The sense of discomfort I feel about the atmosphere that 'using specialized soil is safe'
When you grow caudex plants or succulents, you will inevitably stop here at least once.
Caudex plant specialized soil.
Agave specialized soil.
Hard red clay soil.
High-grade red clay soil.
Professional grade, indoor use, high durability.
The words lined up on the bags all seem correct.
On social media, too, many photos of people growing plants with them appear.
But there must be many people who feel this way somewhere in their hearts.
'Why are the results different for different people even though we use the same specialized soil?' 'Why did the plant I used expensive soil for rot this year of all years?'
'Conversely, why are there plants that have been healthy for a long time in soil I just threw together?'
The true nature of this discomfort is that the axis of soil selection has become 'product names'.
This is not a story about fertilizer or choosing brands.
It is a story about ending the era of stopping your judgment at the word 'specialized'.
The real problem is not that the soil is bad.
It is that you are trying to get results with soil alone while the order of environment, rhizosphere, moisture, EC, minerals, and phase is broken..
*EC (Electrical Conductivity) is a guide that indicates the concentration of fertilizer components and salts. In this article, it is used to mean almost the same thing as 'how strong the fertilizer is'.
───────────
2. The inconveniences of the old world: The judgment axes used until now are stalled
First, let's list the 'common sense of soil selection' used until now.
□ It's safe because it's specialized soil
□ It's safe because it's hard red clay and doesn't break down easily
□ It's hard to fail because it's high-grade red clay soil
□ It's the correct answer because it's supervised by a famous shop
□ It says it's for agave or caudex, so I should just use it as is
□ For root rot prevention, just choose 'soil with good drainage'
There is something missing here that is common to all of these.
That is 'your environment' and 'the phase of your plant'.
Even with the same bag of soil, the 'time the soil stays damp' is completely different when used outdoors on a balcony in summer versus in an indoor, windless environment with LEDs in winter.
Even with the same soil, the amount of oxygen required is completely different when used for a rooted mature plant in its growth phase versus an unrooted imported plant.
Choosing based solely on the product name means making a judgment while ignoring all these 'environmental differences' and 'phase differences'.
In other words, what was lacking was not the courage to buy expensive soil, but the order in which you look at things.
───────────
3. Why we want to rely on 'specialized soil'
This is not a story to blame the reader. On the contrary, I understand very well the feeling of reaching for specialized soil.
Caudex and succulents are plants with high unit prices. There are plants that cost 10,000 yen, 30,000 yen, and sometimes even over 100,000 yen.
If that plant rots after just one repotting, the loss is tens of thousands of yen, or worse, even more.
Moreover, you only get one chance at rooting management or nursing an imported plant.
In situations where failure is not an option, people crave 'the right words'.
If words like 'Caudex soil', 'Agave soil', or 'Professional grade' are lined up there, it is only natural to reach for them.
On social media, only photos of successes using specialized soil circulate.
Stories of failure are rarely posted.
There is a tendency not to post failure stories on social media.
That is why the atmosphere that 'specialized soil equals the correct answer' is easy to solidify.
But plants do not grow roots based on product names. What plants are reacting to are:
・Temperature
・Oxygen
・Moisture
・EC (fertilizer strength)
・Minerals (inorganic components contained in the soil)
・Phase (stages such as seedling, rooting, growth, dormancy, etc.)
These six things only.
The term specialized soil is merely a label attached to a 'mixture that has been adjusted to be somewhat appropriate' for these six factors.
It is not the label that works, but the contents.
───────────
4. Causal structure: Root rot is not caused by 'water' alone
Here, I will enter into the most misunderstood topic when discussing potting soil.
It is often thought that the cause of root rot is only 'overwatering'.
But in reality, in research on growing media for container cultivation (the soil inside the pot), root rot is categorized as a complex factor.
There are four main factors.
