my thought on the matter is that most substrates have advantages and disadvantages.
The problem is finding what suits the orchids, our schedules watering them and doesn't need replacing too often.
Some people like fast drying but it requires more watering. Moss is quite water retentive so generally needs less watering but it depends a big chunk of moss dries slower than a thin layer of moss.
When starting out my orchids were all different sizes and in different substrates and different pots and it was hard to work out when to water each one without overwatering others. It took a while to find what worked best for me and it is different for everyone.
So the big disadvantages with moss and bark is that they degrade faster and they change their structure as they degrade.
If one doesn't like that idea inorganic substrates are a good choice.
I think perlite is a bit underrated myself, it compacts over time but added to a substrate it can't compact that much and even if it does a little I find it beneficial, as it retains more moisture so you can't fill a pot with too much perlite or it compacts and stays too moist but a little adds aeration, moisture retention and makes the pot lighter, meaning roots can grow through the substrate easier. It's also very cheap.
Leca has got some of the biggest disadvantages in my opinion but it is very cheap so it is worth using for that reason. The problem might have nothing to do with alkalinity which was probably just a poor choice of word to describe the problem.
The problem with leca is that it can absorb water a bit too well. I have the same issue with moss. What this means is that water and nutrients are constantly absorbed and transported to evaporate on the surface and what this does is it leaves the nutrients behind as a deposit while the water evaporates.
I do not like using leca as a top layer for this reason.
I've never used charcoal but heard it can give good results but needs even more watering than bark.
A mix of different substrates usually gives the best results to achieve a mix that dries out in the timeframe that one wants with the environment they are growing in.
To start with it is impossible to know what substrate gives what result and will need to be watered in what way, some only need light
misting on the surface, other substrates like charcoal probably need a full soak every once in a while. It can be very confusing but over time one learns what the disadvantages and advantages are and how to use them to ones advantage.
Pumice is the most expensive and it is a natural product so it can be very pure grade or contaminated with other rock. If it is low grade generally it will be less good for orchids but high grade pumice is a very good substrate even completely on its own. I find it hard to source - especially the very good grade.
But even the low grade pumice has its advantages. It is still pumice, it just holds a bit more water so it can be used if a pot needs to be kept wetter for longer.
The only real problem is that this is all good in theory but how does one convert this knowledge in practice. Because one can't try something, see if it works, repot if it doesn't and find what works best on the 5th attempt but the only thing one can do is learn as one goes along and improve from that knowledge.
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