Horticulture Magazine

Compost vs Soil – What Is The Difference?

small seedling growing from healthy soil

Are you confused about the difference between compost and soil? You’re not alone.

Learn about these two materials, and the reason for the confusion, in this simple guide.

When looking at compost and soil, many people may just see ‘dirt’.

But while compost and a soil may look similar, they are definitely not the same thing.

Understanding what compost and soil are can help you to manage both composting systems and the soil effectively and organically in your garden.

What is Compost?

a wooden compost heap with organic materials

Compost is made up of decomposing and decomposed organic matter. [source]

In order to understand what this means, we need to delve a little deeper and talk about organic matter –

Organic matter is:

  • Something that both compost and healthy soil have in common.
  • Derived from living things. In the simplest sense, it is dead plant matter, animals and animal waste.
  • Present in both compost and soil.

In essence, it can be helpful to think about organic matter as nature’s recycling system.

When deciduous trees drop their leaves and herbaceous plants die back at the end of the year, the material is broken down through a range of processes on the soil surface.

leaves composting in a garden basket

The nutrients those materials contain are fed back into the system.

When we create compost, we are essentially trying to refine and shortcut that process.

We can compost at home to turn biodegradable, compostable materials into a soil amendment and slow-release fertiliser which we can use in our gardens. [source]

What is Soil?

hand running through garden soil

While compost contains only organic matter in a state of decomposition (and the micro-organisms and other life that help break it down), the soil in your garden has a mineral component too.

Healthy soil is made up of:

  • Minerals (clay, silt, sand)
  • Organic matter (to various degrees and in various states of decomposition)
  • Water
  • Air
  • Organisms (from microscopic bacteria, fungi etc, to earthworms and other soil life)

It is important to understand that soil is not simple.

Soil is a complex ecosystem, a web of life with various fascinating and complex interactions. [source]

This is a natural phenomenon which is truly amazing, and something upon which all life on earth depends. [source]

Caring For Soil

Taking care of the soil in your garden should be your number one priority as an organic gardener.

Soil that is not cared for becomes degraded and depleted over time.

All the elements in healthy soil work together to allow plant growth, which, in turn, allows us to grow too.

Everything comes back to the soil.

Adding compost to the natural soil in our gardens brings a range of benefits, both to us, as gardeners, and to the planet as a whole. [source]

hands adding compost to soil with sprouting seedling

This is one way to boost the soil organic matter, which raises the amount of carbon stored in the soil.

Sequestering as much carbon as possible in the soil and in plants reduces the amount in the atmosphere, helping to combat global warming. [source]

Why The Confusion?

When talking about homemade compost (which is added to soil) and soil itself, it is easy to see and understand the difference between these two materials.

But confusion creeps in when we start to talk about the ‘potting soils’ or ‘potting composts’ that you can buy.

organic tomato plant in a terracotta plant pot

The word compost is sometimes used to refer to ‘potting soils’ or ‘multipurpose composts’, but it should be noted that these are really rather different.

  • A compost, as mentioned above, is made up exclusively of decomposing and decomposed plant material (and the life that breaks those materials down).
  • A potting soil usually contains a proportion of compost, but it also has other materials in the mix.

Many potting soils will contain mineral constituents, like sand, for example.

Bought potting mixes may also contain other ingredients, like perlite, vermiculite, or additional synthetic or organic fertilisers.

The problem is that while commonly referred to as ‘composts’, multipurpose or all-purpose composts are often not pure composts at all.

As well as containing the decomposed and decomposing organic matter, they also contain other things.

Choosing The Right Option

It is important to understand the difference between true compost, and ‘potting soil’ or ‘multi-purpose compost’.

  1. A true compost can be made up of a range of different decomposed and decomposing organic matter.
  2. You can purchase composts which are made from woody waste, coir, green municipal waste, and a range of other organic materials.
  3. Peat based mixes should be avoided for environmental reasons. For environmental reasons, it is also best to avoid mixes which contain mined, finite materials. [source]
  4. The most sustainable and eco-friendly option is to make your own potting mixes and soil amendments for your garden.

It is also important to understand that ‘potting soils’ are not really soils either – many of these potting mixes are soil-free.

And those that are soil or loam-based only contain a small proportion of soil, which is sterilised and which shares only some of the characteristics of the real soil in your garden.

Where to Use Compost

True compost, whether you buy it in or make your own, is used to enrich the soil in a garden, or as a component of a homemade potting mix.

earth covered with compost, mulch and other organic materials

It should not be used on its own as a medium for filling containers.

It typically needs to be mixed with other materials to create a stable mix which has the right water retention, drainage and aeration characteristics.

Remember, this true compost should not be confused with the ‘multi-purpose’ composts that can be bought, which often contain these other materials already.

These are commonly used not only as a soil amendment but also to approximate the characteristics of healthy soil in pots and containers.

stacks of multi purpose compost in a garden centre

But the additional materials used can sometimes come at a cost, and quality can vary considerably.

So it is important to be informed and to make the right choices, for the planet as well as for you, and the plants in your garden.

Where to Use ‘Potting Soils’

A potting mix is specifically designed for filling containers, and is not as beneficial when added to the soil in your garden.

While we can try to approximate natural, healthy soil in containers in a garden, there really is no substitute for a real, healthy garden soil that we build and care for over time.

Container Growing

Where we are growing in containers, we can simply aim to get as close as we can to approximate optimal soil growing conditions.

Whether we make or buy a potting mix, our goal is to create a mix which, like healthy soil, helps plants grow as well and as healthily as possible.

courgette plant growing in a container

Remember, you can avoid the environmental harm of bought composts and potting mixes by making your own from the organic materials already present in your garden.

You can create healthy, organic mixes for filling containers and to replenish the fertility in your garden soil.

Set up a composting system, and compost in place through mulching and other no-dig gardening methods.

Cherish and care for the soil in your garden.

And stay as close to nature as possible when you are not actually growing in the soil by making your own potting mixes or buying healthy, organic, non-harmful options.

Sticking to real compost and real soil is often the best option of all.

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