First things first, lets talk soil composition. Cacti generally require a very gritty mix to allow for quick drainage of water as the roots do not like to be wet for very long. One of the biggest causes of cacti death is over watering or roots staying wet for too long (substrate is too organic).
As a loose, general rule, you should aim for a minimum of 40% inorganic material to your mixes, especially for Trichos (Trichocereus). Trichocereus can handle more water than, say, Lophophora, which, require, little to no organic material.
To make things easy for you, we do have Trichocereus Substrate Mixes available, I do recommend doing it yourself but the offer is there for convenience.
Examples of Organic Material
- Sieved Compost (John Innes No 2)
- Worm Castings
- Peat Moss
Inorganic materials are normally derived from non-living sources, it is composed of matter other than plant or animal (ie. mineral). Inorganic material normally aids in drainage and in some cases, aids in water retention.
Examples of Inorganic Material
- Perlite (Not a personal fan but a popular choice)
- Washed Pumice (3ml to 6ml)
- Molar Clay
- Lava Rock
- Horticultural Sand
- Horticultural Grit
- Vermiculite (Seeds / Seedlings)
Perlite is a white, lightweight granular material which is made from expanded volcanic glass. It is heated at very high temps till it expands (pops) increasing its base size by a lot.
Personally, I dislike it for a few reasons outside of seed germination. Firstly, its light so doesn’t stay in the organic material very well, loves to float to the top of the pot when watering. Secondly, because it is white, it is hard to spot a mite infestation very easily. Lastly, its light and floats to the top :-).
To end, Seedlings are probably where my use of Perlite stops.
Pumice is one of my favorite inorganic materials and I use it for every stage of growth. It is similar to Perlite in that it is derived from volcanic glass, the difference being that Pumice is in its natural state and does not need to be heated up to expand. It is also heavier than Perlite, so, it doesn’t float to the top easily when watering.
Pumice is great for cacti in that they allow for drainage but also hold moisture and nutrients long enough for the cacti to adequately absorb and meet the needs of the plant.
Akadama (赤玉土, akadamatsuchi, red ball earth) is a naturally occurring, granular clay-like mineral used as soil for bonsai trees and other container-grown plants.
Akadama is one of the best inorganic materials around for both Cacti and Succulents. Much like Pumice, it is quite porous, so, while it aids in drainage, it also retains moisture and nutrients for your Cacti.
Crushed Lava Rock
Lava Rock is another popular soil additive to help with drainage. Generally used more in the succulent world, but, also provides a very versatile growing medium for cacti.
It is pore rich, breathable, water permeable and resistant to rot.
The nice thing about Horticultural Sand, is it is available pretty much everywhere. Commonly used with seed germination due to it’s size, it is also a very good addition to any adult cactus substrate as it aids in drainage and resistant to rot.
Another extremely common inorganic mineral to use for substrate, it is made up of mainly quartzite, silica, crushed rocks etc. It is then washed and drained for use in gardening. Horticultural Grit is the cornerstone of most cacti hobbyist’s substrate arsenal.
Seeds / Seedlings
Starting from scratch with seedlings is a very rewarding process, albeit, a very slow process to start. Seeds do not require a nutrient profile in the substrate as all the nutrients are supplied from within the seed.
I would recommend a 40% Organic / 60% inorganic mix for seeds, it doesn’t have to be fancy at this stage in the game. Something like 40% sieved compost with 60% grit (Horticultural Grit, Pumice, Molar, Akadama etc) will do the trick for your early days of seed germination.
Vermiculite is commonly used as a top layer for the seeds to sit on, the water retaining properties of vermiculite work well for seeds.
Trichocereus Substrate Mix
For Tricho (or Echinopsis for you werid folk) variants, I would recommend min 50% / max 70% inorganic mix to allow for quick drainage of water, this is especially relevant in the UK and our weather.
Organic (4 parts)
- 70% sieved compost (its important to remove any pieces of wood and / or chunky bits).
- 30% Worm castings
Inorganic (6 Parts)
- Horticultural Sharp Sand – 2 parts
- Pumice – 1 parts
- Kyodama – 1 parts
- Horticultural Grit – 2 parts
If you are on a budget, no fear, the most simplest mix would be:
- Sieved Compost – 4 parts
- Horticultural Sand – 2 parts
- Horticultural Grit – 4 parts
Lophophora Substrate Mix
Lophophora williamsii (Petyote) are rock loving cacti that require little to even less watering. I would recommend a max of 10% to 20% organic material, when in doubt, go less.
Lophophora are unique in that they are able to absorb nutrients from rocks, albeit very slowly. Most of the plants energy is focused towards its tap root.
The major difference in substrate choice is how much inorganic is used, I have also included crushed lime stone as Peyote naturally grow around lime stone.
So, for my go to Loph substrate mix:
Organic (10% – 20% Total Volume)
- 70% sieved compost
- 30% worm castings
- RHS Sharp Sand (30%)
- Pumice (10%)
- RHS Horticultural Grit (30%)
- Kyodama (10%)
- Molar (10%)
Pocket Efficient Lophophora Substrate Mix
Organic – 10% to 20% sieved John Innes No2
- 30% RHS Sharp Sand
- 40% to 50% RHS Horticultural Grit (any grit will do, a lot of people use straight molar or pumice etc)
Feel free to experiment based on what is available to you and what you can afford budget wise. Akadama as an example, has become very expensive since Brexit, mainly due to shipping costs. This will make it prohibitive for a lot of people.
If on a budget, pretty much any grit will serve as an inorganic material. As long as it doesnt break down too easily, it should work.
Growing cacti is easy, it is just a bit more tricky in UK climate where too much water becomes the enemy.
When in doubt, less water is better than too much. With well draining substrate, your risk of over watering is greatly reduced.