Soil Mixes for Potted Cactus and Succulents
There is a wealth of information out there regarding soil mixes for potted C&S plants and as many recipes to go with it. In this article we hope to help you narrow down your choices and help you make an informed decision on what to use for your plants.
Growing conditions, pot choice (plastic vs. clay AND size of pot) climate zone, the plants needs and availability of the raw materials will all factor into the mix you use.
NOTE: If your current mix is working well for you, don’t mess with it! Of course, experimentation is great and encouraged. But don’t go repotting your entire collection without testing out the mix or mixes on a few plants first.
Store bought pre-packaged soil mixes are not very suitable for growing C&S to their potential, especially the dry desert types. They are however, a good starting point. Amendment is obligatory.
What’s in a C&S potting mix?
Commercial Mix (Store bought): (ie. SuperSoil, Miracle Grow, Kellogg’s, etc.) Usually contains Spagham peat moss, tree bark, leaf mold, sand and pumice. The sand and pumice amounts are not nearly enough for C&S. Sometimes I really wonder how they can market it as Cactus mix? However, the same stores that sell these pre-packaged bags (ie. Big box stores, Home Depot, Lowes, etc.) also sell the ingredients needed for amendment. Never have seen pumice for sale at these stores, but perilite can be substituted.
Peat: Is partially decomposed organic matter. Decomposition occurs in anaerobic conditions, usually due to being waterlogged. It can be made from almost anything, but commonly is made from moss, occasionally sedge. Peat varies a great deal from fluffy fiberous stuff almost like fresh moss except it’s brown, to dark almost black sludge.
Pumice: Pumice is a semi-hard rock that has some water retention capabilities and helps aerate the mix. Comes in different sizes from small granules to large ¼” or ½” chunks. Usually available from your local roofing company or places that mine/quarry the rock.
Perilite: Similar to pumice, but much lighter in weight (one advantage over pumice) it retains slightly more moisture than pumice (another advantage) however, being so lightweight, it tends to blow easily in the wind and will eventually make its way to the top of the pot floating around as you water. This is a disadvantage over pumice that a lot of growers just can’t overlook. Perilte works and works well, but the hassle of constantly dealing with it makes pumice that much more desirable. Price is usually about the same and the slight advantage perilite has over pumice is negated.
Vermiculite: Similar to perilite. Retains moisture, helps aerate the mix and similar in its properties and effectiveness, but comes in smaller granule sizes. We tend to not use this for cactus, but we do for some succulents.
Coarse Sand: Sand comes in a variety of forms. You want to stay away from any sandbox/playground/beach sand or very finely grained sand (used for brick mortar). You want the sand that you can actually see different sized granules. Usually sold as builders sand for mixing with cement. This type can also have small gravel rocks added. This is a good thing!
Diatomaceous Earth: Diatomaceous earth (fired clay granules) is great for potted C&S. Usually found in very small granules 2-3mm. They retain quite a bit of moisture and also help aerate the mix. DE is also used on sports fields to soak up water before the big game and used as an amendment to the fields’ grass or dirt areas. Also used as a motor oil absorbent.
Native Soil: If you live in an area where cactus are native, using your own garden soil (mixed with ingredients from above) is a great alternative. A quick jaunt out to the desert for a few bucket fulls makes for a nice day out and you get to look at some plants while you’re at it. But leave the plants where they are! Be sure the soil you are collecting is not clay or has large amounts of clay in it. Clay tends to clump together and suffocate the roots not allowing the water to actively get to them.
Grit/Gravel/Decomposed Granite: Grit, gravel or decomposed granite is used to aerate the soil and provide good drainage. Good drainage is essential to growing C&S plants successfully. Used in conjunction with pumice/perlite/DE. Since grit/gravel has no moisture retention, it’s strictly for drainage. Chicken Grit, lava rocks, etc. can be used. Be sure to clean/wash any grit/gravel used from your local areas or anything found outside in nature. Stick with small granules ¼” to ½” in size. Using a kitchen sieve, you can screen out all the fines (sand, clay, etc.) and just use the chunks.
All these ingredients are available from only a few different sources, but most can be found at local stores. Pumice, DE and grit/gravel may require an internet search and a trip to a roofing company, rock quarry or mineral mine.
