What Makes a Good Potting Mix? The Two Numbers That Matter

The short answer: two measurements tell you almost everything about how a potting mix will behave in your pot air-filled porosity (how much air the mix holds right after watering) and water-holding capacity (how much moisture it retains for the plant). Get the balance right and roots breathe and drink at the same time. Get it wrong and the plant either drowns or dries out. Almost no brand publishes these numbers. The ones that don't, usually haven't measured them.

Everyone argues about ingredients bark vs coir, perlite vs pumice. Ingredients are just the means. These two numbers are the result, and the result is what your roots actually experience.

What is air-filled porosity (AFP)?

Air-filled porosity is the percentage of a mix's volume that remains filled with air immediately after it's been watered and drained. It's the single best predictor of whether roots can breathe.

Roots respire they take in oxygen, just like you do. When you water, water temporarily pushes air out of the pore spaces; as it drains, air returns. AFP measures how much air comes back. A mix with high AFP refills with air fast and keeps the root zone oxygenated. A mix with low AFP stays waterlogged, the air never returns, and roots suffocate, then rot.

Ranges to look for:

• Below 10%: dangerously low. Typical of dense, cocopeat- or compost-heavy mixes. High root-rot risk, especially indoors. (See why 100% cocopeat fails.)
• 10–18%: workable, but you have to water carefully.
• 20% and above: what air-loving plants like aroids want.

The plants you're growing change the target. A monstera wants a lot of air; a moisture-loving fern is happy with less. There's no single "good" number there's a right number for the plant.

What is water-holding capacity (WHC)?

Water-holding capacity is how much water a mix can retain for the plant after the excess has drained away. It's usually measured as grams of water held per kilogram of substrate (g/kg). It's the other half of the balance: it determines how long the plant has access to moisture between waterings.

Too low, and you're watering constantly and the plant still dries out. Too high, and the mix stays wet so long the roots are at risk high WHC with low AFP is the classic recipe for a drowned plant. The art is holding meaningful water and meaningful air at the same time.

Why is the balance the whole game?

Because air and water compete for the same space in a pot. A pore is either holding water or holding air at any given moment. The reason a good substrate is hard to make and worth paying for — is that it has to do two opposing things at once: hold enough water that you're not watering daily, while keeping enough air that roots never suffocate.

Cheap mixes solve this by ignoring half of it. Pure cocopeat has huge WHC and almost no AFP — it drowns plants. Pure perlite has huge AFP and almost no WHC it dries out instantly. The skill is in the structure: coarse particles that create permanent air channels, combined with a controlled fraction that holds moisture inside the particles while the spaces between them stay open. That's an engineering problem, and the only way to know you've solved it is to measure.

How do you actually find out a mix's numbers?

You ask. And here's the uncomfortable truth for the Indian market: almost no one can tell you. Brands sell "premium," "well-draining," "airy" mixes by the bagful without a single measured figure behind the adjectives. "Well-draining" is a vibe; 24% air-filled porosity is a fact.

We publish ours because we test ours. Every Terrene House substrate is independently analysed at the Department of Soil Science & Agricultural Chemistry, University of Agricultural Sciences (GKVK), Bengaluru, using the standard FAO method air-filled porosity, water-holding capacity, pH and more, for each of the eight mixes. Our air-filled porosity runs from around 19% to 24% across the range, and water-holding capacity from roughly 1,380 to 1,985 g/kg each mix tuned to a different plant family rather than a one-size-fits-all average. (You can see the full lab table on our Science page.)

That's not a flex for its own sake. It's the difference between buying a result and buying a hope.

What should you do with this?

When you're choosing a mix ours or anyone's:

1. Match the plant, not a generic "good." Air-lovers (aroids, epiphytes) want high AFP. Moisture-lovers (ferns, calatheas) tolerate lower AFP and higher WHC. Drought plants (succulents, snake plants) want fast-draining, lower-WHC mixes.
2. Ask for the numbers. If a brand can't give you AFP and WHC, they're selling adjectives.
3. Watch the behaviour in the pot. Water it: does it drain freely within seconds and feel airy, or does it sit wet for days? The pot tells you what the label won't.

Ingredients are the recipe. These two numbers are the meal. Judge the meal.

FAQ

What is a good air-filled porosity for potting mix? It depends on the plant. Air-loving plants like aroids want 20% or more; moisture-lovers are fine with 10–18%. Below 10% is risky for most indoor plants. There's no single ideal match it to the plant.

What is water-holding capacity in soil? The amount of water a mix retains for the plant after excess has drained, often measured in grams per kilogram. It sets how long moisture is available between waterings.

Why is air-filled porosity important? Because roots need oxygen. AFP measures how much air a mix holds after watering low AFP means waterlogged, suffocating roots and root rot; adequate AFP keeps roots breathing.

Can a potting mix have too much drainage? Yes. A mix that's all air and no water-holding (like pure perlite) dries out almost immediately and stresses the plant. The goal is balance, not maximum drainage.

Why don't most potting mixes list these numbers? Because measuring them requires lab testing most brands never do. Marketing words like "premium" and "well-draining" are cheaper than data. Brands that publish AFP and WHC are signalling they've actually measured their product.

How do I test my own potting mix at home? You can't measure AFP precisely at home, but you can observe behaviour: water the pot and watch how fast it drains and how long it stays wet. Fast drainage and an airy feel suggest decent porosity; standing water and long-lasting sogginess suggest the opposite.