What is soil?

And why feeding your soil is the most important thing you can do in your garden.

Asking “What is soil?” might sound like a pretty dry topic, but what if I reframed it like this: what is the most biologically diverse ecosystem on Earth? What is home to more than half of all species on the planet? What is absolutely vital to 95% of all food production? And what is disappearing up to 40 times faster than it’s being replenished?

The answer is, of course, soil.

Dirt vs soil

Although most people will use these two terms interchangeably, dirt and soil are not actually the same thing. Dirt is made up of sand, silt, and clay - it’s not living, and it won’t support plant life on its own. Soil on the other hand is a living, breathing, ecosystem of which dirt is just one part.

Soil has four components; minerals (which the dirt is a part of), decomposing organic matter, water, and air. Each of these components combine to create the foundational layer of any forest, field, or farm. Plants (and in turn animals, including humans) wouldn’t be able to survive without soil.

The 4 components

So, what does each component of soil do? And why is it important to understand in the context of gardening and the food system?

1. Minerals

Soil is normally made up of about 45% minerals, which can be broken down into two categories. The course fraction, which is any particle greater than 2mm in size (stones, coarse sand, gravel), and the fine earth fraction, which is anything less than 2mm (sand, silt, clay).

Each of the minerals in soil have different characteristics and, because the percentage of each mineral varies greatly, it’s important to understand what type of soil you have. For example, sandy soils tend to hold less water and can be low in nutrients. A soil that’s mostly clay can hold a lot more water and has charged surfaces that can attract and hold nutrients.

While that might sound ideal, there are downsides to clay too. Having too much clay in your soil can lead to poor drainage and water logging. Every soil will have some mix of sand, silt, and clay, and knowing what that mix looks like can tell you what you may need to do to improve the quality of the soil.

2. Organic matter

Organic matter is any substance in the soil that contains carbon and could be anywhere from 1-10% of soil mass. It could be living organisms (like bacteria and fungi), dead animal or plant matter, or organic substances that have broken down over long periods of time in the soil. As you might have guessed from the intro, the soil is absolutely teeming with life; bacteria, fungi, nematodes, insects - they’re all part of the most diverse ecosystem on the planet.

You’ll tend to see much lower organic matter percentages in conventionally farmed fields, and much higher percentages in undisturbed landscapes like a forest. Organic matter is prized in any garden or farm (or at least should be) because it can act as a source of nutrients, increase water retention, provide food for other organisms in the soil (like worms), and act as the agent that binds the minerals together to form the soil that we know.

3. Water

Around 20-30% of soil is actually water. Besides the obvious job of serving as a water source for plants, the water in soil plays an important role in dissolving nutrients and turning them into a form that’s readily available for plants. Without water, even a soil rich in nutrients would be unable to sustain plant life, because the nutrients would be locked away in inaccessible forms - water increases the bioavailability of the nutrients.

4. Air

The final 20-30% of soil is made up of air. Air works in tandem with water to increase the bioavailability of soil nutrients. Many of the microorganisms in soil that provide nutrients to plants need oxygen, so without air the nutrients would again be unavailable.

These microorganisms also expel carbon dioxide, so proper air exchange is vital to bring new oxygen in and release the carbon dioxide, in effect making the soil breathe as we do (it’s not quite the same, but you get the point). The balance between air and water levels is important, as too much or too little of either can lead to a starved plant.

Why does any of this matter?

Healthy soil is what makes healthy food. If the soil is starved of nutrients and life, then so is the plant that you eat (or the animal that you eat - they rely on healthy plants too).

And unfortunately, due to a whole range of things including poor farming practices, we’re losing healthy soil up to 40 times faster than it can be made (it takes a long time for all these minerals and organic matter to breakdown into soil). If we don’t change the way we do things, it’s estimated we could run out of topsoil in as little as 60 years. It goes without saying that this would be absolutely detrimental to our food system and the health of the planet, and would raise serious questions about how we continue to feed ourselves.

If you’re a home gardener, then the most important thing you can do is get your soil right. Knowing it’s composition (of sand, silt, and clay) will tell you a lot about what it might need. Making sure it’s watered (but also well aerated) will ensure that the little ecosystem living inside of it will thrive. And speaking of that ecosystem, those microorganisms need food too! Make sure to feed your soil with compost, manure, or other amendments because that life is what makes nutrients available to the plant and, ultimately, to you.

If you’re not a home gardener, hopefully this gives you some pause to think about where you buy your food from. If it comes from a farm where the soil isn’t cared for, where it’s relentlessly sprayed, dug, and turned, then it may not just be the environment that’s losing out - it could also be impacting the quality and the nutrient density of the food that you eat, and putting the future of our food system at risk.

Buy food from people who care about the soil, it’ll pay you back in spades.