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Is it GOOD Compost? Or Bad Compost?
Q. I have been going to the recycling center in Fairmount Park and bringing home buckets of their free compost. But as I listen to you more carefully I'm wondering if the compost is full of pesticides. An analysis from the soil lab at Penn State is posted at their website , but I don't know how to make sense of it. Can you help? I know I should just make my own. And I'm working on getting a worm bin set up!
---Lucinda in the Germantown section of Philadelphia
Ok—first: Nobody makes enough of their own compost! I have about 20 different 'systems' working—a giant circle of metal fencing filled with shredded leaves, smaller circles of welded wire filled with shredded leaves and coffee grounds, four black plastic composters with locking lids that are filled with shredded leaves that I mix kitchen waste into, an indoor stackable worm bin that processes most of my kitchen waste, and last summer's tomato cages that I filled with shredded leaves in the fall—and I'll barely have enough for my needs. In fact, I'll probably have to 'import' some myself this Spring for a special project I'm doing.
I have three local sources to choose from—a nearby garden center, a big mulch dealer that sells supposedly premium compost and my local municipal supply. I'm getting a five gallon bucket from each and giving them all the enhanced 'duck test'.
Retired University of Maryland compost guru Dr. Frank Gouin taught me the basic three-step duck test: "Does it smell like rich black soil? Does it look like rich black soil? And does it feel like rich black soil when you pick it up and squeeze it?"
If all three answers are yes, 'it's a duck'—aka good compost!
Over the years I added two other 'tests'—one to detect the persistent lawn grass herbicides that weren't around when I first spoke with Dr. Gouin and one I created to test for possible weed seeds. You fill two containers—regular plant pots with good drainage—with the compost. In one you plant a good amount of fresh seeds; be sure to count them. The other one you leave naked. Then you keep them at room temperature and water them every day. If the naked one stays naked, it's not weedy. If the seedlings germinate well—in the 80% range or higher—and the young plants look healthy, herbicides are not an issue.
Pea and bean seeds are supposed to be the best indicators of herbicides, but the Penn State soil lab did a very similar germination test with the seeds of a classic cucumber variety—Marketmore 76. My advice is to use the freshest seeds you have on hand.
The results of that professional germination test? The compost scored 100! It sprouted just as many seeds and the seedlings were just as vigorous as seeds planted in a 'control' of a bagged soil-free seed starting mix. And that's what Al Rattie of the US Composting Council said was the most significant number on the 13 page report: The Philly compost grew seedlings perfectly. And if seedlings grow perfectly, everything else is almost certainly going to grow well. Seedlings are the canary in the coal mine; and Al said that that '100%' was really the only number he needed to see.
But the rest of the report did have what we'll call "listings of concern": Things like soluble salts, fecal coliform, lead, mercury, zinc—all very scary words. When I read them I yelled, "I don't want any fecal coliform or lead in my garden!"
Well good luck to me, because it turns out that they're present in every gram of soil on the planet. The Fairmount Park compost had 25 mg/kg of lead—the EPA limit is 300. "Fecal coliform" was 4.5 on a scale of one thousand EPA units allowed. So it could have been a poopy hundred times 'worse' and still be on the side of the angels.
My head was spinning trying to figure this stuff out. I looked up a lot of numbers on my own, but finally realized I also had to ask for help—and I'm the guy who wrote a best-selling book on compost and whose Tedx Talk on composting just passed 700,000 views on YouTube!
So my relative inability to immediately interpret these test results showed me that 'civilians' have almost no chance. And I reacted just as emotionally to line items like lead and arsenic, that in reality, were detected at levels the EPA and Composting Council consider "incredibly low". But they were just scary words and random numbers on a page without some kind of context.
So what's a poor gardener to do?
The enhanced duck test. Don't even think about using compost that smells bad or looks bad or that grows weeds or distorted seedlings.
Remember that all gardening is a crap shoot. There are no pristine areas left here on Krypton. And even compost that's only reasonably 'good' is still 100 times better than chemical fertilizer.
And read up on these important subjects! You'll find lots more info on these topics in our "A to Z Answers" section under C for 'Compost" and 'S' for Soil Tests.