What Type of Soil Do Cacti Plants need?
Cactus soil is a mix of 60-70% organic material (e.g. leaf compost/coconut coir/ peat-free soil/ carbonised rice hull) and 30-40% inorganic material (e.g. Perlite/ Lava rock / coarse sand/ River Sand/ Pumice/ Crushed granite/ Vermiculite or Limestone).
For general desert species the cactus soil PH range should be between 5 - 6.5 (acidic). However, it's important to note that some genera (e.g. Ariocarpus) actually require alkaline potting soil.
The cacti soil mix should be made to imitate the species natural habitat. The right cactus soil recipe will help promote healthy plants growth, flowering and fruiting; this is especially important if you wish to collect seeds!
Cactus Soil vs. Houseplant Potting Soil: What's the Difference?
The difference between cactus soil mix and regular potting soil is the property and structure of the soil. For cactus plants and succulent plants, the right soil recipe will make the soil drain faster, to prevent root rot.
Difference between Soil Cactus Mix and Regular Potting Soil:
Many houseplants come from tropical climates where it rains frequently and thus, require damp soil. In contrast, almost all genera of cacti come from desert climate regions of the Americas, which are known for long-dry spells. Cactus potting soil has additional inorganic
matter to improve drainage quality, which allows soil to dry out in-between watering's and this will reduce the risk of root rot.
What Is the Best Soil for Cactus plants?
A wise guy once said, "If your cactus had legs, it would step right out of that soil!"
The best cactus soil recipe depends on the cactus species, age of the plant and properties of the soil in the species natural habitat.
A younger plant may benefit from additional organic material to support growth as they need more moisture and nutrients. In contrast, an older cacti may benefit from less organic matter and more inorganic material in the cactus mix, since they will require less water and good drainage.
Most importantly, cactus potting soil should have good drainage and be able to dry quickly. Too much moisture can cause root rot. The chosen pot should also have drainage holes. When watering, most of the water should visibly drain from the bottom of the pot!
The DIY cactus soil mix should also be able to slowly release nutrients as required by your cactus needs and for this reason, should not be made entirely from inorganic matter. Good cactus soil mix must also have good aeration, providing enough channels for the roots to grow and anchor for stability. It should also not hold excessive nutrients such as Nitrogen, as this will lead to a weak and undesirable looking cactus that is at high risk of root rot.
Benefits of making your own cactus soil:
The benefits of making your own cactus soil mix, is that you are in control of the ingredients and able to add additional elements, depending on your desert plants species. You can choose to use high quality, environmentally friendly ingredients and additional nutrients which your plant may need at different stages of growth. Ultimately, you can make your own cactus potting mix to suit your needs.
DIY Cactus soil Ingredients:
1. Organic Matter (animal or plant detritus):
Organic materials in the potting mix provides nutrients to the cactus and helps to retain water. Examples are leaf compost, carbonised rice hull, animal manure (i.e. rabbit droppings) and regular potting soil (easily obtained from most garden centers).
There is much debate around peat moss releasing harmful greenhouse gasses and many regular potting soil still contains peat! Peat moss is a decomposed organic matter, taken from the top layer of soil in peatlands and this can take thousands of years to form!
“Current research suggests that peatlands store twice as much carbon as all the world’s forests combined and are critical for preserving biodiversity, reducing flood risks and maintaining safe drinking water”
We would encourage people to replace peat moss with a peat-free cactus soil, which is natural and environmentally friendly! Alternative products to peat moss typically contain botanical and forestry cuttings! By using peat-free potting soil, you will help reduce the destruction of peatlands and in most cases, reduce landfill pollution as these components are usually fast recycled products!
2. Inorganic Matter (substances of mineral origin):
The delicate roots of the plant naturally become compacted as the roots absorb water, swell and grow. It is therefore important to ensure your cactus mix allows good aeration, to prevent soil compaction around the cactus roots. To achieve this, you could choose from Perlite/ Lava rock / coarse sand/ River Sand/ Pumice/ Vermiculite/ crushed granite or Limestone.
How to make cactus soil mix
In a large bowl or tub, combine a potting mix ratio 60-70% organic materials (e.g.peat-free compost) and 30-40% inorganic matter (e.g. Perlite/ Lava rock / coarse sand/ River Sand/ Pumice/ crushed granite or Limestone).
Depending on your cacti species and age, you may wish to consider adding additional material to your own cactus soil mix (see examples below).
Potential Hydrogen (PH)
Lets not forget the importance of the cactus soil PH! General cactus plants require a soil PH between 5 - 6.5. If your potting soil is too alkaline (PH 6.5+) you could add sulphur powder, carbonised rice hull, vinegar or rain water to reduce the PH.
Some cacti like Ariocarpus, Aztecium and Geohintonia are sensitive to cactus soil conditions and typically grow in calcareous alkaline soil (mainly limestone) in their natural habitat. Thus, are more likely to thrive with the addition of limestone chips (calcium carbonate) in the cactus soil.
The UK has a humic temperate oceanic climate. This means that we experience cool, wet winters and warm, wet summers. An important factor to consider when formulating your cactus soil. The additional inorganic matter is therefore essential to reduce the risk of root rot, caused by slow drying soil, which is a result of slow evaporation in the cactus soil.
