[Kristen]

I'd like to big up the need for aeration, and my experience tells me something inert like sand/grit/gravel does the job

I've found that increasingly necessary on commercial brands - perhaps as they have started using less Peat, or more council-compost / rubbish!! and I have found that 10% (by volume) of Perlite works well for me.  Only snag is when potting on I find that the rootball is inclined to fall apart, although apart from the inconvenience I think that is a good thing as there is clearly plenty of air in the mix and it has not compacted, as multi purpose compost is inclined to do, and (subjectively) it seems the rate at which a good root network is created is very good / fast.

Having run out of Perlite during the summer I did a large batch of cuttings and potting-on with sharp sand instead.  It seemed to work well, but it was much heavier (e.g. judging pot-weight to decide when to water), and when potting up cuttings the mix was very dense and I think that meant more root damage (whereas cuttings in a high Perlite mix (I use nearly 50:50 for cuttings) falls off the roots easily, making separate them a doddle).

I fancy trying a side-by-side trial next year, as I would prefer [environmentally speaking] to use Sand rather than Perlite

I'd argue against top watering in favour of bottom watering, delivered through capillary matting.

All greenhouse benches here are capillary matting, when I created them it was a toss up between capillary matting and flood-and-drain. Capillary matting was a lot easier to "construct", but with hindsight I wish I had gone for flood and drain. I find that the pots are too well irrigated with capillary matting, and I think flood-and-drain would let me increase the time between waterings as necessary - particularly at this time of the year where I want to reduce the water at the plant's roots.  But in terms of reducing time during irrigation its definitely done the job - pot up a plant and plonk it on the capillary matting and then pretty much forget about it!

(For anyone considering Capillary Matting I would strongly recommend putting micro-perforated sheet over the matting (under the pots)

[goodtogrow]

Good point about (sharp) sand, Kristen.  It does make for a denser medium than perlite, but also greater water-holding, which greater water-holding may be too much, at the expense of air!

Always on the lookout for a grit somewhere between sharp sand and 5mm pea shingle.  I use pea shingle at the moment, which makes the mix very free-draining, hence water delivery through capillary matting.

Hey ho!  Formulating our own mixes is fun!  A bit hit-and-miss, but one day I'll get it absolutely right!

Best wishes