What’s in potting compost?

Potting compost - also known as ‘potting soil’, ‘potting mix’ or ‘artificial potting media’ - is the stuff your houseplants grow in. It may look similar to the dirt in your garden, but it’s not the same. Garden soil is made up of sand, silt, clay, rotted plant matter and other such stuff. 

Some people say compost and some people say soil. They mean the same thing and are often used interchangeably. 

Potting compost, for indoor and outdoor plants, is usually made up of a variety of different substances.

• Peat or peat substitute. Peat is decayed vegetation, long used in compost because it retains water and is nutritionally rich. However, peat takes thousands of years to grow and harvesting it is very environmentally damaging, so it’s better to use alternatives like soft wood chips and coconut fibre. We only sell peat-free compost.
• Perlite. These little white grains (they look a bit like polystyrene balls) are super-heated volcanic glass. Perlite helps with aeration and keeps the compost loose.
• Vermiculite. Hydrated laminar magnesium aluminum iron silicate...Let’s not even get into what that means. It’s a mineral composite that aids aeration and drainage, and efficiently retains water and nutrients. It may also contain some sand or small stones, but that tends to be in cheaper potting soil.

It’s not advisable. You don’t know what your outdoor soil contains so it’s unlikely to give indoor plants the best start in life. It may be clay-based, which is too heavy for indoor plants. It’s also likely that outdoor soil will contain weeds, even if you can’t see them.

Technically, yes. If a potting mix or compost is labelled ‘multipurpose’ or ‘all-purpose’ and says it’s suitable for indoor or outdoor plants, then you’re safe to use it. 

Something to remember is that a lot of multipurpose compost has a high percentage of organic matter. As that breaks down it becomes denser, which means less air will circulate. 

Most indoor plants like well-aerated compost. They’ll be best with a mix specifically made for indoor plants, which will have a lighter texture. Alternatively, you could add perlite or vermiculite to a multipurpose compost, to improve drainage and aeration.

It’s a good idea to replace your plant’s potting compost every couple of years, even if it hasn’t outgrown its pot. Potting compost is usually enriched with nutrients, but over time your plant will gobble all these up. 

You can top up the nutrients by regularly feeding your plant, but eventually the soil will still lose its nutritional strength. That will lead to less healthy plants.

In the wild, soil has its nutrients constantly refreshed with rotting leaf matter, rainwater and other good stuff. That doesn’t happen indoors, so changing the soil completely is the best way to give your plants a major nutrient boost. 

The best time to repot is at the start of spring, when your plant is getting ready to do a lot of growing.

Most can, with the exception of cacti, succulents and orchids. 

Cacti and succulents like a soil that contains a lot of sand, stones and anything that’s going to make water drain off very quickly. They hate to sit in wet soil. Look for a cactus specific soil.

Orchids similarly hate sitting in water, but they like a soil with lots and lots of air circulation. Look for an orchid specific soil.