What is in the bag
- Volcanic Pumice 3–6mmPrimary mineral skeleton; porous, light and wicking30%
- Calcined Moler Clay 2–5mmFired diatomaceous clay granules; hold water inside, stay rigid15%
- Washed Horticultural Grit 3–6mmLime-free quartz grit for weight and fast drainage10%
- Coarse Perlite 3–6mmLight permanent air space10%
- Zeolite 2–4mmHigh CEC; holds ammonium and potassium in a mineral matrix5%
- Buffered Coco Coir <2mm screenedSmall organic fraction for water film and root contact20%
- Organic Worm Castings <2mmSlow nutrition and microbial activity10%
| Physical metric | Value | What it means |
|---|---|---|
| Air-filled porosity | 40% | Share of the pot volume holding air after a full watering has drained |
| Water-holding capacity | 27% | Share of the pot volume holding water at container capacity |
| pH range | 6.0–6.8 | Measured in a 1:1.5 water extract after blending |
| Particle grading | 0.5–6mm | Screened range of the structural fraction |
| Starting EC | 0.3–0.6 mS/cm | Low enough for tissue-culture and freshly cleaned roots |
Who it is for: root morphology
Desert cacti and most succulents have shallow, widely spreading root systems designed to grab water from brief, heavy rain. Within hours of watering, many species produce fine, short-lived "rain roots" that absorb moisture rapidly and then die back as the soil dries. Below them sits a more permanent framework of thicker roots, and in many species a taproot or swollen water-storing base.
Caudiciforms such as Adenium, Pachypodium and Dioscorea elephantipes take this further, storing water in a swollen stem or root, the caudex, which can rot from the base if it sits in damp substrate. All these structures share one weakness: they are built for drought and almost none of them have any defence against prolonged wetness, particularly when cold.
Plants this blend suits: Echeveria, Haworthia, Gasteria, Aloe vera, Aloe aristata, Crassula ovata, Mammillaria, Gymnocalycium, Astrophytum, Echinocactus grusonii, Opuntia, Euphorbia obesa, Euphorbia trigona, Adenium obesum, Pachypodium lamerei, Dioscorea elephantipes, Beaucarnea recurvata, Dracaena trifasciata, Senecio rowleyanus, Kalanchoe.
Why this formulation works
A desert mix should behave like a gravel bank after a storm: water passes through in seconds, the particles hold a thin film for a few days, and then everything dries. Our blend is 70% mineral by volume, pumice, calcined moler clay, washed grit, perlite and zeolite, with 30% screened coir and worm castings. That ratio gives an AFP of 40% and a WHC of just 27%, so a 12cm pot on a sunny windowsill dries out in four to seven days in summer. Standard "cactus compost" sold in garden centres is often a multipurpose compost with a little sand added, and sand, which is fine, actually reduces air space by filling the gaps between larger particles.
Each mineral does a slightly different job. Pumice is light and porous, holding water inside its vesicles and wicking it gently through the pot. Calcined moler clay, a fired diatomaceous earth from Denmark, holds water inside granules that stay rigid indefinitely. Washed, lime-free quartz grit adds weight, which matters for top-heavy columnar cacti and tall Euphorbia, and perlite adds air. Zeolite contributes something the others lack: a high cation exchange capacity. Most mineral components hold almost no nutrients, so a purely mineral mix leaches fertiliser straight through, but zeolite holds ammonium and potassium and releases them slowly, smoothing out feeding in a substrate that is otherwise very lean.
The 30% organic fraction is small but important. Screened coir gives fine roots a moist contact surface after watering and makes the mix easier to rewet; a 100% mineral substrate can leave rain roots without enough water film to work with. Worm castings at 10% supply a slow trickle of nutrients and a microbial population. We keep the pH at 6.0–6.8, slightly higher than our aroid mixes, which suits most cacti and succulents, and because the mix contains no limestone, it will not drift alkaline the way chalk-based grits can.
The British problem with cacti is not summer, it is winter. Most desert species need a cool, bright, dry rest from November to February, ideally at 5–12°C, which encourages flowering the following spring. In a warm, dark room the plant stays semi-active, stretches towards the light and is very vulnerable to rot if watered. In this mix, stop watering almost entirely from November, give a light drink only if the plant starts to shrivel, and resume in March when daylength returns. Fungus gnats and root mealybug find little to work with in a substrate this lean and fast-drying, which is a welcome side effect.
When and how to repot
- Repot cacti and succulents in spring, just before the growing season, and only when roots fill the pot or the plant has become top-heavy.
- Make sure both plant and mix are dry. Unlike tropical plants, cacti are repotted dry so that damaged roots can callus before they meet moisture.
- Wrap spiny plants in folded newspaper or use thick gloves and kitchen tongs to lift them out of the pot.
- Shake off the old substrate and inspect the roots. Cut away any that are black, soft or hollow, and check for the white, woolly patches of root mealybug.
- Leave the plant bare-root in a shaded, airy spot for two to seven days so any cut or damaged roots callus over.
- Choose a pot only slightly larger than the root system, with a drainage hole. Terracotta suits this mix well. Set the plant at its previous depth, keeping the base of the stem or caudex above the surface.
- Fill around the roots with dry mix, tapping the pot to settle it, and top-dress with grit if you like.
- Wait seven to ten days before the first watering, then water thoroughly and let the mix dry completely before watering again.
Open the repotting calculator pre-set for Cactus & Succulent Grit Mix to work out litres, bag sizes and pot sizes for your whole collection.
