Garden School:


Garden School:
Teaching this week: Rose pruning (as always!) and leaf mold.

Wednesday, 3 December 2014

Compost contamination

Time for a grumble now: shop-bought compost.

I get through a lot of compost: propagating seeds, potting up the seedlings, and for potting on plants while they are growing, ready to be sold. People often ask me which compost I use, hoping to find a trade secret.  Alas, no secret, I buy the cheapest multi-purpose I can find, in the biggest possible bags.

The best value is usually to be found at Homebase or B&Q,  and for some time the B&Q own brand has been the best.

Gigantic 125 litre bags, tightly packed, and although I certainly can't lift them, I can "walk" them on their corners for short distances, and I use my sack-barrow for longer distances.

On buying them - about £7 each this year - I ask a member of staff to load them into my car, and when I get home I can pull and tilt them out of the back by myself.  Their heaviness means there is a slight penalty in buying the biggest size, but they are the best value: and often you can get a better price for buying three at a time.

To use them, I slit the top as close to the seam as possible, and about half the time they open up really easily with a "lip" of plastic which prevents the contents spilling out all over the floor. The other half of the time, the seams are welded too tightly, and you end up with - you've guessed it - the contents spilling all over the floor. But that only affects the top inch or two...

My procedure is to scoop the compost out of the bag into a bucket, then take the bucket over to my potting bench and sieve it using a coarse ¼" mesh. This removes all the contaminants, and leaves me with three piles: firstly a big pile of lovely fine sifted compost, ready for use: secondly a small handful of assorted rubbish, and thirdly a big double handful of compost "nuts" which are too large to go through the mesh. They are usually rounded woody bits, and they are perfect for top dressing after potting.

"Top dressing?" you say.  Yes, I find that a layer of compost nuts on top of the pot prevents a crust forming, making it easier to water them for months afterwards - the water goes straight in to the nut layer, then slowly soaks in, instead of running off the top.  It's also easier to get weeds out!

So what about the rubbish? Here you are, the contents of one single bag of Verve compost, patiently collected as I sieved each bucketful in turn:

Going round clockwise from top left - a large assortment of bits of plastic film, various colours and thicknesses.

Then a stack of little sticks - not exactly unexpected, but too big to want to leave them in my compost, most of them are 2-3" long (drat, should have put in a ruler for scale, sorry - it's one piece of A4 paper, if that helps). Under those, a selection of stones, all too large to go through the sieve.



Then, bottom right, some more wood, this time chunks rather than twigs. Bottom middle, glass.

Yes, GLASS. One very large clear bit, and several sharp slivers. This is why I always wear gloves when sieving my compost, and this is why, once sieved, I can pot up bare handed without accidents.

Just above the large piece of glass is a section of hard plastic with very sharp edges - glad to have got that one safely out of the way - then to the left, more assorted plastic shards.

Last year I went on a series of trips to various recycling plants, and I was intrigued to hear that at the garden waste processing plant, they are not allowed to bag up and sell the compost to the public, because they can't guarantee that it does not contain contamination - of the sort mentioned above. So they sell it to local farmers at a low price that just covers the cost of processing. Thus, incidentally, reducing our council tax and making that part of the process self-supporting, which is a very sensible way to run it.

But I am staggered (yes! I'm sitting down, but I'm still staggering!) that the council are not allowed to sell their material, yet B&Q are allowed to sell theirs, which contains a whole pile of non-organic matter.  Every single bag of bought compost I have ever had contains contaminants at about the level shown above. I always sieve out the nasty stuff, and by the end of a bagful, I always have about this much material.

To be fair to B&Q, they do print a warning: "wear gloves while handling this compost" on the bag, but to be fair to the users, I do think that as we are paying for this material, we should not expect it to contain glass, sharp plastic, and quite such a lot of plastic film. Wood and stones I can accept, but wherever they are getting this stuff from, I don't think much of their quality control.

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