The mineral ingredients

The mineral ingredients share one virtue: perlite, pumice, coarse sand and calcined clay don't decompose, or do so only slowly, and blending them with organic ingredients can slow the loss of large pores as the organics break down (Bilderback et al., 2005). They share one limit too, which Lesson 4 covered: most of them hold almost no nutrients. What separates them is weight, water, and how they behave in the pot.

Fig. 10. The mineral ingredients side by side: perlite, pumice, vermiculite, LECA (expanded clay), calcined clay, coarse sand, horticultural charcoal and zeolite. These are illustrations, some from our glossary set and some newly generated, not photographs of our own stock.

Perlite

Perlite is a mined volcanic rock, expanded 4 to 20 times by heating into a light, sterile, porous aggregate with a pH of about 6.5 to 7.5; it's added mainly to increase air space (Owen & Lopez, 2015). The bigger the perlite particles, the lower the water holding and the more air (Evans, n.d.). It contributes no nutrients and has no exchange capacity to speak of (Pennisi & Thomas, 2023), and its lightness is also its quirk: it tends to float to the top of a substrate (Owen & Lopez, 2015). We wet it before mixing and wear a mask; the dust is fine and unpleasant.

A note on fluoride, since Module III raised it. Perlite was long blamed as a source of fluoride injury in sensitive plants. The NC State researchers who tested it concluded that fluoride toxicity didn't stem from the perlite sources they examined and suggested the label may be undeserved (Everett & Nelson, 1996); Clemson's fact sheet still advises growers of fluoride-sensitive Dracaena to avoid potting soils high in perlite (Russ & Pertuit, 1999). We lean toward the research, and we also wouldn't argue with someone who keeps perlite low under a sensitive plant.

Pumice

Pumice is a porous volcanic rock: lightweight, normally sterile, chemically inert, pH about 7.0 to 7.4, and unaffected by heat treatment (Owen & Lopez, 2015). Its particles hold a small amount of water, and because they're relatively large, pumice provides drainage and air-filled pore space (Evans, n.d.). In a study of pumice crushed for use as a concrete aggregate, larger pieces absorbed more water as a share of their weight than smaller ones (Lura et al., 2004); how much a horticultural grade holds depends on the grade and how it was wetted, so we don't put one number on it. It's the aggregate we reorder most. One caveat: adding a stable mineral doesn't freeze a mix in time. In a study of Douglas fir bark with 15% or 30% pumice, air space still fell over the growing period overall, pumice or not (Altland et al., 2011). Lesson 7 takes that up.

Vermiculite

Vermiculite is expanded mica. Its plate-like structure holds a lot of water and also holds cations, potassium, magnesium and calcium among them, so it adds both water and some exchange capacity (Robbins, 2018). Its weakness is fragility: expanded vermiculite shouldn't be pressed or compacted, especially when wet, or its structure is destroyed (Robbins, 2018). It belongs in seed-starting and fine mixes more than in chunky ones.

Expanded and calcined clay

Fired clays come in two forms you'll meet. LECA (lightweight expanded clay aggregate) is clay fired in rotary kilns into porous balls; its properties depend on the parent clay and the firing temperature (Owen & Lopez, 2015). Purdue's guide says LECA provides pore space, doesn't float to the top the way perlite tends to, and doesn't break down, so it could help stabilize a long-lived substrate, with cost, availability and weight limiting its use (Owen & Lopez, 2015); a 2025 review of soilless media notes it can float in a badly designed flood-and-drain system, and that growers often pre-soak it before use (Malík & Tlustoš, 2025). Purdue lists Turface and Profile among the fired-clay products (Owen & Lopez, 2015). Calcined clay is a related fired-clay product. In a study summarized by Bilderback and colleagues, arcillite, a calcined clay, added to pine bark raised container capacity and available water and lowered air space from 29.2% for bark alone to 20.8% at the highest rate (Bilderback et al., 2005). That's the thing to remember about any fine mineral: it holds water by adding small pores, and it costs you air to do it.

Sand and grit

Sand is the ingredient most often added for the wrong reason. Fine mortar sand fills the pores between bark particles and lowers air space; in an NC State trial, pine bark amended with either mortar or builders sand held the most water and had the least air of the mixes tested, because the sand's small particles made small pores (Bilderback et al., 2005; Jackson et al., 2010). UMass is blunt about it for home mixes: coarse sand gives anchorage and some air space as a cheap filler, but isn't desirable for most plants except cacti, or to weight a pot (Smith & Cox, 2015). If a recipe says "sand for drainage," read it as "sand for weight."

Charcoal and zeolite

Horticultural charcoal and biochar (charcoal made for soil and growing-media use) are hard to generalize about on purpose. Their physical and chemical properties vary substantially with the feedstock, the production temperature and the method (Zulfiqar et al., 2022); a Tennessee extension guide gives the pH of different biochars as anywhere from about 4.6 to 9.3, rising with production temperature (Basiri Jahromi et al., 2019), and Georgia's guide notes that biochars from different sources have different effects on a substrate's air space, container capacity and bulk density (Yu & Gu, 2024). Orchid mixes use large bark particles alone or with sphagnum, perlite or charcoal for drainage and aeration (Owen & Lopez, 2015). Zeolite you met in Lesson 4: a clay-type mineral some growers add at 2 to 15% for nutrient retention (Robbins & Evans, 2005), with a strong hold on potassium and ammonium (Mumpton, 1999).

Where the Guild stands

Our recipes lean on pumice and perlite, with charcoal counted in the mineral fraction, because the aggregates keep their structure as the organics around them break down. That's a choice. Air and drainage are what the evidence supports; a chunky, mineral-heavy mix is one way to get them, not the only way and not a scientific requirement, and it has a cost: a highly porous substrate needs more frequent watering and holds applied nutrients poorly (Fields & Owen, 2021). If your plants are thriving in a peat-based bagged mix, this lesson isn't telling you to change. It's telling you what you'd be trading if you did.