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Gardening with Mat Coward Making the most of your compost

IF YOU grow many plants in containers you’ll have noticed that potting compost isn’t getting any cheaper — and it wasn’t particularly cheap to start with. (Apparently, the rising cost of everything is the fault of Covid, Russia, Brexit and possibly astrology, and nothing to do with rampant and opportunistic profiteering by oligarchs, gangsters, capitalists and kleptocrats, so we’ve just got to accept it and definitely not riot like our ancestors have always done in the past.)

Several people have asked me lately whether compost — meaning in this context the stuff you buy in bags, not the stuff you make on the allotment — can be reused. 

The answer is definitely yes, I’ve been doing it for many years, but there are some provisos.

The most obvious is that you don’t want to keep compost from a pot that has been severely affected by pest or disease in case you pass that on to the next plant. I have done that, inadvertently, with vine weevil and the results were not pretty.

I also tend not to reuse seed compost. An open texture is everything in a compost designed specifically for germinating seeds, and that quality is likely to be impaired in used compost. 

Finally, compost which has completely dried out is almost impossible to re-wet without it becoming a soggy, airless mess.

Otherwise, I reuse all my compost — but not without giving it a bit of refurbishment between each job.

Replacing fertility in used compost is the easiest bit. Any kind of powdered or granulated general-purpose fertiliser will do, applied at the rate recommended on the label. 

Dried chicken manure is a popular choice. I prefer an animal-free pelleted product made by the long-established coir specialists, Fertile Fibre (www.fertilefibre.com).

Mix the fertiliser into the compost, using a garden fork, in a wheelbarrow, large tub, or some other roomy container. 

This helps with the most crucial part of compost refreshing: getting plenty of air back into it. Plants need air around their roots, which is why the structure of compost is so important. Compacted compost is of no use; it won’t “breathe” or drain properly.

Take a handful of used compost and squeeze it. If it stays compressed when you release it, a bit like a lump of clay, then the structure needs some attention. 

If the squeezed and then released compost is still open and loose, the way it was when you first used it, then it’s ready for reuse.

To restore the structure you can mix perlite or vermiculite into the compost, both of which are available from garden centres or online. 

Horticultural grit or sand will also do, but being much heavier may cost more if you’re buying them mail order. 

Bark mulch is commonly used for this purpose, but it can use up nitrogen in the compost as it decays so you’ll need to feed more heavily. 

Aim for a ratio of about one part perlite (or whatever you’re using) to three parts compost.

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