Some time ago, I posted about horse manure, saying that it was possibly the best food you can feed your worms. I am having to rethink that, not because of the horse manure itself, but because of what else might be in it.
I had found a stable near my home, and was given permission to take all of the manure I wanted. I was feeding a lot of it to my worms, and mulching my garden beds with it. My vegetable garden went crazy. My worms enjoyed feasting on the manure.
Then I brought home a batch of manure that my worms would not move into. Not only were the worms not happy with the manure, my worm population seemed to be declining. In one of my bins, I had a significant die-off.
I wanted to figure out why. I posted on a worm forum, and people responded with different possible culprits. Dewormers were one possibility, but the research I am familiar with shows that composting worms aren’t harmed by dewormers. Also, the worms in my garden beds didn’t seem to be affected. Another possibility was salts in the manure. The weather had been dry for quite some time, so any salts in the manure would not have had a chance to leach out. I decided that that was the best guess.
After a good, hard rain, I picked up more manure. The worms responded to this manure the same way. The worms would not move into it, and my worm population was declining. Later, when harvesting some of the castings, I found pockets of manure that had not been processed by the worms.
I ran a test for herbicides by planting pinto beans in a mixture of starting mix and manure. The pinto beans showed no signs of herbicide damage. In fact, the pinto beans grew much better in a mix of potting soil and manure than in potting soil alone.
Later, when I was growing tomato starts for my garden, I added some worm castings to the starter mix. My tomatoes showed obvious signs of herbicide damage. At this point, I decided to stop using horse manure in my worm bins and garden beds altogether. It just isn’t worth the risk.
It is too bad, really. Untainted horse manure is wonderful worm food, and a great mulch in the garden. I could get all the horse manure I wanted. But I have decided to stop using it. I will be focusing on other food, bedding and mulch sources.