But the volume of compost has shrunk to 1/8 of the original
volume. The compost now has twice as much weed killer (per volume) as the
> starting material. (The shrinkage is not exaggerated, either, based on my > experience.)
I don't want to start a flamefest, but I don't think I have ever lost anywhere near that much volume. Probably no more than 1/3 lost, 2/3 kept. I use, almost exclusively, grass clippings and tree leaves with some kitchen scraps for good measure. I turn perhaps twice a week (depending on what the internal temps are), watering each time.
Does anyone have a link to a study on such shrinkage?
Perhaps 1/8 is a bit low. However, my compost regularly shrinks in volume to less than 1/4 of the original amount. I think that is probably because of the large amount of shredded leaves that goes into each batch and the fact that the curing pile is regularly worked by redworms. Many of my batches also contain a large amount of chopped cornstalks. (Both of these materials really compost down in volume a LOT.)
Here's a study using various material combinations that shows reductions in volumes varying by mix, with yeilds as low as 17% (reduction in volume as much as 83%):
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basic problem with highly persistant herbicides in the composting process is that the volume reduction of the composting process equals or exceeds the breakdown percentage of the herbicide. Thus the resulting compost is still toxic to some plants. Clopyralid is the most problematic herbicide and is widely used on lawns and hayfields. Picloram and triclopyr can also persist through the composting process. Clopyralid can persist even in composted manure.
Potatoes, tomatoes, peas and beans are the veggie garden plants most likely to show signs of herbicide toxicity at low doses. (Which is why I brought up the issue in this thread.)
I do not bring in any grass clippings from outside my property and avoid using commercially made compost in my veggie garden (though I sometimes run out of homemade compost and use bought compost on my squash beds only).
There it is. I don't use a curing pile. Until recently, I've not had enough material to wait. I still don't have enough room. You are seeing further shrinkage in the pile that I experience on the beds (but can not measure). I try for a very hot compost pile, turned often (I watch for a temp. plateau and turn when it drops 5 deg.), but applied immediately afterward. This kills 'enough' of the seeds / diseases while leaving a generous portion of 'the good stuff' in the mix.
Good link, BTW. Thanks.
I do use 'outside' grass clippings from two sources. 1) The alley that runs behind my house and 2) neighbors whom I KNOW apply no chemicals at all to their weedbeds, er, um, yards. This spring, venturing further down the alley, I found the mother-lode of dandelion patches ... where the grass grows tall, the dandelions do, too! Can you say "salad"? MMMMMMMMMMMMMmmmmmmmmmmmmmm!
Usually, finished compost assays out at about 1-1-1. As a fertilizer, it's pretty lame. It's more impressive for its trace minerals and changes to the soil structure that allow it to handle water better and for providing food / habitat for a number of soil denizens.
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