przcvctm, hey, how you doing? i've looked at the links provided, thank you. it looks to me that axis would be a good medium, but it's cost is a little high and it does not out perform turface. they are both are mostly quartz silica, even though one is made by geologic forces and one is made by sea creatures.
they have big differences in water holding capacity. turface holds it own weight in water when saturated. axis holds 110% of it's weight in water. but turface is 35 lbs per cu ft. and axis is 16.8 lbs per cu ft. 35 lbs divided by 7.9, which is the weight of a gal of fresh water, equals 4.43 gal. axis at 16.8 lbs times 110% is 18.48 lbs. divided by 7.9 is 2.33 gals. by reference a cu ft is 7.49 gals.
i paid $9 a 50 lb bag for turface several years ago, the price today is still only $11 per bag.
turface, which is mined from one site in georgia, has a fair amount of ca and mg and acts like a storehouse for these as well as p and k. axis does not have the storage capacity of turface.
Delta9, Axis is more expensive. I guess I'm fortunate enough that paying $.50/lb doesn't seem that much. Especially when it can be re-used.
But you've answered what it is that I've been trying to understand from your past experience using Turface in a Hempy. I'm sure I read at another site long ago that Turface held too much water for a 2" res., and that you eventually drilled a shallow res. or none at all, allowing your nutes/watering to drain to waste for better results. By you calculations above, with the same volume hempy bucket, the Axis would hold about hold about half as much water and looks to be more appropriate for the application.
Thanks to all who took time to look at the links.