1. Oxygen deficiency due to excessive moisture
2. The damp period being too long
3. Roots not being active due to low temperatures
4. Conditions that make it easy for pathogens to multiply overlapping
In other words, it is not the water itself, but when
a state where oxygen does not return around the roots after the water has drained
continues for a long time, the roots weaken.
Here, for the first time, the ambiguity of the term 'well-draining soil' becomes visible.
If you only look at drainage, it passes if water drains smoothly from the bottom of the pot.
But that is a different story from whether or not oxygen has returned inside the pot.
This is what researchers are looking at.
□ How much of the gaps are filled with air after irrigation
□ How much water remains in a form that the roots can use
□ How uniform the particle size is
□ How heavy or easily compacted the soil is
□ Whether the particles break down over time
This is where the concept of air space comes in.
Air space refers to the gaps filled with air that remain inside the pot after irrigation.
In explanatory materials from institutions like Oregon State University, it is reported that the wider the air space in the soil, the lighter the damage from pathogens that cause root rot (such as Phytophthora) tends to be.
However, if you increase the air space too much, it becomes too dry, making it difficult to manage irrigation frequency. Here, there is a clear line of 'going too far'.
Another thing that must not be forgotten is the story of the 'layer where water easily remains at the bottom of the pot'.
This refers to the phenomenon where, even if the same soil is used, water easily accumulates at the bottom if the pot is low and shallow.
Just by changing the height of the pot, the amount of water the soil holds changes.
In other words, even with the 'same specialized soil', if the pot shape and environment are different, the result will be something else entirely.
Here, I will rewrite the axis used until now in one line.
× Soil with good drainage ○ Soil where oxygen returns around the roots after irrigation
This is the starting point for everything.
───────────
5. What do they use to grow caudex and succulents overseas?
Let's shift our perspective outward for a moment.
Japanese caudex and succulent culture is built primarily around Akadama soil.
This is not a bad thing.
It is a skeletal material that is easy to obtain, inexpensive, and offers a balance of water retention and aeration.
However, Akadama soil is not a 'standard material' overseas.
In fact, it is treated more like a special, high-end material derived from bonsai.
What is commonly used in Western soil for caudex and succulents are mineral-based, or inorganic, granular materials.
Specifically:
・Pumice-based mineral granules
・Lava rock
・Porous volcanic stone (scoria)
・Perlite (light granules made by expanding volcanic glass)
・Zeolite (mineral granules that easily retain fertilizer components)
・Fired clay granules (granules made by baking clay at high temperatures)
・Fine crushed stone (grit)
These are the main ones.
The Royal Horticultural Society (RHS) in the UK suggests the concept of 'adding fine crushed stone to peat-free cactus potting mix' for growing cacti and succulents in pots. Akadama soil is not a prerequisite.
In Germany and Central Europe, mineral-based soil that combines lava rock, pumice, and zeolite is commonly used even in ordinary households.
The cactus and succulent growing media produced by the German company LECHUZA, as well as the succulent media from SERAMIS, are also designed by combining pumice-based mineral granules, lava rock, zeolite, and fired clay granules, rather than using Akadama soil. The goal is to be 'resistant to breakdown, easy to reuse, and manageable in a clean indoor environment.'
What I want to emphasize here is not that 'overseas is right and Japan is behind.'
The reverse is just as sloppy an argument.
There is only one fact.
Akadama soil is not the only correct answer for caudex and succulents.
Overseas, 100% inorganic is not the only choice either.
Depending on the climate and the plant, I sometimes mix in a small amount of organic matter.
Here, too, the reality is that it is not a matter of 'black or white'.
───────────
6. Scientific evaluation of Akadama soil
From here on, I will evaluate Akadama soil objectively.
Akadama soil is a volcanic ash-derived granular soil originating from the Kanto Loam layer.
According to published manufacturer measurement examples, the pH (a measure of soil acidity/alkalinity) is around 5.9, the EC (a measure of fertilizer concentration) is around 0.1, and the CEC (a measure of the ability to retain fertilizer components) is reported to be around 20 meq/100g.
