Soil Genesis: How Soils are Made

Summary:

• Soil formation is a natural process that occurs as a result of physical, chemical, and biological processes.
• The process of soil formation is known as pedogenesis.
• Soil formation is influenced by five factors: climate, topography, biota (organisms), parent material, and time.
• Humans can, and have, impacted soil formation.

Outline:

1. Why do soils form?
2. How are soils formed?
3. Can humans affect pedogenesis?

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Soils, just like everything else on planet Earth, have origin stories. Like rocks, plants, or people, a soil’s properties depend on the conditions under which it formed. The soil formation process is complex in nature, but relatively easy to understand.

Why do soils form?

Before we can discuss how soils form, we have to understand why soils form. Soils don’t simply pop into existence on a whim. Much like how gravity is what causes rivers to flow or the rotation of the Earth causes wind, soils are formed as a result of natural processes beyond the scope of human control.

Soils are formed from different materials, usually rocks, minerals, and biota, via weathering and decomposition that happens in situ (i.e., in one place). The term ‘weathering’ describes the process by which rocks and minerals are broken down over time either by physical, chemical, or biological processes. Some examples:

In addition to in situ weathering, soils may also form when materials are transported from one place to another via erosion or another transport event, like a landslide, rockfall, or tree throw.

These processes naturally occur all over the Earth, leading to eventual soil formation. Now that we understand that soils are formed as a result of natural processes, we can discuss how natural processes, in concert with one another, form soil from other materials.

How are soils formed?

Because soils are formed as a result of natural processes, the process of soil formation, called pedogenesis, is influenced heavily by the surrounding environment. That’s why soils in Puerto Rico look very different from soils in Alaska, and both of those soils are different from the ones found in Egypt.

Pedogenesis is influenced by five things, called the five factors of soil formation. Hans Jenny, 1899-1992, published the book Factors of Soil Formation. In his book, he described the five factors of soil formation as an equation:

S = f(Cl, O, R, P, T)

Where:

It really is that simple! Who would have thought that something as complex as soil could be influenced by just five factors? These five things have led to a great diversity of soils across the globe. Let’s discuss them in a little more detail.

Let’s start with climate. Climate, in the context of pedogenesis, is critical because it impacts the temperature and moisture regimes that influence the soil formation process. Soils that form in wet environments are likely to experience more chemical weathering. Soils in cold environments will endure freeze-thaw cycles, while soils in hot environments will experience intense daily heating and cooling.

Living organisms impact the biological weathering processes happening. Lichens and mosses have the power to grow on and break down solid rock. Other organisms have different impacts: sphagnum moss may build up and create peat, bugs in the soil break down organic matter on the surface and bring it into the soil, and fungi take nutrients from soil minerals, making them useful to plants.

Relief, or topography, is very important in soil formation and its impact can be seen in how soils change across a single landscape. In general, soils higher on the landscape (i.e., on hills) are drier, whereas soils lower on the landscape (e.g., in a depression or valley) are wetter. Materials move readily from higher areas of the landscape to lower areas due to gravity, causing lowland soils to accumulate material and upland areas to be thinner.

The chemical and mineral makeup of a soil is largely dictated by the parent material. This term refers to the stuff from which a soil originates. It could be deposits from glaciers, particles transported by water or wind, or rocks that made their way to the surface of the Earth and broke down in place.

Time dictates everything that happens on Earth, even soil formation. The older the soil, the longer a soil has had to weather. However, just because one soil is older than another does not mean it’s more weathered. The degree of weathering depends on the other factors of soil formation.

These five factors change as you move across the globe, or even across a single landscape. Of course the soils in the Gobi Desert are different from the soils in Greenland. But even within your own neighborhood, the parent material and organisms on a local riverbank are likely very different from the ones on the sledding hill nearby. That’s what makes soils so interesting: they are inherently diverse and complex systems, requiring that we understand each soil according to where and how it was formed.

Can humans affect pedogenesis?

The short answer to this question is “yes.” Humans have the power to physically change landscapes by moving earth. We can also grow whatever plants we want almost wherever we want. Over time, our actions have led to changes in climate that we don’t yet fully understand. The only soil forming factor over which we have no influence is time.

Over the millennia of human existence, we have moved earthen materials to change the topography of the land, disrupted hydrological cycles, and torn down natural ecosystems to make room for agriculture and cities. This has devastated some soils and created others.

To conclude, soils are the result of natural processes that occur over time. However, they can be altered, created, or even destroyed by human activity, and we must be careful to ensure that the soils which support us receive our support, too.