Fohat85 wrote: ↑Sun Feb 26, 2023 6:55 pm
Dear friends, my soil mix is composed by:
20% zeolite
40% lapillus
20% dolomite
5% coal
10% coconut fiber soil
5% fine gravel
For older plant (>30 years) i usually replace organic component with fine gravel.
Before Mike gives us an evaluation of your mix and what it means for determining the right amount of Mo, I just wanted to point out a few things...
Zeolite has good CEC (for members who don't know the term, "CEC" is cation-exchange capacity), so you won't need to fertilize with each watering as I do using my pumice-granite gravel mix. (Pumice has very low CEC and granite has none.) Once I show you the recipe, the nutrient dosages in parts-per-million for your watering solution will give us an idea on how often you should fertilize. Any recommendation I make will be nothing more than a guess, so I would defer to Mike if he has a better one. We don't have enough information for that yet, but we will soon.
5% coal -- do you mean charcoal? From Better Homes and Gardens Australia (
https://www.bhg.com.au/charcoal-in-potted-plants):
- "Another great way to keep your plants healthy is by adding a layer of horticultural charcoal to the pot or planter.
"This absorbs excess water from the roots of your plant and keeps the soil “sweet” by guarding against bacteria, fungus and rot. It also helps to eliminate odours, and is particularly effective in terrariums."
I can see that in nonxeric plants, but I don't know if it does anything given the fact that your mix is 90% mineral. Which brings me to the other 10%...
Coconut fiber = coir, and I believe you're using ground coir as your soil component. Not sure why you're using it in the mix for cacti that are less than 30 years old. It causes more problems than it solves, as Mike explains here...
viewtopic.php?p=398697&hilit=coir#p398697
...and here:
viewtopic.php?p=398705#p398705
If you need the 10% component for better water retention in the mix, try this instead:
viewtopic.php?p=398714&hilit=sphagnum#p398714
My recommendation at the moment:
- 20% zeolite and 40% lapillus have roughly the same water retention capability as the pumice in my 60% pumice-40% granite gravel mix, so you're fine there unless people who use both say otherwise.
- You and I share a similar Mediterranean climate -- get rid of the charcoal and dolomite (they're actually working against each other), and go with 25% coarse gravel. Mix it with 5% fine gravel -- remarkably close to the 40% granite gravel in my mix.
Because pumice retains a surprising amount of water, straight pumice in a humid climate takes too long to dry out. Unlike the high porosity of pumice, granite has a much lower porosity, so I use granite gravel as a "moderator" to let the mix dry out in a timely manner. Your gravel doesn't necessarily have to be granite as long as it's relatively nonporous. Cactus roots need something they can grab onto -- smooth aquarium pebbles won't do the job, so your gravel needs to have some roughness to it. I'm not convinced that you need a 10% component for additional water retention, although I don't know the climate in Rome well enough to say if you do or not. If you don't, the only reason for repotting is when cacti outgrow their pots and require bigger pots -- a nice problem to have!