These are very manageable figures for home gardening.
【Strengths】
□ Weakly acidic and easy to handle
□ Good balance of water retention and aeration
□ Has the ability to retain fertilizer components (CEC), providing a certain level of nutrient retention
□ Facilitates the development of fine roots
□ Easy to use for seedlings and during the growth phase
□ Availability and price in Japan are overwhelmingly advantageous
【Weaknesses】
□ Breaks down over time
□ Prone to producing fine dust
□ As it breaks down, the large gaps decrease
□ The inside of the pot becomes clogged, making it difficult for oxygen to return
□ During low-temperature periods or dormancy, water retention becomes a risk
□ Using only Akadama or too much Akadama can be dangerous depending on the phase
Here, we can also organize the value of hard Akadama, fired Akadama, and premium Akadama.
The true value of these is not that 'they work because they are premium.' The essence is much more modest and is as follows:
□ The granules do not break down easily
□ There is little fine dust
□ Physical properties are stable and long-lasting
In other words, they are only valuable because 'their physical properties are maintained for a long time.'
And the fact that physical properties are maintained means that air gaps remain inside the pot for a long time.
There is rationality here.
Conversely, this also means that the word 'premium' itself does not create value.
───────────
7. What is the Japanese specialized soil market actually selling?
When looking at the contents of specialized soil for caudex plants, agave, and succulents sold in Japan, most are combinations of these.
Akadama soil, hard Akadama soil, Kanuma soil, pumice, Hyuga soil, perlite, zeolite, coco fiber.
In other words, the materials themselves are not secret weapons. This is what specialized soil is actually selling.
□ Uniform grain size
□ Low dust content
□ Selection of materials that do not break down easily
□ Pre-mixed, saving the trouble of blending it yourself
□ Designed to be easy for beginners to manage
□ Reproducibility that helps avoid losses of expensive plants
□ Cleanliness suitable for indoor management
This has genuine value. You are buying time and reducing the risk of failure.
However, there are some points to note.
□ Mixing ratios are often not disclosed
□ Little quantitative data on physical properties
□ Terms like 'professional grade' or 'specialized' are not scientific information
□ Products containing fertilizer can actually be a risk during rooting management or dormancy
□ Products containing coco or organic matter tend to stay damp for a long time in low-temperature, indoor, or windless environments
In short, the term 'specialized soil' is convenient, but if you stop your judgment there, you won't notice the discrepancy with your own environment.
───────────
8. How to view price differences
For reference, here are some examples of market prices.
□ Regular Akadama soil: approx. 28.4 yen/L example
□ Hard Akadama soil: approx. 62.9 yen/L example
□ Fired Akadama soil: approx. 107.1 yen/L example
□ Specialized soil for caudex plants: approx. 326.5 yen/L example
□ Specialized soil for Agave: approx. 301.0 yen/L example
When comparing regular Akadama soil and specialized caudex soil, there can be a difference of more than 10 times in the price per liter.
You cannot simply say this is 'too expensive.'
What expensive soil is selling is, as organized earlier, 'grain stability,' 'low dust,' 'time saved by pre-mixing,' and 'reproducibility.'
With that in mind, it can be organized as follows.
【People who find high value in buying specialized soil】
□ Growing expensive plants
□ Have a small number of plants
□ Have little experience in making your own mixes
□ Want to reduce the hassle of removing dust and selecting grains
□ Want to manage plants cleanly indoors
□ Want to buy the reproducibility of the mix itself
【People who have little need to buy specialized soil】
□ Cultivating in large quantities
□ Accustomed to making your own mixes
□ Can manage grain size and dust yourself
□ Can create different soil mixes for each growth phase
□ Can create sufficient physical properties with regular materials
If you can protect a plant worth tens of thousands of yen with one pot, the cost of specialized soil is cheap.
