Results rolling in from the first batch of samples, and it's not too impressive. I'll be posting more detail later, but I'm sure you're all dying to know, so, the 5 inexpensive retail products looked like this:
Organic matter (loss on ignition) 3.2 to 15.7%, average 8.4%
Water 28 to 52%, average 38%
Everything else (?????), by subtraction, average 54%.
Now, really good garden soil is about 10% OM. I would have thought compost would be much higher than that, although the mineral fraction will be higher than in uncomposted vegetation after being concentrated in the compost.
I found results online for our city landfill compost. It's 15-19% OM, which still seems low. They do use drywall scrap, so there is going to be significant mineral content there, but the gypsum is actually beneficial for the clay soil here, so they don't get dinged for high mineral content. At least you know what it is, and it's a waste reduction approach.
I am sending another batch of samples including some high dollar products - I got a bag of cotton burr compost that seems very fluffy, and some more expensive mushroom and cow manure from nurseries (egad, 10 bucks a bag! Still pays to make your own). And, my own backyard stuff from last winter's batch. This way there will be something to compare the low quality products with.
Thinking about a 1-pg fact sheet to put out at our city compost demo trailer at the Earth Day celebration.