that's pine bark mulch

I've seen this at my local Lowes. the bags probably vary, but there were quite a lot of large particles in the bag I poked into.

others seem to report variability. you should inspect each of the bags before you buy,

http://forums2.gardenweb.com/discussions/1391723/timberline-pine-mulch?n=3

Thanks Andi!

I'm fairly certain the bark is 1/8-3/8". No dust in potting mix.

the link in the op has a post by Al stating down to dust.

im not sure if pine bark dust is something that is tolerated or desired in 5-1-1. the dust seems to function similar to peat moss but I'm not sure what the subtle differences are.

too much pine bark fines will raise the PWT. there definitely has to be a limit to fine particles

I was confused - my apologies. I got busy making Gritty and forgot about the 5:1:1. Thank you for keeping me straight!