Conversely, for someone managing over 100 pots a year, a 10-fold difference in price per liter will normally put a strain on operations.
*I grow about 700 pots, but I use different soils depending on the phase.
The decision is not based on the price of the soil, but on 'whether it can reduce failures for your plants and scale of cultivation'.
In other words, it is not that expensive soil is good, but that you should look at 'how much that soil can reduce failures in your environment'.
───────────
9. The correct answer changes depending on the plant
From this point on, we can no longer speak in terms of 'specialized soil'. This is because the required physical properties differ for each species.
【Pachypodium】
For unrooted, imported plants, and overwintering, low EC, high aeration, and not keeping them wet for long are the top priorities.
Conversely, seedlings require a certain degree of uniform moisture retention.
Adult plants can be grown in mineral-based soil, but if the environment is too dry due to strong light and high temperatures, the moisture buffer of Akadama soil is effective.
【Adenium】
During the high-temperature season, a moisture buffer is effective for enlargement.
During the low-temperature season, that same buffer turns into a risk of rot.
This is a group of plants where the interaction with the amount of irrigation is greater than the potting medium alone.
【Agave】
Basically mineral-based.
They are shallow-rooted and are the type that quickly produces fine roots after rainfall.
Therefore, the wet-dry cycle and oxygen recovery are vital.
Akadama is not the main component, but should be used as an auxiliary material to prevent excessive drying.
【Cactus】
There are large differences between species, and they cannot be lumped together.
The idea that 'sand-based is safe' is dangerous.
Fine sand can sometimes act to make the soil heavy and prone to compaction.
What is needed is not 'sand', but a proper variation in particle size and a wet-dry cycle.
【Haworthia/Echeveria】
If you use a completely mineral mix intended for Agave as is, it may cause excessive dryness or a lack of wet-dry variation in an indoor LED, windless environment.
Here, there are situations where the water and fertilizer retention properties of Akadama and Kanuma soil actually become useful.
In other words, you should not put everything into one drawer called 'succulents'.
───────────
10. The correct answer changes even more depending on the phase
Even for the same plant, if the phase changes, the required soil also changes.
【Sowing/Early Seedling Stage】 Cleanliness, uniform moisture retention, low EC, and a balance of water and oxygen are necessary. A mix of only coarse particles is not always the correct answer.
【Rooting Management】 Low EC, no fertilizer, high aeration, and not keeping it wet for long. However, hardening it by complete drying is not always the correct answer either.
【Growth Period】 If there is temperature and light, some leeway in moisture and nutrients is necessary. Change the mix depending on whether you want to fatten them up or tighten them up.
【Dormancy Period】 Since the roots are not moving, moisture and fertilizer become risks as they are. High aeration, quick drying, and low EC are on the safe side.
【Coming out of Dormancy】 Do not suddenly apply strong water. Soil that allows air to return quickly even with a small amount of irrigation is suitable.
In other words, the very idea of 'reusing one type of soil throughout the year' is unreasonable.
───────────
11. The AGRIX decision-making order
Those who have read this far should already see the order.
When choosing soil for caudex and succulents, you must always think in this order.
1. Environment (temperature)
2. Rhizosphere (around the roots)
3. Moisture
4. EC (fertilizer strength)
5. Minerals (the roles of Akadama, pumice, Kanuma, Hyuga, zeolite, etc.)
6. Phase (stages such as seedling, rooting, growth, dormancy, etc.)
Specifically, you ask yourself these questions:
□ At the current temperature, are the roots active?
□ After watering, is the structure such that oxygen returns to the pot?
□ Is this a phase where I should increase water retention, or a phase where I should drain it?
□ Is the EC too high for the plant's current phase?
□ What is the role of each of the Akadama, pumice, Kanuma, Hyuga, pumice-based mineral grains, volcanic rock, and zeolite?
□ Is the plant a seedling, currently rooting, a mature plant, or dormant?
