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Sterilising and re-using potting compost


I have recently acquired an electric soil steriliser (and no longer feel guilty about using it now the solar panels are up and running image ) . My question is - could I just keep using my potting mix year after year by adding, say, garden compost and nutrients and then sterilising it? I can't think of any reason why not - over time I can see that the structure of, say, the coir or leaf mold in it may deteriorate over time - but if I keep adding fresh ingredients that should solve that. Any thoughts?


  • PalaisglidePalaisglide Posts: 3,414

    Why sterilise potting compost? I sterilise seed compost if there is no fresh to use, seed compost does not need a lot of nutrient , in fact seeds will set in washed sand then pot on to a compost with nutrient.

    Old potting compost gets spread on the beds as a mulch as the nutrients are used up, fresh compost mixed with grit is then used to repot plants with a hand full of home made compost, some bone meal and depending on the plant some granular fertiliser. Using old potting soil over and over seems like false economy, we want  plants to thrive, they are often expensive both in cost and labour so why chance it.

    Soil is full of microbes that help in breaking down the soil excreting nutrients and in some cases symbiosis with the plant roots, do you really want to kill them off?


  • Well Frank - I'm talking about a LOT of potting compost - and contrary to what many people say, my experience of 20 year with raised bed gardening in a restricted space tells me that the soil level in beds DOES go up with the addition of manure and compost. Every couple of years I have to give several bags of topsoil away! I've taken to making my own potting mixes but worry about the build up of fungal diseases if I re-use it - and I can't bear to just throw it away!. I made my first big batches of home made potting mixes last year and sterilised them - and I have to say my baskets and pots have done amazingly well in them. I used a mix of some of that spare topsoil, old potting compost and garden compost with Blood fish and bone. This year I want to try a soil-less mix.

  • PalaisglidePalaisglide Posts: 3,414

    Steve, in all my years of gardening you are the first to say you have too much soil, my experience from a small holding during wartime to now is never enough. 30 odd years of mulch and old potting compost have not raised my beds very much, the raised beds I made seemed to drain soil, they were always being topped up. Dad only sterilised soil for seed trays, ordinary garden soil in a bucket sitting on the greenhouse stove, it worked.

    Using a lot of pots containers and trays there is plenty of old compost so next to the compost boxes there is an area it gets dumped mixed with some of my compost and bone meal then left to weather and the worms, it becomes the base compost for pots after sitting around resting a couple of years.

    Guess we all have our own ways and who is to say they are wrong, good luck with what you decide.


  • Hi Frank - Yes - I know - *everyone* says it and I don't doubt their word - I must just be lucky! Never any shortage of soil for a new project. I only have four 16ft x 4ft raised beds in the veg garden. I double dig each bed ONCE every four years when it comes round to potatoes on the rotation, and add manure (used to be just garden compost, but my crops have improved dramatically since I started buying in manure 3 years ago) The other three bed get nothing except maybe a bit of compost and FBB, and just raked up with an excellent and ancient three-tined cultivator that I inherited from my father-in-law. EVERY time, the potato bed overflows with soil and I have to bag it up and put it on one side! First few years I thought it would find it's level - but I've been on this ground since 1984 and it shows no sign of slowing down (unlike meimage  ).  I thought the home-made-potting compost would help use it up - but while last year's mix was reasonably successful, it occurred to me that, as I re-cycle the mix year on year it would gradually have more and more soil in it and upset the balance. So today I've made my first soil-less mix - 2 parts garden compost to one part old potting mix (which already has plenty of drainage in it) with a cupful of FBB and a cupful of worm casts from the wormery to each 30 litre batch. Put 60 litres through the steriliser today and it seems to have a pretty good texture. Incidentally - another good reason for me to sterilize is that I make my garden compost by COLD composting - it's supposedly a better quality compost in terms of nutrients but it does have a lot of viable weed seeds in the finished product, couple of years back I spent a good part of the summer WEEDING my pots and containers image  - the steriliser takes care of that, plus any build up of disease organisms. Have a good season!


  • Hostafan1Hostafan1 Posts: 25,196

    In 40 odd years of gardening ( 25 of which professional ) I've never sterilised soil or compost,not ever felt the need to.

    Each to his own I suppose.

  • PalaisglidePalaisglide Posts: 3,414

    Steve, there are no right or wrong ways in gardening, what works for one will not for others. We learn what works over time, show me a gardener who never made a mistake I will show you a liar. Over sixty years man and boy taught by a real gardener you get to know your methods, and tend to stick to them. I hot compost having the room and place in full sun to make it work, probably why there is no need to sterilise.

    If it is working for you stick with it and good luck.


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