Sand and grit are forever, but bark breaks down in 1-3 years dependence on size and probably climate.
>> I have been adding the red lava rock for $65 a yard. It's 5/16" or 3/8" in size for the largest particles.
Hmm, 8 to 9.5 mm for the largest particles. That's pretty coarse.
You might be getting more benefit from the gritty fraction (1 mm - 3 mm) than from the gravelly fraction (over 4 mm). I think 1/10" is great (2.5 mm) and really 3 mm and 4 mm particles probably do create some air spaces that don't flood easily.
But grains above 4-5 mm might give you more aeration per dollar if they were ground down to 2-3 mm. How easy would it be to crush some of the bigger lava pebbles?
P.S. Technically, by some standards, the dividing line between very coarse sand and very fine gravel is 2 mm. That doesn't leave any category for "grit", the most useful gardening size!
I think of sand as smaller than 1 mm grain size, and gravel as larger then 4 mm. That way, I can call anything from 1-4 mm "grit". 4 mm is 0.157 inches, around 1/6".
