A cactus mix has to do 2 things: let water run straight through, and dry out again within days. How fast it dries depends on the mix, the pot and the weather together, so a pot on a hot, dry patio and a pot on a cool, humid windowsill need different mixes.
All of this applies to cacti in pots, including columnar cacti, the tall upright kinds such as San Pedro (a Trichocereus).
The short version
- Make the mix from 2 kinds of ingredient: organic (bagged cactus mix or potting mix) and mineral (pumice, perlite or coarse sand). Measure by volume.
- General mix, for a pot outdoors in a warm, dry climate: 1 part organic to 1 part mineral.
- Cool, humid or indoor mix: 1 part organic to 2 parts mineral.
- Test the mix in your own pot and spot. Water it, then count the days until it is dry at the bottom. About a week in warm weather is right.
- In cool weather every mix stays wet longer. Do not water again until the bottom of the pot is dry, however long that takes.
- A gravel layer in the bottom of the pot is not needed. Use the same mix from top to bottom, in a pot with a drainage hole.
- If you cannot decide between the 2 mixes, use the cool, humid or indoor one.
What the mix is for
Roots need air as well as water. In a mix that stays soaked, the air spaces stay full of water, and roots can suffocate and die. Also, some of the fungi that rot plants can take hold in standing water.
So a good mix is free-draining: water poured on top runs out of the drainage hole instead of pooling on the bottom. It holds some of that water for the roots, then lets air back in as it dries.
What makes a mix dry fast or slow
Particle size. Coarse pieces leave large gaps that drain and let air in. Fine material packs down and stays wet. The size of the particles matters as well as what they are made of.
Mineral or organic. Mineral ingredients are rock or sand: pumice, perlite, grit, coarse sand. Organic ingredients come from plants: peat, coir (coconut fiber), composted bark. Organic ingredients hold more water. They also break down with time, so replace an old mix when you repot.
The pot and the weather. The same mix dries faster in a small pot than a large one, and in an unglazed clay pot than a plastic one. It dries faster in heat, dry air, sun and wind, and slower in cool weather, humid air, shade or indoors.
Ingredients
For the mineral part, use one of these or a blend:
- Pumice. A light volcanic rock, sold in bags as a soil ingredient.
- Perlite. White volcanic rock that has been crushed and heated until it puffs up. It is light, full of air spaces and sold in most garden centers.
- Grit. Crushed rock, sold as horticultural grit. Pieces about 1/8 inch across are a common size.
- Coarse sand. Sharp sand with grains large enough to see and feel one by one, sold washed or as horticultural sand. Leave out fine sand, the kind that feels like sugar or flour.
For the organic part, use bagged cactus mix or ordinary bagged potting mix. Most bagged potting mixes are mainly organic and hold a lot of water, and neither kind is ready to use alone. The ratios below are the same for both.
Do not fill a pot with soil dug from the garden. In a pot it sets hard, and clay soil drains slowly.
The 2 mixes
Measure by volume. A scoop or a cup works as a "part".
General mix: 1 part organic to 1 part mineral. Use it for a pot that lives outdoors in a warm, dry climate and gets direct sun for part of the day.
Cool, humid or indoor mix: 1 part organic to 2 parts mineral. Use it if any of these is true:
- The plant lives indoors.
- Your summers are humid or rainy.
- Your summers are cool.
- The pot stands in shade for most of the day.
Many growers move to a mix about this gritty when humidity is high or if brown spots or rot start to appear.
If you cannot place yourself, use the cool, humid or indoor mix. To work out your conditions in more detail, see Growing cactus for your conditions.
A cutting with no roots yet is handled differently. See How to root a cactus cutting. For a plant that has just been delivered, see After your cactus arrives.
The dry-down test
- Fill the pot you plan to use with your mix. No plant is needed.
- Water it until water runs from the drainage hole.
- Stand it where the plant will live. Indoors, stand it on a saucer and empty the saucer once the pot stops dripping.
- Each day, push a wooden skewer to the bottom of the pot and pull it out. If moist mix clings to the wood, the bottom is still wet.
- Count the days until the skewer comes out clean and dry.
In warm weather, aim for a mix that is dry or nearly dry at the bottom after about a week. That matches watering about once a week in the growing season.
- Still wet after a week. Add more mineral material, or use a smaller pot. If you are on the general mix, change to the cool, humid or indoor mix.
- Dry in a day or 2. The mix is safe. If you would rather water less often, add organic material a scoop at a time and test again, up to the general mix.
When the mix stays wet in cool weather
A mix that passes the test in summer dries more slowly once the weather turns cool or grey. That is expected, and the answer is to water less, not on a schedule.
- Check with the skewer before every watering. Water only when it comes out clean and dry from the bottom of the pot.
- In winter most cacti rest and use little water. About once a month is enough for a small pot, and a pot 8 inches or wider may need none.
- Move an outdoor pot that stays wet to a place where rain does not reach it, such as under an eave.
- If the pot stayed wet for most of the cool season, change to the cool, humid or indoor mix or a smaller pot when you next repot.
Sand, gravel layers and acid soil
Sand. Judge sand by its grain. Coarse, sharp sand is a good mineral ingredient. Fine sand is not.
Gravel layers. A gravel layer in the bottom of the pot is not needed. It takes up space the roots could use, and a coarse mix all the way through drains well without it. It never replaces a drainage hole.
A layer of grit on top of the mix is useful. It helps keep the base of the plant dry and discourages moss and algae. And it looks nice. However, it also hides the surface, so use the skewer to check for moisture below it.
Acid soil. A mix near neutral suits cacti. Bagged mixes based on peat are acid. If yours is, 1 to 2 tablespoons of garden limestone mixed into a 6 inch pot of it brings it to neutral.
Cacti in the ground
A cactus planted in the ground does not get a mix. It needs a spot where the soil drains well, and the planting hole is refilled with the soil that came out of it.
Sources
Best method(s) for check houseplant soil moisture. Ask Extension, Cooperative Extension. https://ask.extension.org/kb/faq.php?id=681293
Care for Potted Cacti and Other Succulents in Desert Climates. Arizona-Sonora Desert Museum. https://www.desertmuseum.org/visit/sheets/1Cact_pot.pdf
Container Gardening in the Southwest Desert. Kelly Young, University of Arizona Cooperative Extension, 2016. https://extension.arizona.edu/publication/container-gardening-southwest-desert
Growing Media for Greenhouse Production (PB1618). Elizabeth Will and James E. Faust, University of Tennessee Agricultural Extension Service. https://utia.tennessee.edu/publications/wp-content/uploads/sites/269/2023/10/PB1618.pdf
How to grow houseplant cacti and succulents. Royal Horticultural Society. https://www.rhs.org.uk/plants/types/cacti-succulents/houseplants/growing-guide
How to Transplant a Cactus. Michael Chamberland and Jack Kelly, University of Arizona Cooperative Extension, 2020. https://extension.arizona.edu/publication/how-transplant-cactus
Potting Media. UConn Home and Garden Education Center, 2016. https://homegarden-cahnr.media.uconn.edu/wp-content/uploads/sites/3479/2022/08/Potting-Media.pdf
Succulents (GN 175). Pat Turner, UC Master Gardeners of Sacramento County, 2025. https://ucanr.edu/sites/default/files/2025-08/331073.pdf