The Supply Shelf

The aggregate we go through fastest, and part of every recipe we publish. Pumice is light and porous and holds only a small amount of water (Evans, 2014). It's generally considered physically stable in a mix (Schabauer et al., 2026). But in a trial of bark-based mixes with and without pumice, air space still fell over time, and tended to fall more with a plant's roots in the mix (Altland et al., 2011). Our recipes use the 1/4-inch grade, plus 1/2-inch in the Aroid Mineral Mix for 4-inch pots and up. We also keep the 1/8-inch grade, with the fines screened out, for our establishment mixes and several of our finer mixes; those recipes aren't on the site yet.

Perlite is added to a mix to open up large pores, cutting how much water it holds and raising its air space (Evans, 2014). Our recipes use coarse (#3) and fine grades. It's dusty, so we dampen it before mixing. We don't link a product for it yet.

Clinoptilolite zeolite is the nutrient-holding mineral in our Aroid Mineral Mix and ICU Mix; those pages explain what it does. We use 1 to 3 mm and 3 to 5 mm grades and buy ours where aquarium supplies are sold. Check the size before you buy: in our experience, much of the zeolite sold online is finer than this. We don't link a product for it yet.

Our stand-in for bark in the Aroid Mineral Mix. Look for horticultural charcoal in chunks, 1/8 to 1/4 inch or 1/4 to 1/2 inch, not a fine biochar and not barbecue briquettes. We don't link a product for it yet.

Coir typically holds 73 to 80% of its volume in water (Evans, 2014), which makes it the water-holding part of our mixes. Buy calcium-buffered coir, medium pith: untreated coir tested high in potassium and sodium and low in calcium (Gbollie et al., 2021). We don't link a product for it yet.

In a lettuce-seedling study, untreated worm castings carried live bacteria, and heat treatment cut their numbers sharply (Tavali & Uz, 2025). Castings also add salts, and in the same study, seedling emergence fell as salinity rose. So we keep them to a small share of the mix.

Calcined clay holds more water than pumice: a moderate amount, where pumice holds a small amount (Evans, 2014). How much of that water a plant can use depends on the product and its particle size (Koski, 1992).

Expanded clay pebbles. We use them for semi-hydro setups (plants growing in pebbles over a water reservoir instead of a mix). In a lab comparison of nine mineral media, it held the least water plants can use (Schabauer et al., 2026). Flush it regularly, since salts can build up in it (White, n.d.). We use standard pebbles or a mini grade.

Coir-based mixes need more calcium than peat-based ones (Owen & Lopez, n.d.). Water, limestone and fertilizers that contain calcium and magnesium are the usual sources (Cox et al., n.d.). Water with less than 40 mg/L of calcium typically needs calcium added. Too much calcium, though, can lead to a magnesium shortage (Penn State Extension, n.d.). Check your water report and fertilizer label before adding a Cal-Mag supplement.

What we use to spot fungus gnats and thrips early. Yellow sticky cards trap the adults of both (Pundt, 2024), so they tell you a problem is starting, not how to solve it. For fungus gnats, lay a card face up on the mix or clip it to a stake (UNH Cooperative Extension, 2018). The fungus-gnat protocol shows where they fit.

A bacterial larvicide for fungus gnat larvae in the substrate. BTI (Bacillus thuringiensis israelensis) affects the larvae of a few related flies, mosquitoes and black flies among them (U.S. Environmental Protection Agency, 2026), not insects in general. It tends to work better on young larvae than older ones (Cloyd, 2010). Extension guidance says applying it every three to seven days can provide some control (UNH Cooperative Extension, 2018). Drench per the fungus-gnat protocol.

Dust on leaves can block light, and clean leaves photosynthesize more efficiently (Trott, 2026). These wipes are sold as a cleaner and shine. Extension guidance advises against leaf-shine products, which in large amounts and with frequent use can plug leaf pores and collect dust, and favors a damp cloth or a gentle shower instead (Iowa State University Extension and Outreach, 2024). Fuzzy leaves should be dusted with a soft brush rather than a wet cloth (Clemson Cooperative Extension, n.d.).