I am building 12 grow boxes that are 32' x 10' each out of 2x12 douglas fir (untreated). I have 150 yards of very aged cow manure and 150 yards of a sandy top soil mix. I would also like to use some aged saw dust in the mix as well. I'm thinking of using a ratio of 25% manure (very aged), 25% saw dust (very aged) and 50% top soil (sandy loam). What I don't use will be stockpiled for later use or given to friends. I am planning on growing flowering perennials and I'd like to use the Mittleider method of fertilization and watering. My question is does anybody have any suggestions regarding the percentage of mix of topsoil, manure and sawdust? I am in zone 5b in the northeast corner of Washington state if location matters. Any comments or suggestions would be appreciated....Gary