Soil Mixes
"Have you known two growers to use the same mix?" That was a rhetorical question a fellow grower of mine asked after answering my question about what he used for a potting mix. The point was well taken, since I haven't found the perfect mix in over 30 years and DON'T know of two growers who use the same mix. Given the breadth of plants in my greenhouse, there probably is no single mix that will do the job for every kind of cactus and succulent out there. With that said, below are some ideas about cactus mixes.
Commercial Packaged Soil Mixes
1. Potting Soil
Regular commercial potting soils vary from company to company, but the problem with most is that they hang on to too much water for too long a time for most cacti. Most commercial potting soils are manufactured for houseplants that have lush growth and vigorous roots and as a result, tend to use water more quickly than most cacti. Fortunately, there are amendments that can be added to commercial potting soils to help make them more porous and thus succulent friendly.
Some potting soils contain peat moss which can cause problems for cacti as peat moss breaks down after a year or two compacting the soil. If at all possible, try to get a potting soil that doesn't contain peat moss.
2. Commercial Cactus Potting Soil
Any commercial cactus soil out there on the market seems to work just fine for cactus. They are made to be fast draining and help take the guesswork out for someone who is new to the hobby and is reluctant to try their own mix.
 Commercial Brand "A" |  Commercial Brand "B" |  Commercial Brand "C" |
Custom Soil Mixes
The recipes below were listed in a flyer freely available to Sacramento Cactus and Succulent Society members. I don't vouch for any of them, since I've not used them.
Elton Roberts Mix
Elton Roberts is known among many people in the cactus community. He's been growing cacti for a very long time and is quite knowledgeable about the topic. Here is his soil recipe:
- 50/50 Redi-Gro potting soil and perlite
Arboretum Soil Mix
- 30% redwood compost
- 30% horticultural sand
- 15% peat
- 15% scoria
- 10% perlite
University of California Succulent Mix
- 1 part ground sphagnum peat
- 1 part redwood compost
- 1 part horticultural sand
- 2 parts volcanic pumice
- for every one cubic foot of mix add:
- 5 tablespoons dolomite limestone
- 2 tablespoons gypsum
- 3 tablespoons superphosphate
- 1 teaspoon potassium nitrate
- 1/2 teaspoon potassium sulfate
Amendments
Perlite
Horticultural perlite is the end product of a process that transforms natural perlite. Detailed information about the manufacture of perlite can be found at perlite.net.
Perlite is my choice for making a faster draining soil mix. It is an inexpensive and effective way to go when aeration of soil is needed. It is very light meaning that it won't weigh down a pot. Typically, mixing one part potting soil (or cactus potting soil) and one part perlite serves most of my needs.
Be aware that perlite can have a lot of dust in it. If there are a lot of fine, dusty particles, consider taking a colander and sifting out the very fine grains. If too much fine grained perlite is added to a potting mix, it will act to fill in the air spaces between the soil particles and may cause an amended potting mix to hold more water than just potting soil alone. Whenever using perlite, DO NOT breathe the dust.
Pumice
Pumice is a product of volcanic activity where tiny gas bubbles in lava cannot escape before the lava cools. It's similar to perlite in that it acts to loosen up a soil mix, but unlike perlite, it is mined, processed, and sold in its natural state. It is somewhat heavier than perlite, a little bit harder to find, and tends to cost a little more, but it quite suitable in aerating a potting soil.
The most likely place to find pumice is at a nursery, but it might also be found at a local feed store. It goes by the name of Dry Stall and is used in horse stalls so that horses have some sort of traction underneath them. I've used it as an amendment and like it.
Note: Some growers get passionate about perlite versus pumice. Some swear that perlite, because it's so light, will "float" to the top of a pot. I've found that a well-mixed soil with perlite works just fine and if there's any "floating" of perlite to the top, it happens during watering where some perlite may come to the top because it is so light. For me, perlite is a bit easier to find than pumice.
Red lava rock
Horticultural Sand
Peat Moss
Peat moss is good at aerating soil, but it breaks down relatively quickly and its texture changes as it does. In my experience, high concentrations of peat in a potting mix break down to the point where the soil in a pot becomes a solid mass. Problems arise when this happens, one of which is that the soil is hard to rehydrate if it's allowed to dry out completely. Another problem when the soil dries is that it shrinks and pulls away from the sides of the pot, so unless it's soaked in a bucket, much of the water will run down the sides of the pot and out the bottom. Lastly, a peat based soil that's broken down is hard to separate from the roots of a plant. Peat is fantastic for seedlings and cuttings where it's likely plants will be repotted after a short period of time.
Soil Collapse
Potting soils are made from organic materials that were likely composted to some degree before they were bagged and priced. Composting is the breakdown of organic matter by a combination of moisture and micro-organisms. Composting continues at a slower pace while potting soil is in a pot, but it does continue. This can present a problem for plants that are in the same potting soil for several years (10+ years). For someone planning to repot every few years it’s not an issue, because fresh soil resets the clock on breakdown. But when a mature plant doesn’t require repotting for several years, any potting soil or cactus potting soil may cause problems for a grower because the organic component of it may breakdown too much. When a potting soil breaks down aeration goes down and moisture retention goes up even if the potting soil has been amended with pumice, perlite, etc.
If you have older plants that won’t require repotting for several years, contact a Cactus and Succulent Society about a soil mix that will stand the test of time.
Acidifying Soil
Cactus tend to like their soil a little on the acidic side. This came to light when a commercial grower (who lives about 35 miles from my location) mentioned that he acidifies his water with a touch of distilled vinegar. It appears that the groundwater in this part of California (I'm on a well) is on the alkaline side meaning that eventually the soil in pots will tend toward the alkaline side with each watering. For city folk on city water, the water has probably been "softened" meaning that it too is likely on the alkaline side. pH is the measure of acidity or alkalinity of a substance. pH is measured on a scale of 1 to 14 with 1 being an extremely harsh acid and 14 being an extremely harsh alkaline. 7 is considered neutral.
With the pH of my groundwater on the alkaline side, my plants apparently aren't getting the nutrients they need because they were being bound up by the properties of an alkaline environment. The thought is that a touch of acidity in the vinegar-water mixture should free up nutrients for plants to use, and with any luck plants that like slightly acidic conditions will appear to be happier for it. An added benefit of the repeated application of acidic water is the decrease of hard water deposits on pots and plants.
Generally speaking, applying a mixture of two tablespoons of vinegar to five gallons of water should suffice in helping acidify soil in pots. I used that mixture in 2008 with some change in the plants, but it wasn't until I tested the pH in early 2009 that I found that I wasn't using enough vinegar. I started using upwards of five tablespoons per five gallons to get the water in the 5 to 6 range and I have to say, I'm sold on acidification. Many plants rebounded and look so much happier for the effort. If you want to know where you stand in regard to your water's pH, then test the water. Hydroponics stores offer small test kits that cost less than $10 and it's what I use. Keep in mind that too much vinegar in the mix can lead to a hostile environment for plants by making the soil too acidic. In fact, the roots could burn so don't apply any more vinegar than what's recommended above unless you're monitoring the pH. If plants in a collection are growing fine, then there's no need to acidify.