Your Mix Isn't My Mix — And That's Probably Fine

August 24, 2026

Your Mix Isn't My Mix — And That's Probably Fine

A Teaching Scroll

Somewhere, at this very moment, a plant grower is standing over seven open bags of substrate.

Bark.

Perlite.

Pumice.

Coir.

Maybe some sphagnum.

Something volcanic-looking that came in a very expensive little bag.

And, for reasons known only to the grower, a measuring cup.

Batch No. 47 is about to begin.

Welcome, Mad Scientist.

Plant people can become remarkably passionate about what goes inside a pot.

Ask ten growers for their preferred growing medium and you may receive twelve recipes.

One swears by a chunky aroid mix. Another grows beautifully in sphagnum. Someone else prefers LECA. Another likes mineral media. Another has been adjusting the same homemade mixture for six years and will probably defend Version 14.3 with their life.

And you know what?

Several of them may be growing perfectly healthy plants.

That made us wonder whether we're sometimes asking the wrong question.

Instead of:

“What is the best growing medium?”

Perhaps we should ask:

“What does a growing medium actually need to do?”

Because we haven't found a scientific basis for one universally best recipe for every plant, every container, every environment and every grower. Research on container media instead keeps bringing us back to properties — air space, water-holding capacity, particle size, container dimensions and irrigation management — and the useful ranges can change with the crop and the way it is being grown.

And frankly, that would be a rather suspiciously convenient answer anyway.

The roots don't read ingredient lists

A container medium has several important jobs.

It needs to provide roots with access to water and oxygen, allow roots room to grow and physically support the plant. Depending on the material and the way the plant is fed, the medium can also influence how water and dissolved nutrients are retained and move through the root zone.

And much of that happens in spaces we don't normally see.

The pores.

Between and within the particles of a growing medium are spaces that can contain water or air.

When we water, many of those spaces fill with water and air is displaced. As excess water drains and the plant uses water, air returns to some of those spaces.

Roots need both.

Too little available water and the plant dries.

If too much of the pore space remains water-filled for too long, gas exchange slows and roots can become oxygen-stressed. So “holds lots of water” and “drains fast” are not automatically good or bad labels by themselves. The useful balance depends on the whole system.

So already our question has moved beyond:

“What's in your mix?”

toward:

“What are the roots experiencing inside it?”

And that's a much more interesting question.

Same ingredients. Different behaviour.

Here's where our Mad Scientist may want to put down the measuring cup for a moment.

The ingredients matter.

But they aren't the whole story.

Particle size matters.

Coarser and finer versions of the same material can behave differently because they create different pore structures.

How tightly the medium is packed matters.

In one Iowa State Extension demonstration using the same potting medium and the same size container, simply filling the pot and brushing it level left about 15% air-filled pore space. Tapping the pot twice reduced that to about 9%. Pressing the medium down and topping the pot off reduced it to about 4%.

Same medium.

Same ingredients.

Different root environment.

Apparently even potting soil objects to being squished.

The container matters too.

Put the same medium into containers of different heights and its water-and-air balance can change. After watering and drainage, the lower part of a container generally remains wetter because capillary forces hold water in smaller pores against gravity. With the same medium, that wetter or near-saturated zone can occupy a much larger proportion of a short pot than of a tall one. So container height is part of the root environment too.

Then there is the grower.

Ah yes.

The variable nobody puts on the substrate label.

Your watering can is part of the system

Imagine two growers using exactly the same medium.

One waters frequently.

The other waits considerably longer between waterings.

One grows in a warm environment with strong airflow.

The other grows somewhere cooler with slower drying.

One uses small containers.

Another uses larger ones.

Is it reasonable to expect the same medium to behave identically for both?

Probably not.

A medium that retains relatively little water may be perfectly manageable for someone who waters frequently.

Give that same medium to someone who travels constantly and occasionally remembers the plants exist on Sunday evening...

We may have a different outcome.

Conversely, a water-retentive medium that performs beautifully in one environment may remain wetter for longer than desired somewhere else.

This is why “fast draining” by itself doesn't tell us everything.

