Compost is the ideal soil conditioner because of its high organic-matter content: 30 to 60 percent. But we found that many bagged composts contained less than 30 percent organic matter (OM). As I studied our lab reports, I was amazed by the wide variation in the OM content of the brands. And I was frustrated that we didn’t have an easy way to help home gardeners assess this key characteristic of compost. So I decided to do a little compost testing myself-with some help from Woods End Research Laboratory.
I knew that compost gets heavier (denser) as it ages, so I asked Dr. Brinton to run some tests to see whether there might be a way to predict the amount of OM based on the variation in the weight of given volumes of dried composts. Sure enough, when I dried and weighed our samples and then Dr. Brinton ran my data through his computer, we found a nifty way you can easily get an good estimate of the OM content:
Spread out a shovel full of compost indoors on a thin layer of newspaper and let it air-dry for about a week. Then, measure out exactly 1 pint (2 cups) of the dried compost and weigh it. If it weighs from 8 to 12 ounces, then it contains the desired level of 30 to 60 percent OM. If it weighs less than 8 ounces, the compost is probably immature; if it weighs more than 12 ounces, it is too old or has been diluted with soil.
Reprinted with permission from Organic Gardening Magazine