Most people in the UK will keep their cactus plant indoors or in a heated greenhouse. Indoors the cacti will receive less sunlight exposure and less air flow, therefore it would benefit from additional inorganic matter to help the cactus soil dry quickly between watering's. This will also help prevent the cactus from becoming etiolated (unhealthy tall growth).
Slow Release Fertiliser
Slow release fertiliser are small sized pellets, designed to release small amounts of nutrients each time they are wet. The ideal slow release fertiliser for cacti plants should be low in nitrogen and an increased amount of potassium. This can easily be sourced from your local garden center and added to your cactus soil.
Diatomaceous Earth is a naturally occurring rock, formed by fossilised algae. It is highly porous, inexpensive and helps improve the plants resistance to a wide variety of pests/diseases.
Carbonised Rice Hull
Carbonised rice hull is a natural material derived from partial burning of rice hull. The addition of carbonised rice hull to your cactus soil will create superior drainage and aeration of the composition. It is also an excellent fertiliser as it contains Phosphorous (P), Potassium (K), Calcium (Ca), Magnesium (Mg), and micronutrients vital to healthy plant growth! It can also be used to lower the PH of the potting soil.
Humic acid are organic molecules, derived from the natural decay of plants and animals. It plays a key role in improving cactus soil condition, plant nutrition enhancement and cactus roots development. It can be used alone or in combination with fertilisers.
Plant probiotics are beneficial bacteria, which convert unusable nitrogen to nitrate and this form of nitrogen is useful for potted plants! Nitrogen plays an important role in both internal and external metabolic processes. Supporting the development of cactus and succulent plants tissues and cells.
Mycorrhizae is a beneficial fungi that lives symbiotically with the root of the plant, absorbing sugar from their host plant, in exchange for moisture and nutrients. This ultimately increases the absorptive area of the cacti, becoming “extensions” to the delicate roots system within the cactus soil!
“Keep the soil alive, with plant probiotics and Mycorrhizae”
When should you fertilise Cactus?
You should fertilise your cactus a minimum of once per year minimum in Spring (March), as this is typically the start of the growing season. However, cacti (especially younger plants) will thrive if also fertilised in Summer (June) and fall (September). If your cacti appears unhealthy or is becoming prone to disease, a small dose of fertiliser to the cactus soil will likely be key!
There are many options when it comes to fertilisers and you may have heard people talk about the fertiliser analysis of NPK ratios? This is important when deciding what type of fertiliser you need, depending on your species and its specific requirements. Each element is important; too much of just one element in the cactus soil can kill the plant and too little can result in an unhealthy plant.
“N is the chemical symbol for Nitrogen”
Nitrogen is an essential element out of the amino acids in plant structure and it plays a critical role in both internal and external metabolic processes. Supporting the development of plant tissues and cells. Nitrogen is essential for cacti, but too much can lead to soft tissue damage and result in the cacti being prone to disease.
“P is the chemical symbol for Phosphorus”
Phosphorus promotes the structural strength of a plant, encourages root growth, seed germination and the formation of flowers. It is therefore an essential nutrient from the earliest stage of the plant's growth cycle. As a result, a deficiency of Phosphorus can lead to the absence of blooms and poor seed production.
“K is the chemical symbol for Potassium”
Potassium is termed the “quality nutrient”, for it is paramount in plant survival and considered imperative, after Nitrogen. It promotes an all round well-being of a plant. This chemical plays an important role in the movement of water, nutrients and carbohydrates throughout the cells of the cactus plants.
Cacti are renowned for their ability to store water and survive months in the desert without water. But to understand just how much water a cacti can hold, here's an interesting fact!
“A mature Saguaro cactus can soak up and store over 1000 gallons of water! To put this into perspective, the Saguaro can hold over 23 bathtubs of water!”
Chemical fertilisers usually contain significantly more nitrogen than phosphorus and potassium, this should be reversed for cacti fertilisers! The cacti fertiliser should therefore contain less nitrogen than phosphorus or potassium.
As an example an N-P-K ratio of 5-10-5 is typically a good measurement to go for and the general rule of thumb is to fertilise just before grow season in spring (March onwards).
However, when it comes to succulents, they can be opportunistic growers! Succulents like Echeveria's will thrive during spring and fall when the temperatures are mild. Extreme heat or extreme cold can cause Succulents to go dormant, this is their survival mechanism.
Succulents typically go through a mini dormancy in Summer and their growth rate will slow down. We would typically recommend fertilising succulents when new growth begins in spring and late summer to early fall.
Sterilising the Cactus Soil before use:
In general, sterilising cactus soil is more important for seed germination and seedlings; becoming less important for well established cacti that have a strong defence mechanism e.g. are more tolerant to poor cactus soil.
Storing your leftover cactus soil mix
Empty your leftover cactus soil mix into a sealed plastic container. This should be stored off of ground level (e.g. low shelf) and away from water and significant temperature changes that could cause humidity/ excess moisture. The cactus soil should ideally be used within 6 months.