We use sawdust in our Mittleider garden for our grow boxes and in our seedling trays. Each year we top off the grow boxes before the growing season begins. Today we made the trip over to a nearby town where a sawmill allows us to shovel our own sawdust for free. They make pallets and are largely cutting inexpensive cottonwood trees for the lumber. Any tree expect the black walnut provides a usable sawdust for a Mittleider grow box or seedling tray
The sawdust is mixed with sand for both the grow boxes and the seedling trays. For the grow boxes it is mixed at 3 parts sawdust and 1 part sand. For the seedling trays it is 2 parts sawdust to one of the sand. Both are mixed by volume. I’ll try and take some pictures that show the size of the particles. Too large and too fine are both bad.