Soil is just as, if not more, important to the health and success of a plant, as is watering, light, temperature or any other factor. Store bought soil is extremely expensive and frequently comes in small quantities. So we are are going to go over three different formulas for cactus soil, one for hemiepiphytic cacti or Dragon Fruit, one for epiphytic (tree dwelling cacti like Epiphyllum, Christmas cactus, Rhapsalis) and one for xerophytic (desert dwelling cacti), that can be made cheaply at home.
First a little bit about soils, roots, infections and water…
Well-draining soil is important. We hear the term “well-draining” often, but all it means is that water can exit the soil. It does not mean that the soil should not retain moisture, it just means it shouldn’t be waterlogged. It is essential that the soil be able to aerate (allow air to diffuse throughout the soil). Plants respirate (breathe in Oxygen and release Carbon Dioxide and plant roots are no exception. When air is unable to percolate through the soil, the plant roots effectively suffocate. This suffocation causes a dying back of the roots and their protective layers, allowing fungi to capitalize off this and infect the plants rhizosphere (root system). How each plant reacts to this suffocation and subsequent infection varies, but cacti as a rule are very sensitive, thus it often leads to the death of the plant.
The question is then, how to avoid this?
This leads us into soil amendments. A soil amendment is a substance added to soil to change its properties. In our case we want to soil to be less compact, so that air percolates it better, and have a courser, rockier, siltier texture, so that water drains more thoroughly. We will accomplish this by added varying quantiles of paver base, mulch and perlite to generic potting soil.
The materials and soil amendments you will need are:
Paver base
Hardwood mulch
Perlite
Potting soil
5-gallon bucket with lid
Mycorrhizae and/or fertilizer (not required, but certainly helpful)
Latex, nitrile or rubber gloves
So what is all this stuff and why?
Paver base is an excellent non-reactive material (will not affect soil pH or chemistry) that contains a mixture of silt and variously sized rocks. It can be used to greatly improve soil drainage as it increases the average particle size of the soil substrate and does not retain moisture. We have found that the best paver base for this is made by
Hardwood mulch acts, much like paver base to improve drainage of the soil by increasing the average particle size of the substrate, however unlike paver base it is organic and able to retain some moisture in the soil. Mulch can act as a sort of very long release fertilizer for the soil, as it provides a food source for beneficial soil dwelling fungi (called mycorrhizae) which can give the plant nutrients. NOTE: We recommend hardwood mulch. Do not use pine bark, it contains turpentines and other substances that may not be so beneficial to the soil.
Perlite a very soft volcanic rock resembling foam. It is incredibly light weight and difficult to compact. It is used to allow more air to percolate through the soil but reducing the soils’ ability to compact and lowering its average density.
Potting soil, such as those found at any hardware store in one cubic foot bags will do fine. It is important that the soil is specifically for use in indoor or outdoor pots, other soils, like topsoil or raised bed soil may retain too much moisture to work effectively.
Five-gallon bucket is for thoroughly mixing the soil. The lid can be placed on it and the bucket can be rolled or shaken about (in addition to kneading) to thoroughly mix the soil.
Before you start!
I would recommend the use of rubber or latex gloves. Soil contains minerals called phyllosilicates (I studied mineralogy before I studied botany), these minerals can dry out your skin to the point of almost cracking. Wearing gloves will spare you a lot of irritation!
The Recipes
Soil and amendments should be measure out volumetrically. We do not provide exact measurements, as the recipe is a ratio of soil to the added soil amendments. If desired, add a pinch of mycorrhizae or fertilizer to each batch of soil before mixing it (although not necessary, it can be helpful).
Hemiepiphytic Cacti Soil (Dragon Fruit):
14 parts gardens soil
2 parts perlite
3 parts hardwood mulch
3 parts paver base
Epiphytic Cacti Soil (Epiphyllums, Christmas Cactus, Rhapsalis):
7 parts garden soil
1 part perlite
1 part hardwood mulch
Xerophytic Cacti Soil (Columnar Cacti):
11 parts garden soil
2 parts perlite
3 parts mulch
3 parts paver base
Finally
Once you have proportioned all of the ingredients, mix them together both by shaking the bucket and by kneading the soil. Formulas do not have to be obeyed thoroughly, their exact ratios can vary, however these ratios should give you a soil mixture with properties tailored to each group of cacti. Larger cactus specimens or for cacti which require extremely well drained soil, place a layer of large to medium sized rocks in the base to the pot or container, before filling with soil, this will greatly improve drainage.
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