Nor does “water retentive.”

The useful question is how those properties fit into the entire cultivation system.

There are many ways to build a root environment

Traditional and modern container media use an enormous range of materials.

Peat. Coir. Bark. Perlite. Vermiculite. Pumice. Sphagnum. Mineral aggregates. Expanded clay and many combinations in between.

Different materials contribute different physical and chemical characteristics.

And growers combine them because they're trying—whether they describe it this way or not—to influence things such as:

water retention
aeration
drainage
pore structure
physical stability
nutrient availability and retention
pH

There is no requirement that every successful grower arrive at the same recipe.

Which means we're not particularly interested in starting the Great Substrate War of 2026.

If your plant is thriving, we're certainly not coming to your house to confiscate its potting mix.

So what do we use?

In the Grove, we currently use Akadama and Fluval Stratum as our primary growing medium.

That's not a recommendation that everyone should immediately dump their existing substrate into the compost bin and copy us.

It's simply the medium we've chosen to work with inside our cultivation system. And our reason for choosing it is actually rather ordinary: simplicity.

We didn't want to stand over seven or ten different substrate components every time we needed to make another batch, trying to remember whether this scoop was number three or number four. The more ingredients we add, the more measuring and mixing there is to repeat. Akadama and Fluval Stratum give us a two-component medium that is straightforward for us to prepare consistently.

There is another reason that has nothing to do with plant physiology at all: we like the way it looks. The visual texture and appearance are pleasing to us. That's a preference, not scientific evidence of superior performance — and we think it's perfectly fine to say so. A grower is allowed to enjoy looking at the medium too.

So our choice wasn't based on discovering a magical two-ingredient formula. It was based partly on how the medium appears to function within our system, partly on ease and repeatability, and partly on aesthetics.

And our two ingredients didn't exactly arrive yesterday. Akadama comes to us from the world of Japanese bonsai, where it is a well-established granular growing material used alone or as part of bonsai mixtures. Bonsai growers value properties such as its granular structure, water retention and porosity, although — appropriately for this Scroll — even experienced bonsai growers do not universally agree that Akadama belongs in every tree, climate or growing system.

Fluval Stratum comes from a very different plant-growing world: the planted aquarium. Its manufacturer describes it as a lightweight, porous, non-compacting volcanic-soil substrate intended to let aquatic plant roots penetrate and spread through the granules. Aquarists have therefore been growing rooted plants in Stratum in an environment very different from a houseplant pot.

We find that meeting of two established growing traditions rather interesting: one material familiar to bonsai culture, another designed for planted aquariums, meeting in one of our pots.

But here is the important limit to our claim: a history of successful use in bonsai does not prove Akadama is ideal for Alocasia, and successful use of Fluval Stratum in planted aquariums does not prove it is ideal for terrestrial houseplants. Nor have we found controlled research showing that our particular Akadama + Fluval Stratum combination is superior for Alocasia, Scindapsus or houseplants generally. We use the combination because it has worked practically for us — not because either tradition, or the scientific literature, has crowned our particular recipe the winner.

Our watering practices, nutrition, containers, temperatures, humidity, airflow and other environmental conditions all interact with that choice.

Change one part of the system and the behaviour of another part may matter differently.

And our choice isn't sacred either.

If observation gives us a reason to change it someday, we'll change it.

Science and cultivation don't become more useful because we refuse to reconsider them.

The recipe isn't the answer

Perhaps this is the part we find most interesting.

Two growers can use completely different ingredients and still create root environments that function well for their plants.

And two growers can use the exact same recipe and get very different results.

So when someone asks:

“What's your mix?”

Maybe the ingredient list should be the beginning of the conversation rather than the end.

What size are the particles?

How much water does it retain?

How much air remains after watering?

How quickly does it dry in that environment?

What container is it in?

How frequently is it watered?

How is nutrition supplied?

How does the structure change as the medium ages?

And most importantly:

What are the roots telling us?

Healthy roots are useful practical evidence that something in the system is working. They are not, by themselves, proof that one ingredient or one recipe caused the result — and certainly not proof that the same recipe will be best somewhere else.