Soil Horizons

The most important resource in your garden is also the most abused. So, before you go all Ike Turner on your soil this season, let’s learn how to preserve and support a healthy community of life beneath your feet. I couldn’t possibly covereverything on this subject but I will touch on a few important topics. I encourage you to learn more online or even by cracking a book. How many of you have taken the time to get to know your soil. Foolishly, we often plan gardens by colors and seasons of interest. We’ve forgotten to look at what’s important. Not long ago a farmer could slip some soil in his mouth and determine many important characteristics depending on the texture, taste and presence or absence of fizzing.

I want to start by saying that soil is alive and comprised of solids, liquids, gasses, living organisms and organic matter. A handful of soil has billions of living fungi, nematodes, bacteria and arthropods. If you hold a handful of soil, you are holding more living organisms than there are humans on this planet. These organisms are continuously breaking down organic matter and enriching the soil.

Soil is classified into horizons. They are arranged like a stack cake. Many have different thicknesses and colors. Soil horizons look different in different parts of the country. In the top few inches you will find topsoil. This is where the majority of life is found and what I will spend most of my time discussing.

O Horizon (humus): The thin top layer. You can find leaf litter and humus here. This organic matter helps to hold moisture and prevent erosion during light precipitation.

A Horizon (topsoil): This is below the O Horizon and referred to as topsoil. Roots and worms are found here. This is also where seeds germinate.

E Horizon (eluviation layer): This layer is often light in color. You will find sand and silt here because the clay has been moved further down through eluviation.

B Horizon (subsoil): Clay and mineral deposits from higher horizons.

C Horizon (regolith): Broken up bedrock with little organic matter. Plant roots usually do not go down this far.

R Horizon: Bedrock or parent material. Stop digging!

The next post will cover soil types and how to determine yours.

Virginia Tech has an informative slide show about soil horizons.


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