Understanding Soil Composition & Tips for Healthier Soil

I cannot stress enough that healthy soil is critical to having thriving plants! Even knowing that, understanding all the variables of soil composition and how to achieve the best soil can seem so convoluted.

You’ll hear a lot of people talking about the components of sand, silt, and clay. And still other people focus on pH, moisture, beneficial bacteria, fungi, or the amount of organic matter in the soil. 

In this guide, we’ll dig deep into the topic of soil to explain its components, how they affect plants, and the 5 pillars of healthy soil that every gardener should know! 

The Science of Soil

Many people consider soil just boring ol’ dirt, but soil is so much more than that. If you head outside and pick up a handful of soil, you are holding the very foundation for all life on Earth!

That’s why, no matter where we are in our gardening journey, we can all benefit from learning about the science of soil. I hope you utilize the following soil resources, so you can grow bigger! 

• How to Improve Soil Drainage

• Best Organic Soil Amendments

• How to Test Soil pH

• Decontaminate Soil With the Power of Plants

• The Role of Nitrogen in the Garden & How to Add Nitrogen to Soil

• The Best Fertilizer for Vegetable Gardens

• Relationship of Beneficial Mycorrhizal Fungi and Plants

Understanding Soil Composition

Image courtesy of NC State Extension Publications

Soil is made up of minerals, air, water, and organic matter. What many consider the ideal soil composition of healthy soil is 45% mineral, 25% air, 25% water, and 5% organic matter.

The mineral part of soil is weathered rocks broken down by nature over time. These inorganic soil particles are put into categories by size: sand (large particles), silt (medium particles), and clay (small particles). The ratio of these will determine your soil's drainage, texture, and water retention capacity. 

Image courtesy of https://extension.umd.edu/

• Sand - Sand is unable to retain water, has high aeration, and because of the high drainage, sand particles have a hard time holding onto nutrients as water washes through. 

• Clay - Clay is poor-draining, which can lead to high water retention. It is very dense with minimal aeration, but it does hold onto nutrients well. 

• Silt - Silt is the middle ground between sand and clay in all aspects: aeration, drainage, ability to hold nutrients, and water retention.

• Loam - Loam refers to a healthy soil structure that contains all three components of sand, silt, and clay. 

5 Pillars of Healthy Soil

Now that you have a basic understanding of what soil is, let's dig a little deeper. The topic of soil composition can be a bit overwhelming so I have broken it down into the five things that you need to focus on to achieve ideal, healthy soil: 

1. Soil Moisture

2. Aeration

3. Organic Matter

4. Beneficial Bacteria and Fungi

5. Good Soil Coverage 

Soil Moisture and Aeration

The first two essential pillars of healthy soil, moisture and aeration, are closely linked. They are directly affected by soil texture and compaction.

You see, you have to have proper drainage in order to have proper soil moisture. Too much drainage, like sandy soil, leaves the plants and beneficial bacteria void of moisture because all the water drains right through.

Not enough drainage, like compacted or clay soil, will cause water to either flow right off the top or soak in and oversaturate the soil. This, coupled with the inadequate aeration in compacted and clay soils, leads to root rot, root suffocation, and not enough oxygen to support beneficial bacteria and fungi.

If you have compacted or poorly draining soil, then How to Improve Soil Drainage is a must-read!

Organic Matter

Decomposed organic matter, or humus (pronounced HEW-MUS), is made from the breaking down of things that were once living. Plants, insects, animals, and microorganisms all decompose eventually to create nutrient-rich humus. 

Humus has a structure that is sort of spongy and soft. As I explained in What Is Compost?, it is vital to healthy soil because it reduces compaction, provides nutrients, retains moisture, and helps keep the soil conditions hospitable for microbes to thrive.

The best way to increase the humus in your soil is by adding compost as an organic soil amendment.

Microbiological Aspects of Soil: Beneficial Bacteria and Fungi

Under your feet is this symbiotic network made up of tens of thousands of species that we rely on in soil to produce vegetation. Beneficial bacteria and fungi are a requirement in any organic garden because plants need this symbiotic relationship. 

Bacteria are single-celled microorganisms that decompose and move organic matter. Different species of soil bacteria can live in aerobic (with oxygen) or anaerobic (without oxygen) conditions. Good soil aeration means the soil has adaquate oxygen to facilitate the beneficial aerobic bacteria that we want to actively break down the organic material. 

Similar to bacteria, fungi play a huge part in soil health! Fungi lack chlorophyll (the green pigment in plants that is responsible for photosynthesis) meaning they cannot photosynthesize.

Instead, fungi use decaying organic matter as a food source, aiding in decomposition, or they can have a symbiotic relationship with plant roots where the plant creates sugars and carbohydrates that the fungi feed on, and the fungi help the plant absorb trace minerals, nutrients, and even water in return. 

Did you know that mushrooms and other fungi in your garden can be a sign of healthy soil ecology?! Here’s more information on garden mushrooms and building a beneficial mycological environment in soil. 

Soil Coverage

The fifth pillar, and arguably the most underappreciated yet most important, is good soil coverage! Having good soil coverage is what protects and preserves all the other pillars of healthy soil from wind, sun, and water damage.

This soil coverage could come from a living mulch like cover crops or densely planted crops that keep the soil shaded. It could also be organic mulches like leaves, wood mulch, pine needles, compost, grass clippings, pine shavings, or straw. 

Embracing the importance of mulching will radically improve your gardening game! Each step you take toward understanding your soil composition will be a win for your garden, for you, and the health of your family.