You do not start with fertilizer or product names.
Materials are the second to last step.
Because you were doing this in reverse order, that is why 'I bought specialized soil but it rotted' has been happening until now.
In other words, you do not start with the name of the soil, but rather start with the temperature and the condition of the roots.
───────────
12. There are only 3 solutions
[Solution 1] Before buying specialized soil, check the blended materials and whether it contains fertilizer. If you look at what is in it and why, you will see whether it fits your environment.
[Solution 2] Separate the soil for seedlings, rooting management, mature plants, and dormancy periods. Do not try to use one bag for everything.
[Solution 3] For expensive plants, unrooted plants, and beginners, buy 'reproducibility.' For mass production, experienced growers, and those who can manage materials, keep costs down by 'making it yourself.' This is not about superiority or inferiority, but about how to balance cost and the risk of failure.
───────────
13. Optimal choice
The optimal choice is not a binary choice between 'specialized soil or homemade'.
□ High-value plants, unrooted plants, beginners → Buy specialized soil to gain reproducibility and save effort
□ Mass cultivation, experienced growers, those who can manage materials → Use custom mixes to gain cost efficiency and flexibility
□ Seedlings, rooting, growth, dormancy → Reconfigure as separate soil types for each stage
Separating these three and managing them accordingly is currently the design with the least amount of loss.
───────────
14. Summary
Let me condense the message here.
What caudex and succulents need is not specialized Akadama soil, but a rhizosphere physical design that allows oxygen to return, suited to the temperature and growth phase.
And one more thing.
They do not grow because of expensive soil. They grow in soil where roots can breathe.
Specialized soil is not bad. It is dangerous to stop thinking just because of the word 'specialized'.is the point.
───────────
15. Immediate action you can take in 5 minutes
Before you close this article today, please try doing just this.
Take one bag of soil you have on hand and check it as follows.
□ Does it contain fertilizer?
□ Is there a lot of Akadama?
□ Does it contain pumice or Hyuga soil?
□ Is the particle size uniform?
□ Is there a lot of fine dust?
□ Is your plant currently a seedling, rooting, a mature plant, or dormant?
□ Is your current location outdoors, or indoors with LED lights and no airflow?
If you can answer these, the way you look at the next soil you buy will already be different from yesterday.
Instead of 'whether to buy specialized soil,' you should now be able to look at it in terms of **'whether it is soil that allows oxygen to return to the roots for your current plant and environment.'**
───────────
16. Finally
In this article, I have organized the 'decision criteria' for choosing soil.
I will continue to articulate in more detail actual mixing examples, soil design by plant type, and how to use soil differently for root development management, seedlings, and dormancy periods.
This cultivation technique is included in AGRIXAI.
When you are unsure about choosing soil, inputting the 'plant name,' 'current phase,' 'location,' 'pot size,' 'watering frequency,' and 'soil currently in use' will make it easier to organize the decision-making process.
If you found this article helpful, I would be happy if you followed me on note. I will continue to articulate the decision-making processes for agriculture and horticulture.
───────────
Main reference materials
・Purdue Extension: Materials on soil pores in pots and layers where water tends to remain at the bottom of the pot ・Oregon State University: Materials on container cultivation media and air phase ・British Columbia Ministry of Agriculture: Materials on aeration in container cultivation ・LSU AgCenter: Materials on container cultivation media ・University of Minnesota Extension: Materials on seedling damping-off and sowing media management ・Royal Horticultural Society: Guidance on cactus and succulent cultivation ・LECHUZA: Product information on mineral media (PON/CACTUSPON) ・SERAMIS: Product information on mineral media for cacti and succulents ・Research on the interaction between media and irrigation levels in Adenium ・Research on Agave root systems and adaptation to arid lands ・Public information from manufacturers of specialized soil for caudex, Agave, and succulents sold in Japan (Hanagokoro, Protoleaf, KINCHO Gardening, etc.)