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Succulents grow in regions ranging from semi-arid to tropical forests, each with a different soil profile.
Most plants, including succulents, need soils that effectively balance moisture and air availability. Sand (inorganic) drains fast but is ineffective at retaining water, while clay tends to be anaerobic (airless). The best succulent soil is a mixture of organic and inorganic materials.
What do My Succulent Roots Need?
Your succulent soil should provide the plant with the following six essentials:
- Air Management: The soil must ensure the succulent has reasonable access to oxygen. Saturation porosity is the percentage of air trapped in the soil even when it’s flooded, and more than five percent should be the target.
- Moisture Management: Succulents prefer soil that is occasionally dry. Ideally, you want to flood the pot, drain it, and leave it to stand and dry out over a week or so. The time it takes to dry depends on the field capacity of the soil, expressed as a percentage of water remaining in the ground after draining.
- Nutrient Management: Soil with a high cation exchange capacity (CEC) can ensure nutrient availability to plant roots. CEC is a product of the soil’s organic content.
- Soil Biology: A diverse population of microorganisms (bacteria, fungi, actinomycetes) improves soil and plant health and resilience to disease and pest attacks. The best way to boost a soil’s biology is by adding compost.
- Soil pH: The plant’s preference for acidic or alkaline soil and soil biology is most effective at ensuring a buffered pH environment.
- Plant Anchorage: Ensuring the media isn’t so light that the plant cannot remain reasonably erect as it grows. Anchorage is significant to cacti which can become top-heavy.
The Function of Organic and Inorganic Materials in Soil
As we can see from the above list, organic matter is vital for CEC, soil biology, pH management, and moisture management.
Most organic elements are interrelated; for instance, CEC improves water and nutrient retention while acting as a buffer to regulate the soil’s pH.
Organic material also hosts the soil biomes, without which they can’t survive.
But organic material also has risks, mainly compaction, which happens when the organic material decomposes and collapses, limiting air availability.
To ensure better airflow, we need some materials that provide porosity. Ideally, we achieve better aeration using odd-shaped grains, like sand, expanded shale, granite grit, pumice, or perlite.
Pine bark, while still an organic material, decomposes slower and will help ensure better root aeration.
Inert aggregates don’t only provide aeration but also contribute to water retention. Below is a table that shows the saturation porosity and field capacity of common inorganic materials used in succulent soil.
|Material||Saturated Porosity – Air||Field Capacity – Water|
|Turface®||28%||40 – 60%|
Growing succulents in inert inorganic aggregates is possible, but I don’t advise doing so. The main challenge is plant health, as resilience is a factor of microbial life in the soil.
Also, inert aggregate requires you to provide a constant supply of nutrients as the “soil” will have zero cation exchange capacity, a state where nutrients leach through the soil with each watering – much like hydroponics.
Creating the Right Mix for Succulents
How to best combine organic and inorganic materials to create the optimal growing environment is an art best discovered by trial and error.
I find it exhilarating to see my plants respond to varying soil composition, which can easily be adjusted when you repot.
Generally, a 75 percent inert aggregate composition and 25 percent organic material is a good starting point. Using the methods I detailed in my The Best Bonsai Soil Composition for Bonsai Health article, consider measuring your mix’s field capacity (water retention capacity) and saturation porosity (air availability).
A basic succulent soil recipe may include:
- One part of a compost
- One part expanded shale
- One part pumice or perlite
- One part of Coconut Coir
For succulents, we can define too much water as “A root state that is seldom allowed to be dry.” Too much water is when a succulent’s roots are often damp.
Unlike most houseplants, succulents cannot tolerate moist soil and require their soil to dry out occasionally.
The well-being of your plants depends on root health, a factor of their growing medium.
The advice above will hopefully guide you in creating the optimal growing environment for your succulents.
With that sorted, you only have to ensure good light and temperature conditions to grow these beautiful plants.