Making Potting Mix with Homemade Compost

There is something deeply satisfying about scooping a trowel of dark, crumbly compost from your own bin and knowing it started life as last week’s vegetable peelings and a pile of soggy cardboard. But once you have that beautiful finished compost in your hands, what exactly do you do with it? One of the most rewarding uses – and one that saves you a surprising amount of money – is making your own potting mix for containers, seed trays, and raised beds.

The trouble is, homemade compost on its own is not quite the same as the bags of multi-purpose compost you pick up from a garden centre. Used straight from the bin, it can be too rich, too dense, or too variable in texture to give your plants the best start. The good news is that blending it correctly is straightforward, cheap, and puts you in control of exactly what goes into your plants’ growing medium. This guide walks you through everything you need to know to make a genuinely useful potting mix using your homemade compost as the star ingredient.

Why Homemade Compost Is Not a Straight Swap for Bought Compost

Commercial potting composts sold in the UK – brands like Levington, Westland, or New Horizon – are carefully blended, pH-adjusted, and often contain added fertilisers and wetting agents. They go through quality control processes that ensure consistency. Your homemade heap, wonderful as it is, does not offer that same reliability. The finished product from a home bin can vary batch to batch depending on what went in, how hot the heap got, and how long it was left to mature.

Fresh or partially finished compost can actually harm young plants. It may contain high levels of ammonia or soluble nitrogen that scorch delicate roots, and it can carry weed seeds or pathogens if it never reached a sufficiently high temperature during decomposition. Dense, unstructured compost also compacts in pots, which restricts drainage and air movement around roots – two things that will quickly make plants unhappy.

None of this means your compost is bad. It simply means it works best as one component in a blend rather than the whole show. Think of it like cooking: a great stock enriches a dish without being the only thing in the bowl.

Getting Your Compost Ready to Use

Before you mix anything, your compost needs to be properly mature. Finished compost should smell earthy and pleasant – like a woodland floor after rain – not sour, ammonia-rich, or musty. It should be dark brown to black, crumbly rather than slimy, and you should not be able to identify the original materials any longer. If you can still see carrot tops or newspaper print, it needs more time.

Once you are confident it is ready, sieve it. A simple riddle sieve – the sort you can buy from any decent garden centre or hardware shop for a few pounds – makes this quick work. Use a mesh of around 10mm for general potting mixes, or 6mm if you are making a mix for sowing fine seeds. The lumpy bits that do not pass through the sieve can go back on the heap for another cycle. What passes through is what you will be working with.

Sieving also aerates the compost slightly and removes stones, woody fragments, and any odd bits that crept in. It is worth taking ten minutes to do this properly – the difference in the finished potting mix is noticeable.

The Key Ingredients to Mix With Your Compost

A good potting mix is a balance of three things: nutrition, structure, and drainage. Your compost provides the nutrition and some structure. The other ingredients bring drainage and aeration, and help balance the overall texture.

Horticultural grit or sharp sand – Not builder’s sand, which is too fine and can actually make compost more compacted. Horticultural grit is coarser and keeps the mix open and free-draining. You can find it at most garden centres or order it in larger quantities from suppliers like Gartmore or Horticulture Direct. A 25kg bag is usually enough for several batches of mix.

Perlite – This is a lightweight volcanic mineral that looks like tiny white polystyrene balls. It is excellent for improving drainage and aeration, particularly in mixes intended for containers or houseplants. It is peat-free and widely available in the UK. Companies like Fertile Fibre or Carbon Gold sell good quality perlite online.

Coir (coconut fibre) – Coir is the fibrous material from coconut husks, often sold in compressed bricks that you rehydrate with water. It has a lovely open texture and holds moisture without becoming waterlogged. It is a sustainable, peat-free alternative to traditional compost extenders and has become much easier to find in UK garden centres over the last few years as the push away from peat gathers pace.

Loam or good topsoil – For heavier mixes suitable for larger containers, shrubs, or trees, adding some loam or screened topsoil gives weight and stability. John Innes-style mixes traditionally include sterilised loam for exactly this reason. If you have decent garden soil, you can use a small proportion of that, though it is worth baking it in an oven at around 180°C for 30 minutes to sterilise it first if you are using it for seed sowing.

