You’ve got half a bag of potting soil, a bag of compost and a plant that needs repotting. Mixing the two seems like an obvious way to use what you already have — but is it actually a good idea?

It can be. Compost and potting soil aren’t necessarily interchangeable, though, and adding one to the other changes the properties of the finished mix. The important question isn’t simply whether you can combine them, but whether the resulting mix will suit the plant you’re growing.

What’s the difference between potting soil and compost?

They’re related, but they aren’t quite the same thing.

Compost is decomposed organic material. Depending on the product, it can contribute organic matter and nutrients to a growing medium.

A potting mix is usually more than one ingredient. Different materials can be combined to balance moisture retention, drainage, aeration, structure and nutrition.

That’s why adding compost isn’t simply a matter of making your existing potting soil “better”. You’re changing that balance.

What happens when you add compost to potting soil?

That depends on what was already in the potting soil and what type of compost you’re adding.

In general, adding more fine organic material can make a mix hold moisture differently. It can also alter its nutrient content and reduce the proportion of larger particles that create spaces within the soil.

Is compost on its own suitable for houseplants?

Sometimes the word “compost” gets used very broadly, particularly here in the UK. A bag sold as multipurpose compost may already be formulated as a growing medium rather than simply being homemade garden compost.

However, that doesn’t mean every compost has the structure every houseplant needs.

Plants with roots that prefer plenty of air and rapid drainage can struggle in a very fine, moisture-retentive medium. Other plants are much more tolerant.

So before using compost by itself — or adding a large amount to another mix — think about the plant rather than treating all houseplants in the same way.

How much compost should you add to potting soil?

There’s no single ratio that’s right for every potting soil and every plant.

If your existing mix is already quite fine and moisture-retentive, adding a large amount of similar compost could make it heavier still. If you’re starting with a very open mixture, the effect will be different.

Instead of choosing a ratio simply because you’ve seen it recommended somewhere, look at what you’re starting with and what your plant needs.

Can adding compost make potting soil too heavy?

It can.

Roots need more than water and nutrients. They also need oxygen, which means the physical structure of the growing medium matters.

If a mixture contains a very high proportion of fine material, the spaces between its particles can become smaller. The mix may also settle and compact over time.

For plants that favour an open, free-draining substrate, that can leave you with exactly the opposite of what you wanted.

What can you add to make compost more free-draining?

This is where larger structural ingredients become useful.

Materials such as bark, perlite, expanded clay and biochar can create a much chunkier texture than compost alone. Different ingredients behave differently, but the basic idea is to introduce more physical structure into the mix.

For tropical plants such as Monstera, Philodendron and other aroids, an open substrate can be particularly useful because their roots benefit from a balance of moisture and air.

Can you mix multipurpose compost with potting soil?

Yes, but again, look at what’s actually in both products.

Multipurpose compost is intended to work for a broad range of plants. A specialist houseplant mix may already have been formulated with particular proportions of compost, coir, bark, perlite or other ingredients.

Mixing the two isn’t inherently a problem, but you’ll alter the properties of the specialist mix when you do it.

If you’ve bought a ready-to-use specialist soil because it suits a particular plant, there’s usually no need to add compost simply for the sake of it.

Can you add compost to old potting soil?

You can, but first check whether the old soil is actually worth keeping.

Adding fresh compost doesn’t undo problems such as severe compaction, pests, unpleasant smells or badly deteriorated material.

If the old soil remains in reasonable condition, refreshing it with new material may make sense. We’ve covered the checks to make first in our guide to reusing potting soil.

Can you mix garden compost into houseplant soil?

This needs a little more thought than opening a bag of commercially prepared compost.

Homemade garden compost can vary considerably depending on what went into it and how thoroughly it has decomposed. It may also contain insects, seeds and other organisms from outdoors.

If it’s still actively decomposing or contains recognisable pieces of undecomposed material, we wouldn’t treat it as a straightforward substitute for a finished indoor potting medium.

Mature garden compost can have uses, but indoor containers are small, controlled environments. What works happily in a garden bed doesn’t always behave in the same way inside a plant pot.

Should you add compost when repotting a houseplant?

Only if it improves the mix you’re trying to create.

If you’re using a suitable ready-made potting mix, adding compost isn’t an obligatory part of repotting. The existing mix may already contain the organic material and nutrients the plant needs.

If you’re making or adapting your own mixture, compost can form part of it. Just remember that you’re building a complete growing medium, not trying to maximise the amount of any single ingredient.

If the actual repotting process is the part you’re unsure about, our step-by-step houseplant repotting guide will take you through it.

What does a good potting mix need?

There’s no universal perfect recipe because different plants have different requirements.

However, thinking about the jobs a growing medium needs to perform is much more useful than focusing on whether it contains one particular ingredient.

Is it better to buy potting soil or mix your own?

Both approaches can work.

A ready-made specialist mix is convenient because the ingredients have already been combined with a particular type of plant in mind. Making your own gives you more control and lets you use materials you already have.

There’s also a middle ground. You can start with ordinary compost or potting soil and alter its structure with additional ingredients.

So, should you mix potting soil with compost?

You certainly can. Just don’t assume that adding compost will automatically improve every potting mix.

Look at the ingredients and texture you already have, then think about what your plant needs. Compost can contribute organic material and nutrients, but the finished mixture still needs the right balance of moisture, drainage, structure and air.

If your starting compost is already rich and fine, the thing it needs may not be more compost at all. Adding chunkier materials can sometimes be the more useful adjustment.

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