Understanding the Essentials of Soil Composition

Understanding the composition of your soil is key to understanding your garden and succeeding as a grower. The best soils for growing nearly any plant are loamy soils. Loam is a balanced mix of sand, silt, and clay, which is considered the best soil for gardening due to its excellent drainage, nutrient-holding capacity, and workability. But what does that even mean? To understand the role soil texture (balance of sand, silt, and clay) you must first understand the composition of soil. Soils are composed of pore space and solid materials that are further divided into minerals, organic matter, air, and water. Each piece plays a critical role in plant health. Let’s dive in!

The Critical Role of Pore Space in Soil Composition

Pore space is the area between the solid materials found in soil. This is where air, water, and plant roots travel through. While pore space may be considered a “negative” space, it is very much so an important portion of the composition of soil. Did you know pore space accounts for 50 percent of soil? When considering the health of your most vital growing material, you must consider this vital space. The average soil composition is made up of 25% air and 25% water. This balance allows developing plants to access water, oxygen, and nutrients that are stored in the solid soil materials. Nutrients are able to reach the growing plant roots by traveling through the pore space via gas particles or attached to water molecules. The amount of pore space you have is dependent on your soil texture, the combination of sand, silt, and clay particles.

Air - 25% of Soil Composition

Have you heard that compacted soils cause problems? Well, that is mainly because they lack air! Air in the soil is essential for root respiration and the survival of soil organisms like earthworms. Good soil structure allows air to penetrate without creating too many air pockets, supports good drainage, and enables healthy plant growth. Imagine how helpful a sponge would be without holes. That would be like cleaning your dishes with a brick. Just as the holes in a sponge allow for water to be transported and for the sponge to dry between uses, so does the air in the soil allow for water and nutrient passage without drowning developing plants.

Water - 25% of Soil Composition

You know this one! Water is critical for plant health and good soil structure. It is needed for plant hydration, nutrient transport, and more! There is a reason why plants fail when the soil is too dry or wet for too long. Water is an important part of a healthy soil composition. The soil’s ability to hold onto water is largely dependent on the minerals present, i.e., sand, silt, and clay. While it is important for soil to be hydrated, the time it spends drying is actually more important to plant development. How frequently water should be applied will heavily depend on the ratios of the solid components your soil is made of.

Minerals and Organic Matter - The Solid Components of Soil

On average, 50% of your soil is made up of mineral and organic matter. These are the solid materials that most people think of. Unlike pore space, these materials are not equally divided. Mineral contents, on average, account for 45% of soil composition, while organic matter accounts for only 5%. The mineral contents include sand, silt, and clay particles that are developed from eroding parent rock materials found several layers deep into the ground. The organic matter, on the other hand, forms from the byproducts of living organisms on the surface and decaying plants, microorganisms, insects, and other animal matter that work their way down. The combination of water, physical tilling or mixing, and root transportation create the arable growing conditions needed for growing crops.

Soil Minerals - 45% of Soil Composition

All soil is primarily made up of a mixture of the eroded minerals of sand, silt, and clay. Sand is the term used for the largest particles, silt is the medium-sized particles, and clay is the smallest. Clay particles are smaller than most bacteria! Each has its purpose, but when any one of them is gathered in too high of a concentration, it can cause problems in the soil. Sand, being the coursest and largest particles, are fantastic for creating space for air and water to exist in the soil, but they are terrible at holding onto nutrients. Silt particles are much better at holding nutrients and water but do not provide much structure for the roots to hold onto. Clay particles hold nutrients well but are so fine and flat that they can easily become compacted and limit root growth. That is why loam soil is the best. Loam soil is a mixture of all three of these elements that creates the optimal abiotic environment.

Organic Matter - 5% of Soil Composition

One of the most important ingredients of soil needed for growing plants is organic matter. When you hear the word “organic” you are probably thinking about the growing practice. In this context “organic” is instead referencing the carbon material that exists in living or once living matter. This includes decomposed plants and animals, such as compost, leaf litter, old mulch, and more! Organic matter enriches the soil with nutrients, improves water retention, and enhances soil structure. It is also necessary to maintain beneficial microorganisms in the soil. There is a growing body of science suggesting that the microorganisms in soil are so critical to soil health and, therefore, plant health that without them, we would need to compensate by adding fertilizers and other artificial amendments. Most of the time, when soil is lacking in some way, from structure to drainage to nutrition, adding organic matter helps! This is done through compost, cover crops, worm castings, mulch, and more.

Soil Amendments to Improve Organic Matter:

Soil Layers

A cross-section of soil will reveal the layers beneath the surface. Soil can further be divided into the six main sections, including humus (organic matter), top soil, eluviation, subsoil, parent rock, and bedrock. Each layer plays an important role in plant health and soil regeneration. Humus is the top layer primarily made up of decomposed organisms. Topsoil is where seeds often germinate and take root. This is where the building humus layer mixes with the mineral particles. The eluviation layer is mostly sand and silt. This layer lacks nutrients due to leaching by water. Subsoil is next and is heavily composed of clay and mineral deposits that have been carried through the layers above it via water. The parent rock is hard, deep, and can’t be penetrated by plant roots. The deepest layer is the bedrock, which is unweathered rock. As gardeners, we generally have the most control over the humus and topsoil layers. These can be amended and changed to suit the needs of our plants with various materials and practices.

How Does Soil Form?

Soil forms through the weathering of rocks (abiotic/non-living factors) and the decomposition of organic matter (biotic/living factors). Over time, rocks break down into smaller particles, creating sand, silt, and clay. Organic matter from plants and animals mixes with these particles, enhancing fertility and structure. Productive soils contain the benefits of nutrient and water-retentive organic matter balanced with proper drainage via a loamy soil texture. It is useful to understand this process in order to understand and improve your own soil for productive plant development.

Maintaining and growing your soil is vital to sustainable food production. Because soil grows at a measly rate of 0.025 to 1 millimeters per year, it is vital that growers protect what soil they have for a sustainable future. Without maintaining proper soil composition, individual plots of land and expansive regions can face devastating consequences. We do not need to look far into our past to see such detrimental effects of neglecting our land. Examples such as the great Dust Bowl of the Great Depression can teach us just how important it is to understand the role that caring for our soil plays in maintaining growable land for generations to come. Continuing to use sustainable growing practices like cover crops and no-till or low-till processes can help us restore and improve the nutritional value of our soil and the food we harvest.

Soil provides essential nutrients and structure for plants. Improper balancing of soil composition results in reduced root growth, unwanted or reduced microbial activity, and poor nutrient uptake, which reduces yields and overall unhealthy plants. In summary, soil is a dynamic mixture of minerals, organic matter, air, and water, all of which contribute to its ability to support plant life. By understanding soil composition and structure, gardeners can create optimal growing conditions for their plants both indoors and out.

Lara Wadsworth, True Leaf Market Writer

I am a native of Southwestern Michigan, where I also reside, and I love all things plants! I got a Bachelor's Degree in Horticulture and found the first work-from-home job I could get. Now, I spend my days writing for TLM, playing with my dog, eating delicious food with my husband, and plotting my next landscape or gardening move. I believe everyone should get down and dirty in the soil now and then. Happy Gardening!