How to Mix Your Own Potting Soil

A parts recipe is a ratio, not a quantity

"Four parts base, two parts aeration, two parts compost" doesn't tell you how much to make. It tells you the mix is half base, a quarter aeration, a quarter compost — and that holds whether you're filling one pot or a hundred.

Fix any single ingredient and the rest follow. That's the whole point of writing recipes this way.

There's one rule: a part must be the same container every time, filled and levelled the same way. A heaped scoop of perlite against a struck scoop of compost is a different recipe from the one you wrote down, and it's the most common reason a mix that worked last year doesn't this year.

Buy in dry quarts

This is the trap, and it's exact rather than approximate:

	Cubic inches
US liquid quart	57.75
US dry quart	67.2006

The dry quart is about 16.4% larger, and it's the one bagged mix, compost and perlite are sold in. So is the cubic foot — 1 cu ft = 25.71 dry quarts.

Size a batch in liquid quarts and buy in dry ones and you over-buy by a sixth, which is harmless. Do it the other way round and you're short on the last pot, which is when you find out.

Everything this tool reports is in dry quarts, because that's what the bag says. Total mix in cubic feet and Total mix in litres are there for the other two labels you'll meet.

The pot volume is a ceiling

Mix per pot treats the pot as a straight-sided cylinder, and almost no pot is one.

Nursery pots taper, sometimes sharply, so the real volume is lower — which is what Taper allowance is for. Set it to 0 for a genuinely straight-sided container and raise it for a strongly tapered one. At the defaults, 20% takes a 12 × 10 inch pot from 16.83 down to 13.46 dry quarts.

If your pots carry a stated volume on the label, use that instead. It's a measurement of the actual pot and it beats any calculation from two dimensions.

Why it never comes out exactly right

Mix is compressible, and substantially so. The same batch loosely tipped and firmly pressed occupies noticeably different volumes — and it settles again after the first watering, which is why pots need topping up a week later.

So: make a little more than the arithmetic says, fill pots without ramming them, water them in, top up. Chasing the last few percent on something this variable isn't worth the effort.

Wet the ingredients first

Peat and coir both repel water when bone dry. Get that mix into a pot dry and it can sit refusing to absorb anything while water runs down the sides and straight out of the drainage holes.

Damp-but-not-soggy is the target, and it's far easier to hit in a mixing tub where you can work it than in a planted container.

Coir bricks need soaking and breaking up first, and they expand a great deal doing it — measure the parts after hydrating, not before.

The ratio is yours

The defaults are an ordinary starting point, not a recommendation.

A mix for seedlings, for cactus, for a shrub that will sit in the same pot for five years, and for a hanging basket that dries out daily are all correct and all different. Drainage, water retention and weight move with the ratio, and the right answer depends on the plant, the pot, the position and how often you actually water.

Base is your bulk and water-holding — peat, coir, composted bark. Aeration is what keeps it from turning to mud — perlite, grit, pumice. Compost is fertility. Amendment is whatever else you add, and it's zero by default because most mixes don't need one.

Adjust from a mix that has worked for you rather than from a number on any page. The printable has a table for logging what you tried and how it did, which is worth more after two seasons than any starting recipe.

When it's worth mixing your own

For a few pots, bagged mix is usually cheaper once you count your time.

Mixing your own starts to pay at volume, and it pays most when you need something the shops don't sell — a very free-draining cactus mix, or something heavier for a windy site where light pots blow over.

One difference worth knowing: commercial mixes usually carry a starter fertiliser charge that a home mix doesn't. An unamended home mix may need feeding sooner than you'd expect — a question about the plant rather than about the volumes here.