Making your own potting compost: the rule of thirds and our recipes

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Why buy bags of potting compost when you have a composter at the bottom of the garden? With well-matured compost, a little soil and a little sand, you can make your own mixes for sowing, potting on and planters. It saves money, it avoids buying peat-based products, and you know exactly what goes into your pots.

You do, however, need to follow two rules: use compost that is truly mature, and never use it on its own. Here is how to do it, with simple recipes for each use.

Compost and potting mix: what's the difference?

The two are often confused, wrongly. Compost is a soil improver: a rich, concentrated material that feeds the soil and improves its structure. Potting mix is a growing medium: a balanced blend in which a plant can take root and find water, air and moderate nutrition. A good homemade potting mix therefore contains compost, but it isn't just compost.

On this point the rule is clear: you should never sow or plant directly into compost. A few very hungry plants, such as tomatoes or pumpkins, cope with it; most cannot tolerate it. Too rich and still active, pure compost can hinder the germination and rooting of young plants.

Choosing the right compost: well matured, even « old »

Not all composts are equal when it comes to potting mix. For use as a growing medium, we recommend compost that is at least 18 months old. That is considerably longer than for simply adding to the vegetable patch: by this stage the compost is stable and its decomposition is well and truly complete.

If your compost is younger, keep it for the garden and set part of your harvest aside, covered, for the following season. Before using it, sieve it: fine compost is easier to work with in pots and mixes better. We explain how in our guide Harvesting and sieving your compost.

The rule of thirds

This is the basic recipe, and the one most commonly recommended: one third compost, one third soil, one third sand, measured by volume.

• Compost provides organic matter and nutrients;
• garden soil, taken from a healthy area and cleared of stones and roots, gives the mix body and holds on to nutrients;
• sand lightens the whole mix and ensures drainage, so that water doesn't sit at the bottom of the pot.

In practice, use a bucket as your unit of measurement: one bucket of sieved compost, one bucket of soil, one bucket of sand, and mix thoroughly on a tarpaulin or in a wheelbarrow. This base suits most uses, and you can adjust it to your plants.

Willy's tip

If your garden soil is already light and sandy, there's no need to add sand: a mix of two thirds soil to one third compost works very well. Conversely, if your soil is heavy and sticky, keep the third of sand, or even add a little more.

Our recipes for each use

The proportions below draw on the recommendations of several reference guides. We have kept the ranges on which they agree.

For sowing

Seeds don't need nutrients to germinate: they have their own reserves. A light, low-nutrient mix suits them better. Allow roughly one quarter well-matured, finely sieved compost to three quarters good fine soil, lightened with sand if needed. Well-rotted leaf mould is also an excellent sowing medium (see below).

For potting on and houseplants

The rule of thirds works well. For plants that like a lighter soil, you can go down to one quarter compost to three quarters soil.

For setting up a planter

For a new planter of flowers or vegetables, the rule of thirds remains a good base. The guides we consulted allow 30 to 40% compost in a new planter: don't go beyond that.

For refreshing a planter

There's no need to throw everything away each year. Remove the old roots, loosen the old soil and add around 20% compost, without exceeding this proportion. Your planter will be ready for another season.

If you also have a wormery, bear in mind that vermicompost is much more concentrated than garden compost: it is dosed differently, as we explain in our guide Harvesting vermicompost and worm tea.

Leaf mould, the ally of seedlings

Fallen leaves, composted separately, produce something different from compost: leaf mould. Lower in nutrients, very light and very fine, it makes an ideal medium for sowing. Most leaves take around two years to break down, and leaf mould more than two years old can be used as seed compost. Younger, it makes a good mulch. Oak, beech and hornbeam leaves give the best results. We describe the method in detail in our guide Leaf mould: what to do with fallen leaves?

Checking your mix with the cress test

Before starting all your spring sowings, do a trial. Fill one or two small pots with your homemade potting mix, sow a few cress seeds, water and keep somewhere warm. If they germinate well and evenly within about a week, your mix is ready. If germination is poor or patchy, your compost is probably not mature enough, or there is too much of it in the mix: add soil and repeat the test. You'll find a more precise version of this test in our guide How can you tell if your compost is mature?

Did you know?

Garden compost rarely heats up enough to destroy all weed seeds. For delicate sowings, you can put slightly moist compost in the oven for 30 minutes at around 80 °C. This is optional, and best kept for small quantities intended for sowing.

Storing your homemade potting mix

Store your homemade potting mix out of the rain, in a bin or in bags that aren't airtight, so that it stays slightly moist without rotting. To move compost, soil and sand to your potting area, our 55-litre compost collection cart saves many trips back and forth. And if you garden in town, on a balcony or terrace, our book « Composter en ville » will help you go further.

Key points

Never use pure compost in a pot. For homemade potting mix, use well-matured compost, ideally 18 months old or more, sieved, and follow the rule of thirds: compost, soil, sand. For sowing, one quarter compost is enough.

In summary

Making your own potting mix is simple: well-matured, sieved compost, garden soil and sand, mixed in equal parts according to the rule of thirds. Use less compost for sowing, a little more for hungry planters, and limit it to around 20% when refreshing an old planter. Leaf mould over two years old is the ideal addition to your sowing kit. Do a quick cress test before you start, and both your seedlings and your potted plants will make the most of your compost.

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