Soil

Soil Functions

Everyone is familiar with what soil - or as it may be more commonly known, "dirt" - is. And most people know at least some of what it does, but soil has an incredibly important role in our world and in all terrestrial ecosystems.

It filters and cleans water that moves through it, provides a habitat for various organisms, serves as an area for plant roots to anchor and provides nutrients and water to the plants, and the soil is an essential location for the cycling of various nutrients.

Soil Formation

Soil consists of a mix of geologic minerals and organic matter, air and water, and various organisms. The geologic process of weathering and the biological process of decomposition are essential to creating soil.

The geologic material that soil forms from is known as the parent material. It is typically bedrock. As the parent material is weathered, or broken down, it adds minerals to the soil and contributes a great deal to the soil's properties and structure. Different parent materials can affect the soil's pH, nutrient content, and texture.

Erosion, the transport of weathered materials, can carry geologic minerals and deposit the soil particles into other areas as well. Areas with increased precipitation will experience increased rates of weathering and erosion. Likewise, topography has a large impact on these processes as steeper slopes lead to more erosion, while flat areas are prone to deposition.

Soils are also made of organic matter. As this matter is decomposed, it forms humus, the darkest part of soil that is nutrient-rich and holds moisture. Warmer temperatures or larger populations of decomposers lead to increased rates of decomposition.

Soil Horizons

As soil forms, distinct layers (known as soil horizons) tend to form due to the movement of particular components and where the processes of weathering and decomposition occur.

  • O Horizon: this layer is composed of organic matter, such as dead leaves

  • A Horizon: topsoil; this layer has humus made from decomposed organic matter, well-weathered minerals from the parent material, and has the most biological activity

  • E Horizon: the eluviation layer; clay and iron are leached to lower layers from here

  • B Horizon: subsoil; this layer is mostly minerals with little to no organic matter; clay and iron build-up here from above layers

  • C Horizon: this layer is the least weathered and the closest to the parent material

  • R Horizon: this is the unaltered parent material, or bedrock

The parent material, as well as the biomes, have large impacts on the soil horizons that form. For example, forests tend to have readily observed O and E horizons, while grasslands rarely do.

There are other horizons that may be present as well, such as the L Horizon, H Horizon, and F Horizon, but they are not considered main soil horizons.

Soil Properties

The texture of soil refers to the composition of sand, silt, and clay in the geologic component of the soil. It is typically referred to in terms of percentages, and the three numbers will always add up to 100. A soil with a texture of 50-20-30 has 50% sand, 20% silt, and 30% clay. The numbers always go in the order of sand -> silt -> clay, and that order is based on the size of the particles. Sand has the largest particle size of the three, with clay having the smallest. The texture of the soil has a strong impact on other properties of the soil due to the varying porosity.

Porosity refers to how much empty space is between the particles. The bigger the particles are, the more pores there are, meaning that sand is the most porous and clay the least. This is relevant because the more porous the soil is, the more space there is for air and water, which affects a soil's permeability and water-holding capacity. Permeability refers to how easily water drains from the soil, and it increases with porosity. On the other hand, water-holding capacity refers to how well the water is retained, and this goes up as permeability goes down. Think about pouring a bucket of water on the sand at a beach, the water will be absorbed very quickly, due to high permeability and low water holding capacity. Different plants are adapted to and require varying levels of permeability and water-holding capacity for their ideal growth.

Soil fertility refers to how well the soil can support plant growth. This consists of two major components: water-holding capacity and nutrient level. In most soils, a medium or "goldilocks" level of permeability/water holding capacity is ideal as it prevents both the soil from drying out and plants from drowning from too much water. The nutrient level refers to the prevalence of nutrients available in the soil, particularly N, P, and K, the three main soil nutrients.

The ph of the soil has a very large impact on nutrient levels, as acidic soils reduce the availability of nutrients.

Why is it helpful to know the characteristics of soil quality?

Soil quality can be tested in a variety of ways, and the information gathered can help inform our decisions when it comes to things such as agriculture and growing plants. It could help us decide things like whether fertilizers should be added, whether limestone should be added to raise the pH, whether the soil should be aerated to increase permeability, etc.

How to Use a
Soil Texture Triangle

A soil texture triangle can be used to characterize the type of soil based on its texture. In essence, you need to follow the lines associated with each mineral to determine which type of soil it is. The lines for sand move toward the top left, the lines for silt move toward the bottom left, and for clay they move to the right. They should all meet together at the type of soil.

Let's say we have a soil with a 40-30-30 texture. I usually go one at a time following the lines and try to find where they all meet. Give it a try!

Click here to check your answer.

Clay Loam

What does it mean when we say that soil is being degraded?

Soil degradation refers to a decline in soil quality, whether it be biological, chemical, or physical. There are a variety of ways that this can happen, and it is happening en masse throughout the world.

One major cause of soil degradation is the loss of topsoil due to erosion. Practices that remove vegetation (such as clearcutting) and those that loosen the soil (such as tilling) result in increased rates of erosion. This primarily results in the loss of topsoil, which removes nutrients from the soil, removes habitats for decomposers, and dries out the soil.

Soil compaction is another threat, and this is due to compressing the soil from heavy things on top of it, such as machines or livestock. This compression reduces the porosity of the soil, resulting in less room for air and water. This negatively impacts plants, dries out the soil, increases erosion, and can lead to desertification.

The third major threat is nutrient depletion, which is when nutrient levels in the soil fall. A major cause of this is monocropping. By growing the same plants over and over, key nutrients are depleted which reduces the ability of future plants to grow.