I don't grow cacti, except holiday cacti. However, I would think for other cacti, coir is not a good medium for them as I thought I heard it's pretty water retentive.

Actually is pretty good, if you mix it properly. cacti dont grow on pure sand, and coir is just coarse enough to make up for it.

How much water retention does the coir have? I've had bark break down quickly - within a year - and turn to compost in the pots. Does coir do that? Like I said on the other thread, I have never used it.

You could increment grit and gravel by 5-7,5% if you live in a humid climate.

I use a coarse gritty mix with limestone, and a bit of topsoil and some organic nutrition - trying to emulate their habitat conditions. But there are as many favourite mixes as there are growers, and most people think their mix is the best.

And most cacti accept what they are put in and grow as well as they can - recommendations to increase a certain constituent by 7.5% are total BS IMHO.

But I do find that desert cacti (and Mexican desert cacti are what I concentrate on) grow best when given plenty of SUN - also emulating their habitat conditions. Far more important than the minutiae of their soil mix. We get loads of people coming on here with sadly etiolated cacti sitting in a dark window or under a feeble light asking what their problem is - SUN SUN SUN is the answer.