Is It Black Gold? Or the Black Plague? Heres How to Test Compost for Weeds and Worries
Q. I've noticed a lot of leaf curl on my tomatoes and pepper plants this year; the first year I've used the free compost that my town makes from all of the yard waste they collect. I've read that some lawn herbicides, two that Dow makes in particular, may not be breaking down during the composting process. What do you think? Is there a lab that will test for herbicides?
---Zach in Phoenixville, PA
A. First, it may not BE the compost. This question came to us in the early summer of 2015, when most of the nation has been having a very challenging gardening season. Where I personally garden in PA—about 30 miles from Zach, who has had the same weather—lots of plants have been severely stressed by our weirdly hot and dry Spring followed by heavy rains. Texas and Oklahoma have experienced dire flooding. And Southern CA continues to be dry as a bone.
So first consider your season. If it's been nothing but cool and cloudy, you're going to see lots of disease. If it's been much too wet, your plants are probably drowning. Too much sun with no rain is just as bad in the opposite direction.
But to answer the question, yes; there are such tests. They can be very expensive; and you need to have some idea of what you're looking for if you want to test for herbicide residues. Luckily, this suspect compost should already have been tested by the township. They should be testing for herbicide residues and toxic metals, like lead and mercury on a regular basis. On a happier note, they should also know the values for the basic plant nutrients…
Typically that would be the big three of "NPK": Nitrogen, Phosphorus and Potassium. But Nitrogen is very ephemeral: "Here today and gone tomorrow", and many soil labs will instead test for "the percentage of organic matter"—especially in a sample of compost. It's a better guide to how basically nutritious the compost is going to be; much more stable than Nitrogen.
That's because the air we breathe is mostly Nitrogen, and every time compost is turned or moved around, some of its Nitrogen is going to escape to play with its cousins in the atmosphere. But that's not a bad thing; frequent turning is an integral part of the process of making really good compost on a large scale.
(Oh, and the Nitrogen that goes into the air? It comes back; it has to come back—"an element cannot be created or destroyed; only changed in form." There are even theories that much inherent soil Nitrogen originated when atmospheric Nitrogen was concentrated by lightning strikes.) Anyway, a basic test for bulk compost should include percent of organic matter, phosphorus, potassium and the pH—a very important number to know. They'll often test for some of the more important micronutrients as well.
Bottom Line: Municipal compost should come with paper, although you probably have to ask for it. (All bulk compost that's sold to the public should have paper, but some smaller suppliers may be many deliveries down the line from the original maker and may not have it handy. Municipalities should always have good paper and you should always be able to easily get it.)
But—will they have tested for herbicide residues?
The odds are good that they will have. But no matter what, YOU can test for them—and test for weeds as well. And you don't need a soil lab. As Adam West might have said while wearing the cape and cowl back in 1966: "Diabolical evil might escape the detection of highly sophisticated scientific apparatus, but even the most fiendish of foes will be revealed with the use of two ordinary plant containers."
"Holy hidden horticulture, Batman!"
Anyway, get a sample of the compost you're thinking about acquiring—enough to fill a couple of plant pots that hold at least a quart or two of soil each. Fill them both with the compost under audition. Just water one of them thoroughly and keep the compost moist. In the other, plant vegetable seeds that you know are fresh and keep that compost moist as well. Beans or peas packed for the current gardening season would be ideal.
Why? Because our listener is correct; there are some relatively new persistent lawn herbicides that are not broken down during the composting process. And they can kill plants by surviving in the finished compost at very low concentrations—a couple of parts per billion is all it takes. Yes--Parts per Billion. There have been cases of "killer compost" that tested clean at the Parts per Million level. The herbicide residues were only discovered when they used more sensitive tests that detected these almost impossibly small amounts. Peas and beans are very sensitive to these plant-killing chemicals, so if they're present, the sprouts will show the signs of herbicide damage.
The unplanted container is to check for weeds. If the compost wasn't piled high enough or turned often enough, it could contain viable weed seeds. You want the 'compost alone' to not sprout anything after being kept moist at room temperature for ten days or so.
And you should always also perform what retired Maryland Extension agent and renowned compost expert Dr. Frank Goin calls 'the duck test'. Sniff the compost under consideration; it should be rich and earthy, with no sour or other bad smells. Look at it; it should be black and free of wood chips and other uncomposted material. And squeeze it; it should be neither dry nor overly wet.
And if it passes the container tests, looks like good compost, smells like good compost, and feels like good compost…
…It's a duck!