Soil is the foundation on which we grow, live, and work.

Soil is a natural resource on this planet as critical as clean water and air and is the medium to anchor plant roots for plants to obtain nutrients, water, and oxygen. However, not all soils are created equal. A soil’s ability to provide water, air, and nutrients is based upon its origin and how the environment has changed over time.

When considering soil formation, we are looking at a geologic timescale. Most soils begin from the inorganic minerals known as parent material. This parent material is weathered rock such as limestone, granite, basalt, shale, and sandstone. Micro- and macro organisms colonize soil and also assist in soil development. Without life living within it, the soil would not be soil. It would be the same inert regolith that covers other rocky planets and moons in our solar system.

Soil formation may take hundreds to thousands of years. The speed of soil formation depends on a location’s geology, site conditions like landforms, and the climate.

There is more to soil than weathered rocks (minerals) and worms. In fact, a typical soil is comprised of 50% solids and 50% pore (empty) space. To break that down even further a typical mineral soil is comprised of 45% minerals, 5% organic matter, 25% air, and 25% water.

Physical Properties

Of course, not all soils hold that ideal ratio of solids to pore space. Or hold 25% water at all times. So how do we determine how well a soil holds water? Or how plant roots can access air in the soil? Soil texture and soil structure are two basic properties of soil that we can use to determine its overall physical properties and suitability for plant growth.

Soil Texture

Soil is composed of three basic mineral particles, of three different sizes; sand being the largest particle (2 mm to 0.02 mm), silt intermediate (0.002 mm to 0.02 mm), and clay the smallest (<0.002 mm). Sand, silt, and clay each have characteristics that are specific to each particle. For example, sand is a larger particle with an irregular shape and does not “pack” as tightly which creates larger pore spaces; because of this, water easily passes through sandy soil. This is good for drainage; however, it is bad for water holding capacity and is why sandy soils tend to dry out more rapidly. Sand also tends to hold fewer nutrients because it has less surface area for nutrients to adhere to.

Clay particles are the smallest particles typically with a flat or plate-like shape which allows them to stack on top of each other packing tight and holding water better; however, clay can hold water too well which leads to poorly drained soil. However, because of its flat shape and ability to pack more clay particles into a volume of space, clay has more surface area for nutrients to adhere to. Clay had another disadvantage of being easily compacted. It is especially important not to work clay soils when they are wet.

The percentage of each particle in a soil determines the soil’s texture as well as its physical properties. An ideal soil texture consists of a combination of sand, silt, and clay, but not in equal parts. Due to clay’s large relative surface area compared with the other particles, it has a more pronounced effect. In essence, a smaller percentage of clay can still have a significant influence on the overall soil texture. To achieve an ideal soil, called a “loam”, a soil contains about 40% sand, 40% silt, and 20% clay. However, typically one component predominates, producing a clay loam or a sandy loam.

Soil Structure

Over time soil particles will cluster together and form aggregates. Many factors go into how these aggregates come together including freezing and thawing of water, cycles between drought and flooding, earthworm activity, and plant roots. Aggregates form into different shapes called peds. A ped can be blocky, platy, subangular blocks, prismatic, columnar, granular, and crumb. Granular and crumb tend to be ideal for more landscape and garden crops. To break up a blocky soil ped structure a gardener may till the ground to create a granular or crumb structure. Tilling does allow for better drainage and airflow, but over time we see tilling or plowing destroys soil structure and compaction can increase. Tilling introduces oxygen into the soil, which accelerates the decomposition of organic matter in the soil. As our soil loses organic matter, decreases aeration, and becomes more compacted the soil begins to lose productivity.

Soil Profiles

Digging through the soil, you may notice the deeper you go, the more it changes. This is because over time soil begins to stratify or form layers. These different layers are called soil horizons. A soil profile is a vertical slice of the soil that shows the different layers. Each layer is different both physically and chemically. Each horizon is named and defined.

O Horizon – consists of the organic matter such as fallen leaves or decomposed plants at the very top of the soil profile.

A Horizon – Likely the most valuable soil horizon to human civilization, this is the primary mineral soil horizon for root growth. Typically, referred to as topsoil. Often darker in color due to higher levels of organic matter.

B Horizon – Under topsoil is the subsoil layer. It is often rich in minerals but contains high amounts of clay that have leached out of the A horizon, making it more difficult to dig and for plant roots to penetrate.

C Horizon – Just above the bedrock is the C horizon composed of weathered parent material. This material could have weathered and broken from the bedrock, deposited by floodwater, or gravel and stone glacial deposits. The C horizon becomes covered in the soil profile and remains relatively unchanged as the soil above continues to change and develop.

Horizon depth varies based on the many factors that are at play in soil formation. The A horizon may only be two inches deep on a mountainside but could be two feet deep on a former Illinois prairie.