Soils for Succulents

 Basic Cactus and Succulent Mix

1 part potting soil for houseplants

2 parts sharp sand 

1 part vermiculite

1 part perlite

This is a good basic soil for most potted cactus and succulent plants, consisting of ingredients that are easily available from nurseries or hardware stores, at least in my part of the world. The commercial potting soil portion holds some water and nutrients, while the sand drains well and provides inert bulk. Perlite and vermiculite are lightweight expanded minerals that help aeration and drainage, with vermiculite also being able to hold some water and nutrients. 

The sand component is the potentially most variable part of any of these mixes; I've seen a surprising range of stuff sold as bagged sand at the hardware store, even under the same brand name at different times. Ideally I look for sand that is mostly quartz, with rough, irregular grains; in New England this is mostly river sand mined from old glacial deposits. Beach sand is usually not suitable, since it tends to consist of smooth, rounded grains that pack together. Sand that consists solely of very fine grains (granulated sugar sized) is also not good. I've occasionally run into "sand" that was actually some kind of industrial ground-up rock, with a mixture of larger chunks and a lot of powdery material; this has proven disastrous for succulent soil usage.

 UConn Cactus and Succulent Mix

3 gallons soilless potting medium 

3 gallons sharp sand

1 gallon perlite

1 gallon vermiculite

2 gallons calcined clay pellets

1 gallon pumice (or sponge rock or expanded shale)

1 gallon pea-sized gravel

3 tablespoons slow-release fertilizer

This is the cactus soil used in the biodiversity greenhouses at the University of Connecticut, made up in big batches in an electric soil mixer. The soilless medium is currently Jolly Gardener C/B, a peat and composted bark mix; in the past we have used peat/perlite Cornell-type mixes from the ProMix and Fafard lines. Coir-based mediums also give acceptable results with this recipe.  

The UConn C&S mix includes some coarse inorganic components that are a little more exotic than vermiculite and perlite. Calcined clay comes from several manufacturers; we have used Turface MVP and Primera One Field Conditioner. It's similar to kitty litter, but fired at a higher temperature so that it does not break back down into clay when wet. 

Pumice has become somewhat difficult to find in the eastern US in recent years; we have a stockpile that was ordered in bulk from Hess Pumice Products. Pumice substitutes that might be easier to find in the Northeast include sponge rock (an expanded volcanic mineral similar to perlite) and expanded shale, which is manufactured in New York and available under the Norlite, Solite and Espoma Soil Perfector brands. Expanded shales and calcined clay can contain harmful quantities of salt, so be sure to get a material that is intended for horticultural use.

Mineral Soil Mix

1 part sandy-loam soil

2 parts sharp sand

2 parts perlite

2 parts vermiculite

1 part pumice, calcined clay or expanded shale

This is a soil-based mix containing essentially no organic material, for use with more delicate succulents, such as Conophytum. I picked up the basic idea for it while working at Mesa Garden, where, back in the day, just about everything was potted in "Mabel Mix," which was equal parts sand, pumice, and native desert soil, the soil coming from the leftovers from road grading. "Loam" is used here in the soil science sense: a soil composed of silt (mineral particles smaller than sand grains) with some sand and a little clay, not in the gardening sense of "any fertile soil." New Mexico desert loam is hard to come by in Connecticut, but sandy loam subsoil, from a few feet down, with very little organic content, is an acceptable substitute. 

Digging a hole in New England will probably yield lots of rocks, and maybe some loamy mineral subsoil that would make a good base for cactus mix. 

 

 

The general idea with all of these recipes is to produce a potting medium that drains well, isn't too heavy, isn't too rich, but that still holds some water and nutrients. The basic theme is: a smaller amount of organic soil or loam, cut with generous quantities of sand and coarser mineral components. There is much leeway for changing the ratios and substituting or omitting ingredients. The flip side of this flexibility is that there are no miracle ingredients that will effortlessly take your succulent plant cultivation to the next level; there is no substitute for patience and close attention to proper light, temperatures, ventilation, watering and fertilizing.