The first summer I grew tomatoes on the balcony I filled the pots with soil dug out of the garden. It looked perfectly good going in. Three weeks later the surface had set into a crust that water ran straight off, and the plants sat there sulking. The following season I mixed my own, and the difference showed up within a month.
How do you make your own potting mix? Combine three parts coconut coir, two parts finished compost and one part perlite, then add water until a squeezed handful holds its shape without dripping. That ratio is light enough for roots to breathe through, rich enough to feed a plant through a season, and open enough to drain after a heavy watering.
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Why Garden Soil Belongs in the Garden
Out in a bed, garden soil has worms, roots and rain working on it constantly, and all of that keeps it open. Shovel the same soil into a pot and none of that happens. Every watering packs it down a little more until the air pockets close up, and roots need air just as much as they need water.
There is a second problem. Garden soil arrives with company: weed seeds, fungus gnat eggs, and sometimes a disease that was already living out there. In an open bed that mostly balances out. In a five gallon (19 liter) pot on a balcony it has nowhere to go.
What a Good Potting Mix Has to Do
A container mix has three jobs, and every ingredient in the recipe below is there to cover one of them. It has to hold water so the pot does not dry out by lunchtime. It has to hold air so the roots can breathe between waterings. And it has to hold enough food to carry the plant through a season.
Get those three balanced and almost anything will grow in it. That is the whole idea, and it is why the recipe is short.
My Potting Mix Recipe
Three parts coconut coir, two parts finished compost, one part perlite. I measure by scoop, not by weight, so a “part” is whatever container is handy. One yogurt tub, one plastic pot, one bucket. As long as you use the same one throughout, the ratio comes out right.
Mix it dry first. Turn it over with your hands or a trowel until the color is even and you cannot pick out streaks of one ingredient. Then add water a little at a time, stirring as you go. You are aiming for the point where a squeezed handful holds its shape and nothing runs out between your fingers. Wet it in the bucket rather than in the pot, because dry coir sitting at the bottom of a container can stay dry for weeks.
The Three Ingredients, One at a Time
Coconut Coir
This is the bulk of the mix and the part that holds moisture. Coconut coir usually comes as a compressed brick that you soak in water and it expands to several times its size, which makes it easy to store. Peat moss does the same job and you can swap it in one for one. I use coir because it rewets easily once it has dried out, and peat famously does not.
Compost
Compost is the food. Coir and perlite have almost nothing in them nutritionally, so without compost you would be feeding with liquid fertilizer from day one. Use compost that is finished, meaning dark, crumbly and smelling like a forest floor. If you can still recognize what went into it, leave it another few weeks.
Perlite
Those white specks you see in every bag of commercial mix are perlite, volcanic glass puffed up with heat. It does not absorb much and it does not break down, so it simply props the mix open and keeps drainage channels there all season. Vermiculite is the alternative, and the two behave differently. Perlite drains, vermiculite holds water. In hot weather on a sunny balcony I want the drainage, so perlite it is.
Storing What You Do Not Use
A batch keeps for months if you store it slightly damp in a lidded tub or a bag with a few holes punched in it. Sealed airtight and wet, it goes sour and starts to smell. Bone dry, the coir shrinks away from the sides and takes ages to rewet. Somewhere in between is what you want, and a quick squeeze test before you use it tells you whether to add a splash of water.
I keep mine in the shade, because a black tub sitting in full sun cooks the compost and kills off the useful life in it.
Adjusting the Recipe
The three to two to one ratio is my everyday mix. A few situations call for a change.
Seed Starting
Seedlings do not need much feeding for the first couple of weeks, and lumpy compost gets in the way of tiny roots. For trays I use four parts coir to one part perlite and skip the compost, then start feeding once the true leaves appear. If you want to know which seeds are worth sowing at all, run a few through the paper towel method first.
Cacti and Succulents
These want the water gone quickly. I go two parts coir, one part compost and two parts perlite, and often add a handful of coarse sand on top of that. If the pot is still damp three days after watering, add more perlite next time.
Hungry Crops in Big Pots
Tomatoes, peppers and squash strip a container fast. For those I push the compost up to three parts, so the ratio becomes three coir, three compost, one perlite. Even then they get liquid feed once flowering starts. My guide to growing bell peppers in containers goes through that feeding routine in detail.
What This Costs Compared to Buying It
One compressed coir brick expands to roughly 2 gallons (8 liters) of usable material and costs a few dollars. A bag of perlite lasts me a whole season across a balcony full of pots. If you make your own compost, the most expensive ingredient in the recipe is free. Mixing a large batch works out at well under half the price of the same volume in bags, and you know exactly what went into it.
FAQ
What is the best ratio for homemade potting mix?
Three parts coconut coir, two parts finished compost and one part perlite. That balance holds moisture, drains after watering and feeds a plant through a season.
Can I use garden soil in a potting mix?
It is better left out. Garden soil compacts in a container until the air pockets close up, and it can bring in weed seeds, pests and disease.
Do I need to sterilize homemade potting mix?
Not if you use bagged coir, bagged perlite and fully finished compost. Those ingredients are clean to start with.
How long does homemade potting mix last in storage?
Several months, kept slightly damp in a container that can breathe and stored out of direct sun.
Can I reuse potting mix from last season?
Yes. Pull out the old roots, then refresh it with about one part fresh compost to every three parts of the old mix.
Is coconut coir better than peat moss?
They perform much the same in a mix. Coir rewets easily after drying out and is a renewable byproduct, which is why I use it.
