Pumice for Shohin
Finding small, quality pumice is not easy. A-1 has pumice that after screening has a very low % of pumice in the 1/8" and smaller range. Dry Stall is small but has so much dust, by weight and volume. Small scoria we can get at A-1 in a reasonable % after screening the smaller size grouping (3/16"). It is my understanding that shohin/mame soil has a particle range of 1/32" to just over 1/8" (1-3mm) for the larger shohin.
I saw pumice at Anderson's but it was expensive compared to A-l, but the sizing looked very good for shohin. My curiosity got the better of me and I bought two bags. One was a 6 liter (<1/4 cuft) of Gardener and Bloom Pumice for $3.99. It screened out at 85%+ shohin/mame sized particles, and it appeared to have been washed so almost no dust or fines were seen. Some dust will probably appear as it dries more. At A-1, if one fills the bag they provide up to where it is just closable with the twist tie, it is about 1- 1/2 to 1- 3/4 cuft. We pay about $8 for a bag which would be $4.57/cuft, or approximately equal to $1.14/1/4 cuft, or putting it another way, at $3.99 for just under 1/4 cuft, that makes the small bag equivalent to $28 for an A-1 sized bag. The small bag is pricey, unless one only needs a small amount of smaller, clean pumice.
The other bag was by UniGro in a 1 cuft bag for $10.95. It screened out at about 97%+ in the shohin/mame range, about 2/3 in the shohin size. There was essentially no dust and the larger particles were the remaining 3%. It too was moist appearing to have been washed, so some dust may appear later as it dries out and when it is washed again or after watering thoroughly at re-potting time.
The particle size % over 3/16" was <2% and under 1/32" was <.5% , basically no fines and dust. The remaining was useable shohin size particles. $10.95/cuft makes it about $16 per A-1 bag. Pre-screening at A-1 makes it nice, then thoroughly screening it for use at home. But pre-screened or not, this does not affect the volume at the time of purchase, just the weight of the A-1 bag. A second bag was purchased and was consistent.
The UniGro pumice is expensive compared to A-1 but it is almost all in the small sizing range for shohin and is very clean and therefore not dusty/messy to handle. Dry Stall is in the shohin size range. However, there is generally not much if any particles 1/8" to 3/16" and there is so much dust and fines that even though a bag of Dry Stall is cheaper per unit volume, after screening it is actually about the same or slightly more expensive than the UniGro.
Dry Stall comes in a 40 lb bag that appears to be about 1 1/3 cuft. Based on the last 6 bags I have purchased over the last 2 years or so, about 25-30% of the weight has been dust and fines. The volume lost due to the dust and fines was less at about 20% since a lot of the finer particles occupy the spaces between the larger particles and the pores in the pumice and therefore do not contribute too much to the bag volume. The last bag I bought was almost 50% dust and fines by weight or about 25% of the volume. I paid about $14, so that makes it about $20 plus for an A-1 bag of1 1/2-1 3/4 cuft of usable material. I am sure the last bag was an anomaly, but the other bags are still wasteful.
Conclusion: It seems that UniGro is a good value for clean, smaller sized pumice as compared to Dry Stall. It is easier to get enough of the smaller particle sizing for shohin than A-1. It is very clean and also compliments the sizing of the A-1 scoria nicely. So if one does not have the time to go to A-1 and/or only needs a small amount, Anderson's consistently carries the 1 cuft UniGro pumice and the 6 liter Gardener Bloom pumice.
Dislcaimer: Shipments do vary for many horticultural products. A-1 had two batches of pumice the last time I was there. Both were signed 1/4". One was primarily 1/4"+ with small amounts in the 1/8"-3/16" size, while the other was at 3/16" to 1/4" primarily, with about 5-10% in the 1/8" size. Not much of the small particles in either batch. That could change, but history shows their source of pumice is consistently on the larger end.
Shohin Study Group
SDBC member Charlie Mosse lets you know of interesting bonsai posts from around the web but especially shohin topics as he is leading the shohin group.