Recipes for Different Uses

There is no single perfect potting mix recipe. What works brilliantly for tomatoes in a grow bag will not suit a tray of lettuce seedlings. The following proportions are guidelines rather than rigid rules – adjust based on what plants you are growing and what materials you have available.

General Purpose Potting Mix

This is your everyday mix for containers, hanging baskets, and established plants being potted on.

• 40% sieved homemade compost
• 30% coir
• 20% horticultural grit or perlite
• 10% good topsoil or loam

Seed Sowing Mix

Seeds need a finer, lower-nutrient mix. Too much fertility at the germination stage can actually inhibit sprouting and cause damping off – a fungal condition that kills seedlings at soil level.

• 25% well-sieved homemade compost (through a 6mm sieve)
• 50% coir
• 25% fine perlite

Vegetable Container Mix

Vegetables in pots are hungry feeders and need a richer blend, but still require good drainage to prevent root rot.

• 50% sieved homemade compost
• 25% coir
• 15% perlite
• 10% horticultural grit

How to Mix Your Potting Compost: Step by Step

1. Lay out a tarpaulin or use a large trug – Mixing on a flat surface makes the job easier and means less mess. A large builders’ trug (the rectangular flexible buckets from any DIY shop) is ideal for small batches. For bigger quantities, a tarpaulin on the patio works well.
2. Measure your ingredients by volume – Use a bucket as your measuring unit. If your recipe calls for 40% compost and 30% coir, that is roughly 4 buckets of compost to 3 buckets of coir, and so on. Precision is not critical, but consistency helps if you are trying to replicate a mix that worked well.
3. Add your dry ingredients first – Grit and perlite go in before the moisture-holding ingredients. This makes blending easier and prevents clumping.
4. Add coir and sieved compost – Tip these in and begin turning the mix with a trowel or small spade. Work from the outside in.
5. Check the moisture level – A good potting mix should feel slightly damp when you squeeze a handful, then crumble apart when you open your fist. If it is too dry, mist lightly with water and mix again. If it is too wet, add a little more grit or perlite and leave it to air for an hour before reassessing.
6. Smell and texture check – Before potting anything up, take a moment to assess the finished mix. It should smell fresh and earthy. If it smells off in any way, your base compost may not have been fully matured.
7. Use it or store it – Fresh-mixed potting compost is best used within a few weeks. If you need to store it, keep it in a covered container or sealed bag out of direct sunlight. Avoid leaving it exposed to heavy rain, which will leach nutrients and affect the texture.

Comparing Homemade Mixes Against Bought Options

It is worth understanding how a homemade compost-based mix stacks up against what you can buy off the shelf. This helps you make informed choices about when to use your homemade version and when a commercial product might be worth reaching for.

The cost savings are real and meaningful, particularly if you are growing
on a larger scale or managing several containers throughout the season. A gardener producing their own compost and combining it with inexpensive coir and sharp sand can mix a serviceable all-purpose potting medium for a fraction of the shop-bought price. The trade-off is time and some initial trial and error to get proportions right, but once you have a reliable recipe it becomes straightforward to repeat.

It is worth noting that homemade mixes will vary batch to batch depending on the maturity and composition of your compost. A well-rotted, dark, crumbly compost with no recognisable material remaining will perform far more consistently than one that is only partially broken down. If your compost still contains visible woody fragments or smells earthy rather than sweet, pass it through a coarse sieve before mixing and set the rougher material aside for mulching or trench composting instead. Consistency in your raw materials translates directly into consistency in your finished mix.

Commercial products do have their place, particularly for seed sowing where a sterile, fine-textured medium reduces the risk of damping off, and for gardeners who do not yet have a mature compost supply to draw from. Many experienced growers use a hybrid approach, reserving bought seed compost for trays and modules while relying entirely on homemade mixes for potting on, container topping and border planting. There is no single correct method, and your own conditions, compost quality and plant range will shape the approach that works best for you.

Conclusion

Making your own potting mix from homemade compost is a practical, low-cost alternative to buying bags of branded growing media, and it keeps useful organic material circulating within your garden rather than ending up as waste. Start with a basic recipe, keep notes on what works, and adjust proportions as you gain experience with your own compost’s characteristics. Over a season or two, most gardeners find they can produce a reliable, versatile mix that suits the majority of their growing needs without compromising on results.