is made of somewhat decomposed organic matter. This organic matter comes from fallen leaves, small branches, and other forms of plant vegetation. The existence and make-up of this layer depends on the region. In tropical jungles, the O horizon has the most amounts of soil nutrients, while this layer may simply not exist in arid regions with little vegetation. Below the O horizon lies the A horizon, which is a dark and highly fertile soil formed from completely decomposed organic matter. Together, the O and A horizons form topsoil, which can erode easily due to run-off. The next layer, the B horizon, has less organic content, which means that it is less fertile. It is often known as subsoil, and is composed of clays, nitrogen, phosphorous, and other elements that trickle down from upper layers. The final layer is the C horizon, which is composed of weathered rock.

How does the dynamic system of soil operate?

The system began four billion years ago. Early heat-loving bacteria, similar to the ones still found in the thermal pools at Yellowstone National Park, increased the weathering rates of rocks to form a very primordial soil underneath protective bacterial mats. In turn, the bacteria consumed enough carbon dioxide to allow the Earth to be inhabitable.

The dynamic system is self-enriching and self-reinforcing. It recycles decomposing matter, serving as a filter that converts dead organic material into the nutrients for new life. Soil acts as the interface between the earth and plants and animals by regulating the transfer of elements in a continuous cycle. Erosion of soil and of rocks helps maintain the pace of the creation and usage of soil as well.


Montgomery, David R. Dirt: The Erosion of Civilizations. Berkeley and Los Angeles, California: University of California Press, 2012. Print.

Post by Alice Liu, Third-Year, Statistics and Spanish