A Quick Guide to Potting Compost

Potting compost; what is it? How is it different from regular compost? Do you really need it or is it just part of another marketing tactic?

If you’re new to gardening or perhaps a regular who’s never used potting compost before, this guide is for you.

What is Potting Compost?

Potting compost is a light, well-draining growing medium that I’ve found helps plants grown in pots and trays.

There are several types of potting compost:

• Seed potting compost.
• Potting compost for young plants.
• Potting composts for established pot plants.

All three types of compost are very different from regular compost or garden soil which is too heavy and dense for most pot plants.

There isn’t much difference between the three compost types as all are light, well-draining, well-aerated mediums that allow roots to grow strong and healthy.

Potting and Multipurpose Compost – A Comparison

Multi-purpose compost is a nutrient-rich growing medium that can be used as a standalone material to feed established plants for up to a year. It can also be deployed as a soil conditioner.

Most potting composts contain fewer nutrients than multi-purpose composts but have better drainage and are more aerated – perfect for seeds, cuttings and plugs. I’ve found that a light, airy growing medium allows roots to grow while dense heavy soils and composts restrict the early root growth.

Perhaps the image below will help to explain:

When growing young plants (ie from seed, cuttings or small plugs), I believe it’s essential that the growing medium isn’t waterlogged or allowed to dry out, as either can lead to early failure of the plants.

I’ve found that it’s very easy to overwater or underwater a new plant but potting compost is far more forgiving. The extra additives can soak up any excess water so the roots don’t rot. In dry conditions, the additives slowly release the moisture thus watering the plant via the roots.

I believe that regular multi-purpose compost is best for established plants, it contains more nutrients (which larger plants obviously need) but becomes so dense and can hold so much water, it’s not very forgiving to seeds, cuttings and small plugs.

Potting compost mixes are also perfect for both indoor and outdoor pot plants, in fact, almost all pot plants do better in aerated mixtures compared to heavy composts and soils.

What You’ll Need

I’ve been making my own potting compost for years, here’s what you’ll need to make yours:

Compost – I prefer to use our own compost where possible, if I haven’t got enough, I’ll mix it with bags purchased from the garden centre. If you’re new to composting, check out our review of the best hot compost bins. Ideally, the compost should be fine and fluffy, and sieve out any large lumps as required.

Worm Castings or Manure – Worm castings are literally worm poo and are bursting with nutrients. Manure is a nutrient-rich organic farm animal dung and is usually from cattle and horses. It’s very dense and heavy hence why I need to mix it with other ingredients to lighten it up.

Perlite – This material is a volcanic glass that’s heated to over 1500°c where it expands like popcorn and becomes very absorbent. Perlite holds water and air bubbles, so I add it to my potting mixes to prevent soil compaction.

Horticultural Grit/Course Sand – Added in small amounts to improve drainage and create air bubbles in the mixture, I’ve found it also prevents compaction.

Coconut Coir – Made from the husk of coconuts, this material is used in potting mixes as an alternative to peat. It doesn’t have any nutritional value of its own but is very absorbent. I’ve used it several times and it holds up to 1000 times more air than typical garden soil. These qualities make it perfect for mixing with dense compost and soils. Coconut Coir is widely used in hydroponics and many plants grow in it without any compost, soil, manure or any other growing medium.

Slow Release Fertiliser – Granules that release nutrients reduce the risk of over-fertilisation and In feel are perfect for lazy gardeners.

How to Make Your Own Potting Compost

Here are three different mixtures that I’ve made before, one for seeds and cuttings, one for annual plants (that’s plants you expect to keep for one season only) and the last mix is for established pot plants:

Seed Potting Compost

This mix is light, airy and well-drained, and I’ve used it for seeds and cuttings that don’t need as many nutrients as larger established plants:

• 1 parts compost
• 3 parts coconut coir
• 1 parts perlite

Potting On Mix (Ideal for baskets, tubs and containers)

These plants, often found in baskets, tubs and boxes require more nutrients than seeds and cuttings but still prefer a well-drained, airy potting compost:

• 2 parts compost
• 1 part manure or worm castings
• 2 parts coconut coir
• 1 parts perlite
• small amount of slow-release fertiliser pellets

Potting Compost For Established Permanent Plants  

Established plants that I intend to keep in my pots for more than one year require more nutrients and less aeration and inert content. Heavy mixtures also mean the pot is less likely to blow over in the wind. I suggest compost and manure mixed with only a little perlite and coconut coir. I usually add slow-release fertiliser once or twice a year:

• 2 parts compost
• 2 parts manure or worm castings
• 1 part perlite
• 1 part grit/course sand
• small amount of slow-release fertiliser pellets

My Experience With Homemade Potting Compost

I’ve used mixes like this for years with great results. I’ve grown plenty of seeds in light, fluffy potting mixes and I’ve enjoyed growing hundreds of new plants via propagation.

Could I save money and just use regular multi-purpose compost? Sure, but for seeds and cuttings, I feel the success rate will be lower. Also, based on my experience, any over or underwatering can cause problems that I would otherwise get away with had I used a more airy, well-draining mix.

Feel free to adjust the mixes, these are just the three that I’ve found work the best.

Is creating your own potting compost worth the effort? Why not just buy a bag of potting compost from a garden centre?

If you’re only growing a handful of plants then a bag or two of potting compost from the local garden centre would be cheaper and easier. For those of you wanting to grow lots of plants, making your own is timesaving, cost-effective and also educational. 

Don’t forget; it’s far cheaper to grow from seeds and cuttings than to buy pre-grown established plants from garden centres.

Conclusion

Potting compost typically contains fewer nutrients than regular compost and often includes coir, perlite and/or vermiculite to help with drainage, moisture retention and aeration.

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