Soil plays essential roles in our lives and our environment. We use it to grow our food, store our water and plant trees & flowers that can be used as shelter and food for the wildlife. Soil is a mixture of organic matter, rock particles, organisms, air, minerals and liquid which provides the essential nutrients and the structural fondation for plants to thrive. In this article, we provide an overview of the different soil layers and types of particles found in mature soil and a few examples of soil types.

Layers of soil

Soil formation starts with rocks which are broken down into smaller particles overtime. Although soil is forming constantly, it takes hundreds of years to completely form and mature. Mature soil can be divided in several layers, called horizons. The table and picture below describe the different horizons.

HorizonNameDepth
(inches)
ColorDescription
OOrganic- rich hummus0-2″Black, dark brownMix of decaying organic matter
ATopsoil2-10″BrownRich in roots & microorganisms ; mix of minerals & organics from the O horizon
BSubsoil10-30″Yellow, orangeMix of deep rooted plants, minerals (iron, clay) and nutrients from layers above & parent rock from the C horizon.
CBedrock30″-48″Hard rock which breaks down over time ; very compacted soil.

 Gardeners are interested in the composition of the O and A horizons, the two layers closest to their feet. The diagram on the right shows the proportion of each elements for an ideal topsoil :

  • Air is a critical factor to allow roots to grow deep into the ground and the tree to survive in difficult weather.
  • Water is essential in numerous processes in life. For plants, water is essential for photosynthesis, which is the process through which plants produce glucose and oxygen from CO2 (carbon dioxide) in the air, water and sunlight.
  • Minerals provide the key ions and vitamins for plants to thrive.
  • Organics are mainly composed of 85 % humus (i.e. decaying leaves), 10 % roots and 5 % decomposers (i.e. organisms that break down dead or decaying materials).

Soil properties

Soil is made of three types of particles : sand, silt and clay. Knowing the proportion of these three ingredients is important for better soil management as they influence water infiltration (i.e. how fast water is absorbed into the soil), water-holding capacity, nutrient retention and soil erosion capacity.

Each particle type comes with specific attributes which define soil properties :

  • Size with clay being the finest particle and sand being the coarsest.
  • Soil texture which corresponds to the proportion of these three particles in the soil. 
  • Soil porosity which is the amount of pore space between soil particles. When the size of the particles decreases, total soil porosity increases, but the size of the pores decreases. The latter can make it difficult for plants to expand their roots through the soil.
  • Soil permeability which is a measure of the time it takes the water to infiltrate through the soil. Soil permeability increases when the size of the particles increases. High permeability allows water to go through the soil quickly, but can negatively impact plant growth due to low nutrient retention.

Types of soil
Soil texture defines different types of soil which exhibit specific properties and usage. The riveted chart to the right can be used to identify the type of soil based on the soil texture. This triangle is divided per type of soil (eg. clay, silty clay, loam etc.). Each side represents the proportion of clay, sand or silt. Let’s say we have a soil containing 20 % clay, 50 % sand and 30 % silt. To find out which type of soil this distribution corresponds to, we first locate where each proportion is on the sides and then look at where the lines intersect in the triangle. In this example, the lines intersect in a type of soil called ‘‘loam’’. (If you wish to visualize this further, please go to the National Resource Conservation Service website.)

Examples of soil types

Clay soils are, like the name implies, rich in clay. Because clay has the lowest permeability, clay soils drain water slowly. Clay has a high total porosity which results in good nutrient and water retention. However, clay’s pores are small making it difficult for plant roots to penetrate through the soil. Clay soils are easily malleable, lumpy and sticky to the touch when wet, but rock-hard and smooth when dry.

Another common type of soil is sandy soil. Sandy soils drain water well thanks to the large size of the particles but cannot hold moisture and nutrients (high permeability) making it difficult for most plants to grow. Sandy soil feels dry, rough and gritty to the touch and does not hold well together.

Silty soils have bigger particles compared to clay soils and smaller particles than sand. This type of soil is more fertile than clay or sandy soil because it retains moisture and nutrients better than sand and drains water better than clay. Although silty soil is better to cultivate than clay and sand, it can be easily compacted by constant pedestrian traffic or gardening equipement. Silt soil is smooth to the touch, leaves dirt on your hand and can develop a soapy consistency when wet.

Loamy soil is ideal for gardening because it retains nutrients & moisture, drains well and provides enough air for roots to settle. Loam is soft, dry, and crumbly to the touch and holds its shape when lightly squeezed.

Soil testing

To find out the soil texture present in your backyard, you can:

  • Observe the consistency (wet and dry) and drainage properties of the soil ;
  • Test the soil using a kit or by sending soil samples to your local soil testing service. Depending on the service, a soil test can provide more information than the soil texture alone. This information includes measure of the pH (measure of acidity/basicity of a solution), the concentration of several elements necessary for the plant to grow or the concentration of organic matter present. Knowing this additional information can be useful to start improving the soil (amendment) based on the wanted purpose.

Conclusion

Regenerative agriculture places soil health at the center of its mission as a means to reverse climate change. Understanding what type of soil is present in our backyard is thus a useful starting point to improve our soil and figure out which plant or tree will grow best.

Sources :

Introduction videos on soil: 

Triangle diagram: https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/survey/?cid=nrcs142p2_054167

Picture of horizons: https://de.wikipedia.org/wiki/Bodenhorizont#/media/Datei:Soil_profile.png