This is way cool. Would be interesting also to do the same measurement on old peat or peat-based mix. Say, if someone was repotting a plant from a year-old peat mix. Is anyone repotting from peat any time soon? I think I repotted my last peat-only plant only 2 weeks ago :(

Soil porosity values are well documented and can even be calculated from the various particle sizes in the soil. Published values (percent air space within the soil) range from about 40%-60%, usually very close to 50%. This holds all the way to pure sand and even larger solid materials, as shown by your gravel result. Your value of 30% is anomolously low, perhaps because there was a small amount of water already present, perhaps experimental error (since the water capacity was measured at 35%, physically impossible to be more than the porosity).

Published values for the water holding capacity are generally about 10% lower than the total porosity for mineral soils. The value of 35% for your soil is actually at the low end (again there may have been 5%-10% moisture already in the soil), with values up to 50% for heavy soils being quite common. Sand, and fine gravels, are much lower, around the 25% value you show for gravel.

They don't describe peat as "fluffy" for nothing ;) One of the problems with peat is the very large difference between the amount of water retained from a dynamic watering, and the maximum amount of water which can be soaked up, for example from standing in a saucer. This is what you referred to as wettability in another thread. Although peat retains air from a simple watering, it can become almost fully saturated from being allowed t soak up water. You can measure this by a third experiment where the cup is allowed to stand in water for a few hours.

I believe that the same thing applies to the calcined clays like Turface although I can't find any published experiments on this. It is certainly a valuable property in a soil because, as we discussed on the other thread, it will often reduce or eliminate a perched water table after a pot has been watered from above.

Perlite does not show this property because it is a closed cell granule. The air pockets within each particle are not open to the exterior in any way. This shows in the very low saturation number you found and is also apparent in the willingness to float.

Volcanic rocks are very variable, some closed cell, some open, and the cell size and base density of the rock varies. Yours shows the properties of a closed cell rock, or perhaps it is simply a very coarse rock with large internal cells.

Of course it still remains to discuss what ideal values of these properties might be in a succulent container soil. Total porosity is often mentioned and it is claimed that it should be 60%-70%, certainly never below 50%. I feel that this isn't the most important number, rather that the air space remaining after you water is critical for succulents, as indicated by the lower saturation numbers in your table. I suspect that rather dense soils are valuable for many succulents, certainly Lithops and some cacti seem to prefer a soil which would have low total porosity. Others prefer a much higher porosity, I would suggest that fleshy rooted species like Ectotropis, Haworthias, and some cacti, prefer a especially open soil (although possibly require regular watering in such a soil).

Lastly I still claim that the wettability, or lack of it, is important, possibly more important than any of the measurements here. In your experiment where tipping a pot forces extra water to drain, when tipping the pot a few minutes later does not force extra water to drain the perched water table has been eliminated and this is a good thing. There are other ways to quickly reduce or eliminate perched water tables: porous clay pots; only watering in dry sunny weather; and only watering plants which are thirsty.

On the other hand, some growers habitually water from below, I don't know whether this overrides any wettability issues to force a perched water table or actually reduces the amount of water retained within the soil because of limited capillary action or limited time standing in the water. Presumably all these things are possible and each grower has adapted a method which suits their own soil mixes and plants. My experience is that bottom watering is a way to put extra water into the soil and I only use occasionally for plants which are in peak growth or subject to very drying conditions (my soil and pots are chosen for safety in a cool humid climate and dry exceptionally quickly by in rare hot low humidity conditions).

Excellent commentary shrubs.

My soil mix dries out pretty quickly on warm sunny days. What I do when plants are actively growing is to water lightly late in the day or evening so that the the sun does not dry the pots out, giving the roots time to soak up the H2O. I will then water again, heavily, the next morning to leach out any residues.

I agree, plants with thick tap roots probably benefit from a more open, coarse mix. Most mesembs with fine, shallow roots probably are better off in a dense mix that can retain moisture for long enough to give the rootlets time to absorb the available moisture.

x

Turface MVP is about 50% of my mix and I'm very pleased with it's performance.

I've been growing a few Adenium arabicum in pure Turface MVP. They aren't growing as quickly as others from the same batch of seed, but I really like the type of growth I'm getting. Much more compact and pleasing to the eye. I plan on doing a larger group in the spring of obesum, arabicum, and Thai Soco.

Some people grow Adeniums in pure clay granules here too. I've tried it and even with fertiliser in every watering I couldn't get the same growth as with a real soil. I'll save it for when I can't stop something rotting off in soil.

Kudos for the experiment! The peat result surprised me as well.

In addition to other complements I would like to complement you on the presentation in the OP, especially the pictures. The pictures were focused and too large or to small for this venue. Good work.