Well, I return to this thread, with its many helpful suggestions - thank you all very, very much - after a five day trip down to Manila to stand in for the father of a young man - who is my nephew - at his high school graduation from a prestigeous school, and during the trip we happened upon not one, but two, bonsai purveyors who use and had stock available of local pumice in graduated sizes!
O frabjous day! Callooh! Callay!'
He chortled in his joy.
Yes, indeed, I am chortling! Problem solved! Readily available, as a substrate to mix with cement to form concrete! (I'l leave the cement out of my mixes!)
The one fellow, FWIW, says to use a base layer of the "corn" size pumice, then a layer of the "mongo," i.e. medium size, (like "mong" beans? ... "Mongo only pawn in game of life ... "), then a layer of "river sand," and then a topping of the fine stuff - though quite frankly the various sizes all look so similar I doubt one needs to worry so much about things.
There are many areas, BTW, with "river sand" of varying volcanic origins and colors, but all of them are simply too fine and/or dense to make a good soil consistantly for various types of trees as far as I can tell: these sands are filled with nutients, perhaps, but simply too dense, I believe, to make a great soil by itself for all types of trees.
BTW, to add good organic stuff to the mix, many people use the readily available "Pilay," rice husks, to their mix for all kinds of plants, or some nurseries even use it by itself or only lightly mixed for bougainvillea or other plants. My wife grew upon a farm here, where Pilay was an important by-product.
Anyway, thanks so much for all your help and support in my quest to get this most essential aspect of the hobby covered!
