The compost test

Earlier this week I realized that when all our new landscaping plants arrive in the spring, we'll need plenty of compost to amend the soil where they're planted. And since our little compost bin doesn't produce more than a few cubic feet at most—nowhere near the volume we'll need for three plum trees, five cherry bushes, a dozen raspberry canes and five rhubarb plants—that means buying commercial compost. Unfortunately, there is a problem these days with commercial compost: rather than nourishing your plants, it could actually kill them.

I first read about the problem of "killer compost" in Mother Earth News. Basically, the problem is that certain herbicides, known as pyralids, do not break down during the composting process. Even if they've been eaten and digested by a cow, they remain present in the manure, essentially unchanged. So if you put this stuff on your garden plants, it will do what herbicides are designed to do: kill the plants.

When the magazine first discussed in the issue back in 2008, the only way it mentioned to "protect your garden" was to avoid buying commercial compost unless the seller could vouch for its safety. That wasn't very useful advice for those of us who buy the stuff in bags from Home Depot, rather than directly from farmers. Eventually, nearly a year later, the editors got around to providing some information about how to test bagged compost to make sure it's safe to use. The linked article gives the procedure in detail, but the quick summary is:

1. Mark the bags of compost with numbers or letters so you can tell which bag is which.
2. Take a sample of compost from each bag you want to test. (Brian took the samples by piercing the bag with our soil knife, just the way drug dealers on TV cop shows do when they want to test the purity of a shipment of heroin. His reaction to this was always, "Dude, now there's a hole in your bag and all the heroin is going to spill out," but in the case of our compost bags, a small hole shouldn't be a problem as soon as they remain stacked neatly in the shed.)
3. Mix the sample with potting soil in a seed-starting tray or pot. (Brian used our seed-starting mix instead of soil, since he knew seeds would germinate in that.) Make sure to label each pot or section of the tray with the number or letter corresponding to the bag the sample came from. Also fill a couple of sections or pots with plain soil as a control.
4. In each pot, plant two or three fast-sprouting seeds, such as peas or beans. (We use mung beans, which sprout easily in plain water. In fact, while setting up the test, Brian also started a batch of bean sprouts in a jar, which will confirm right away that our seeds are viable—and also allow us to have Pad Thai for dinner next week.)
5. Put all the pots on a tray and water them thoroughly. Keep them in a sunny window for the next two to three weeks, keeping the soil moist. If the sprouts that emerge from the compost-treated pots are as healthy as the control seedlings, you'll know the compost is okay. If they come out with cupped leaves and twisted stems, that's a sign that your compost could be tainted and probably isn't safe to use on your garden plants.

The "Ask the Expert" column in Mother Earth News also describes a version of this test in which you start the seeds first and then water them with a compost "tea" made by mixing the compost with water. But we find mixing the compost directly into the soil is less hassle, especially when you have multiple bags to test.

We've done this procedure every time we've bought compost for the past few years, and so far, we've been lucky enough not to get a tainted bag. But we aren't prepared to trust to luck, especially where our new fruit trees and bushes are concerned. These plants are a long-term investment, and we're not about to risk losing them to a batch of tainted compost.