The compost test: preliminary results

About a month ago I posted about how we were planning to test the compost we'd bought at Home Depot to make sure it wasn't herbicide-tainted "killer compost." Well, the preliminary results are in, and I'm afraid it doesn't look good.

First, let's take a look at the control pots. Brian planted these so we could compare a healthy plant grown in normal soil with one grown in potentially harmful compost. And just to give us the broadest possible basis for comparison, he planted control seeds in three different growing media. The top pot in the picture has straight-up garden soil, the middle pot has a mixture of regular soil and seed-starting mix (the same stuff we're using to grow our garden seedlings), and the bottom one has unadulterated seed-starting mix. And here was our first surprise: the bean seeds planted in the seed-starting mix didn't actually start. They didn't come up and later wilt; they simply never sprouted at all. This isn't the best possible endorsement for the seed-starting mixture, given that the seeds we used are mung beans, which will normally sprout in a couple of days in just a jar with a bit of water. But on the other hand, the seed-starting mix does seem to improve the performance of plain soil; with three seeds planted, we got two healthy seedlings in the soil/seed starter mixture, and only one in the regular soil. So this part of the experiment suggests that our decision to mix the seed starting mix with regular soil for starting our garden seedlings was probably a good call.

The test results on the compost itself, however, were much less satisfactory. This picture shows the seedling tray we used to test the compost, with one cell for each bag of compost we bought (mixed with the seed-starting mix). As you can see, only about half the cells produced healthy seedlings. In one pot (second row, middle cup), the seeds simply didn't come up at all—not even a hint of a shoot poking through the soil. In a couple of others (like the third row left), they started to sprout, but never actually grew into plants. And in a couple more (like the top right and third row middle), the seedlings came up normally, but then started to droop and eventually withered away completely. So basically, of the twelve bags of compost we bought, only six produced seedlings as healthy as the control batch. The other six I would definitely be unwilling to use—especially on our new fruit trees and bushes, which represent a long-term investment.

Now, Brian isn't entirely convinced by these preliminary results. Being a scientist, he agrees that the results do support the hypothesis that the six bags of compost are tainted, but he thinks there could be other ways to explain the outcome. For example, he thinks that it's possible he simply forgot to add the seeds to the one pot that never sprouted at all, and he thinks the seedlings that came up and then withered may be explained by the fact that the cups they were in simply didn't have enough soil to support them. So he has designed a new experiment to test this possibility, adding more dirt to all the pots and planting three new seeds in each of the ones that didn't produce seedlings on the first go-round. If the withered plants perk back up, or if the previously empty cups manage to produce healthy seedlings on the second attempt, he thinks that will indicate that the corresponding bags of compost are safe to use. I'm not optimistic, but I figure it can't hurt to carry out the experiment.

My bigger concern, though, is what to do about these six unusable bags of compost (assuming they do turn out to be unusable). First of all, how do we dispose of them? The bags have all been sampled from, so I don't think we can return it to the store; they're almost entirely full, but I definitely don't want to pass them on to anyone else on Freecycle; and the contents are organic material, but it sure isn't safe to dump it in our compost bin. So what do we do—just put these full bags of compost out with the trash? If so, do we need to put a label on them that reads, "WARNING: TAINTED COMPOST. DO NOT USE," to ensure that no unwitting person scavenges them off the curb?

And second, how do we replace them? It's mid-March now; our trees and bushes will most likely arrive in early April. We don't really have time to buy more compost and test it by then. Of course, we could just use the bags we know are untainted on the new trees, then buy and test more compost for use in the garden beds, most of which won't be planted until early May. But how many bags should we buy? Do we buy six more to replace the six we can't use? Or should we buy a dozen more, on the assumption that 12 bags will yield about 6 usable ones? But then, what if the results of the second test are even worse than the first and only four, or three, or none of the dozen bags are usable? Would we be better off just going with no compost at all in the garden, aside from whatever we can manage to abstract from our one dinky little household bin?