A couple of years ago I started an experiment into this with growbags, whose contents, I suspect, have more or less the same composition as potting compost. Unfortunately I wasn't able to finish it but I did make an initial visual assessment of the compost, including photos.
I could look out my notes if you want, but some brands certainly contained plastic and partly-rotted wood.
I see I've been very lucky - I've been buying Multipurpose with John Innes (Westland, I think) from our local garden centre and it's been perfectly fine. The plants seem to like it too. I decided against peat free compost after reading the test results on the Which Magazine website last spring. (Hmm, just checked, in the 2015 results they recommend two peat-free composts for containers and say "there are now a number of good peat-free composts available".)
I take no notice of Which, last year they said Verve was the best compost around it was the worst I have ever ever bought. I have never seen so much rubbish in a MPC before.
There is also the problem that it is not sterilised. Lasy year the forum had a lot of queries over random seedlings that were coming up out of compost. We are told that commercial composting gets hot enough to kill seeds and bindweed roots etc, but this appears to not be true. If I have spent four quid or so on a few seeds, thats what I want as seedlings, not thistles and bindweed.
This is one of the growbags from the experiment. I fished out the rubbish that was within easy reach of the three holes I cut for the tomatoes. Bits of white plastic bag, blue stiff plastic, painted plaster, bit of a plastic plant pot, piece of bent wire and several pieces of unrotted wood. And twigs. This was the worst but not the only offender. The Levington growbags were the best from this point of view, and also produced the healthiest-looking plants.
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I'll stop buying it when there's an alternative which is of a similar price and works as well .
I found a shard of glass in a bag I'd got from a large national retailer.
Not a good thing when you've just had surgery due to an accident with glass
Buy mine from a local garden centre (own brand) and have only ever found small bits of wood. The compost is really good quality.
A couple of years ago I started an experiment into this with growbags, whose contents, I suspect, have more or less the same composition as potting compost. Unfortunately I wasn't able to finish it but I did make an initial visual assessment of the compost, including photos.
I could look out my notes if you want, but some brands certainly contained plastic and partly-rotted wood.
I see I've been very lucky - I've been buying Multipurpose with John Innes (Westland, I think) from our local garden centre and it's been perfectly fine. The plants seem to like it too. I decided against peat free compost after reading the test results on the Which Magazine website last spring. (Hmm, just checked, in the 2015 results they recommend two peat-free composts for containers and say "there are now a number of good peat-free composts available".)
I take no notice of Which, last year they said Verve was the best compost around it was the worst I have ever ever bought. I have never seen so much rubbish in a MPC before.
There is also the problem that it is not sterilised. Lasy year the forum had a lot of queries over random seedlings that were coming up out of compost. We are told that commercial composting gets hot enough to kill seeds and bindweed roots etc, but this appears to not be true. If I have spent four quid or so on a few seeds, thats what I want as seedlings, not thistles and bindweed.
This is one of the growbags from the experiment. I fished out the rubbish that was within easy reach of the three holes I cut for the tomatoes. Bits of white plastic bag, blue stiff plastic, painted plaster, bit of a plastic plant pot, piece of bent wire and several pieces of unrotted wood. And twigs. This was the worst but not the only offender. The Levington growbags were the best from this point of view, and also produced the healthiest-looking plants.