Organic and Non-Organic/Mineral
Which is which?
Organic: Organic ingredients are made from natural resources. Decomposed plant material (leaves, tree bark, moss) and contain natural minerals and matter that plants use as food. These ingredients are ok for pot grown cacti, but should be used in very small amounts. Most cacti do not grow naturally in these ingredients, so they do not like to be condemned to a pot full of moss and tree bark. Some plants really dislike any of these ingredients and do not do well at all being potted in them.
One disadvantage of using these organic types are pests, -gnats, larvae, algae, fungus, etc. There are no guarantees these bags are completely sterile and these pests can be present and in turn get put into the pot with your cactus. To ensure the mix is pest free, sterilizing is mandatory.
Non-Organic/Mineral: Non-organic ingredients are made from mined minerals. Rock, sand, loam, Decomposed Granite, etc. These do not contain any type of organic matter or nutrients the plants can use. However, they do offer some mineral trace elements that, in conjunction with fertilizer, do help the plant. Non-organic or soilless mixes do require more culture practice, but it is well worth it for a healthy growing cactus.
The Potting mix
What do we use?
We’ve tried many different mixes, some worked, some didn’t. Your geographical climate may be the most dependent on the mix you use, besides the actual plant you are potting and your regimen of culture. Here in So. California it’s much more arid and hot than it is in say, New York. Your growing conditions (Greenhouse, open air, windowsill) will also play a part in your potting mix choices. Here in the desert a potting mix can dry out very quickly, especially when using a mineral mix. Watering schedules increase and fertilizing becomes more frequent. Back in New York that same potting mix could take a few days longer to dry out, due to lower air temps, higher humidity, etc.
The type and size of pot you use also changes things. Plastic pots will retain moisture longer than clay pots. Plastic is not able to breathe so the mix will take longer to dry out. Clay pots are breathable so the moisture can evaporate through the sides. FYI: Glazed clay pots will perform about the same as a plastic pot, as the glazing seals the pot. A full mineral mix in a plastic pot in an arid region will dry out about the same as a slightly organic mix in a clay pot in a more humid region. This is merely a theory, but you get the idea. The pot size will also play a part in the dry time. Makes sense right? There’s more in the pot so it’ll take longer to dry. This is another reason for using the correct sized pot for the plant.
We’ve narrowed our ingredients down to 3, sometimes 4. The amount of each ingredient depends on what we’re potting up. We use a combination of: Commercial cactus mix, pumice, diatomaceous earth and sand. The commercial cactus mix is always the smallest portion and sometimes can be as little as 3 to 1. For example: 1 part Commercial mix, 2 parts pumice, 2 parts diatomaceous earth. Meaning, if we mix 1 scoop of commercial mix, we’ll mix in 2 scoops of pumice and 2 scoops of DE. Small adjustments can be done on the fly to get the consistency we are looking for. Our test is very simple. Grab a handful of your mix, squeeze it tightly to make a fist and then open your hand up. The mix should just crumble away and not clump up at all. If it does still clump, add more pumice or DE.
Experimentation: Testing your potting mix
Once you’ve decided on your ingredients it’s time to test them. This is a very simple test, certainly not highly accurate, but it will give you a good idea of what you’re dealing with. Collect a few clear plastic bottles (water, gatorade, soda) that approximate the same sizes as the pots you use. Cut the tops off to about the depth of a standard pot (4-5inches) and poke a few drain holes in the bottom. To help keep track, label each bottle –Mix1, Mix2, etc. Now mix your ingredients together and put them into each bottle, changing the ingredients and/or the amount of each ingredient. If you use a top dressing, add that last. You may want to test the same mix with and without top dressing. Now water the bottles as you would your plants and set each bottle where you grow your plants. Take note of how the water soaks into the mix, (this is where the clear bottles come in) watch to see how much drains out the bottom and how fast. Then wait to see how long the whole bottle takes to dry out. Adjusting your ingredients and/or the amounts you can fine tune your mix as needed. As said, this is not a scientific test and not highly accurate, but it will give you a good idea of how your mix will react in the real growing world.
NOTE: when growing in clay pots, expect the mix to dry a few days sooner than if growing in a plastic pot.