This is like asking if oxygen and air are the same thing. Many people use the terms interchangeably, which really annoys people who like technical definitions.
Soil and dirt are not the same thing. Soil is an ecosystem with five different ingredients:
- Organic matter
- Biological organisms
Dirt is merely the term for displaced soil that makes your hands or home dirty.
However, dirt is a term commonly used to refer to soil. Does it matter? Only amongst horticulture professionals. Soil is a beautiful, misunderstood world beneath our feet, while dirt belongs in the trash can.
So, what makes soil so special, and why is dirt a dirty word?
Soil Determines Flavor & Nutrition
Have you ever wondered why homegrown vegetables taste better than what you buy in the supermarket?
The answer is soil.
Soil is a biodiverse home to all sorts of animals, insects, and microorganisms that fall into a food chain:
- Animals eat each other and other insects, and produce waste.
- Insects eat each other and plant material, and produce waste.
- Microorganisms eat each other, plant material, and the waste produced by the entire food chain.
When bacteria have broken down waste into its most basic elements, it becomes food for the plants.
This is what the cycle looks like with a dead leaf:
- Leaf falls on the ground.
- Mites, woodlice, pillbugs, and other insects eat the leaf.
- Bacteria eat the waste from the insects.
- Plants eat the waste from the bacteria.
- Plants die and leaves fall on the ground.
All of the nutrients that were in a living plant will break down and return to the soil if the soil environment is conducive to insects and bacteria.
The official name for this process is the nitrogen cycle.
The nitrogen cycle is the process of changing different forms of nitrogen into more basic forms of nitrogen, while also breaking down other chemicals into their basic forms.
The nitrogen found in a leaf is not the same as the nitrogen found in the soil or in the air. There are four forms of nitrogen (source):
- N2: Dinitrogen/Atmospheric Nitrogen. Makes up 78% of the atmosphere.
- NO3: Nitrate. Most common form of nitrogen used by plants.
- NH4: Ammonium Nitrogen. Less common form of nitrogen used by plants to form proteins.
- C-NH2: Organic Nitrogen. Found in living plants.
There are two processes that return nutrition to the soil; mineralization and nitrification.
Mineralization is the first step in the nitrogen cycle. During this process, organisms break down organic nitrogen into ammonia. The ammonia will eventually break down into nitrates during nitrification.
Mineralization is interchangeable with biodegredation. Or, more simply, decomposition.
As bacteria break down organic nitrogen, they release fundamental compounds like phosphate, sulphate, calcium, potassium, and other nutrients.
During nitrification, organisms break ammonia down into nitrates. The plants take in the nitrates, and then turn them back into organic nitrogen.
Synthetic fertilizers do not depend on nitrification, since they are already in plant-available forms. This does make the process easier, but it harms soil life.
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So, what does this have to do with the nutrient content and flavor of your veggies?
Plants can only absorb the nutrients that are available in the soil. The soil can only contain nutrients that result from mineralization and nitrification.
Synthetic fertilizers generally do not hold well in the soil, and they only contribute to leaf and root growth, not overall nutrient content.
So, your vegetables are only as healthy as their soil. Kale grown in healthy soil will be more nutritious than kale grown in overworked soil. Nutrient-dense plants are more flavorful, because the chemicals responsible for flavor depend on trace nutrients.
The key to overall nutrition, then, is thriving soil health.
Soil Breaks Down Contaminants
Another important function of soil is that it can be nature’s filter. In situ bioremediation is the process of using plants to extract harmful chemicals from the soil and break them down or transform them into harmless compounds (source).
Healthy soil is capable of breaking down organic contaminants, including:
- Heavy metals
- Other carbon-based chemicals
This process can happen naturally over time, or it can be accelerated through aeration, leaching, and composting.
One of the more famous examples of bioremediation is the London Olympic Park environmental cleanup campaign that began in 2007 and transformed 500 acres of contaminated brownfields into parks, sporting fields, and an aquarium (source).
Healthy, thriving soil can break down and filter out many harmful contaminants before they reach the water supply. This function has given soil the affectionate nickname earth’s kidney.
Healthy soil also functions as a giant water filter, by binding to contaminants in the topsoil and subsoil layers and only allowing clean, mineral-rich water to leach into the groundwater.
Ecosystems with a diverse plant and soil life can filter out many different pollutants and return clean, fresh water and oxygen to the environment.
What is Dirt?
Dirt refers to displaced soil.
Soil is an ecosystem. It is alive. It is the sum of many ingredients, processes, and life cycles.
If you remove small soil particles and drag them into your home on your feet, they become dirt.
Because these particles are no longer able to partake in the ecosystem. They are just small collections of clay, sand, silt, and organic matter.
It’s as though soil is a forest, and dirt is a sapling. If you remove the sapling from the forest and bring it home, you did not bring home a forest. You brought home one tiny displaced tree.
Why do we make the distinction between soil and dirt?
Because soil is capable of providing nutrition, filtering water, producing oxygen, breaking down dead plant material, providing homes for animals and insects, becoming a solid foundation for buildings, and regulating climates.
Dirt makes your floor dirty.
Perhaps one of the most satisfying things I heard when I was a greenhouse manager was customers who referred to soil as soil. And, consequently, one of the most discouraging things I heard was a customer who said they needed 15 bags of dirt.
Is that fair? Probably not. But, now you know why this is such a sensitive subject for horticulture nerds.