As I have a small garden I rely on general purpose compost from the garden centres for troughs and tubs, but does this need to be changed every year? I would think there are plenty of nutrients left after one season, so can I just remove roots etc and re-use? Should I add anything to it? Thanks for any advice.
You can reuse the potting mix. Just remove debris and supplement it with more mix, if needed. I doubt that there is much nutrient value to most potting mixes. I would get some Osmacote or other slow release fertilizer and mix it in before planting.
Thanks. Everything seems to grows pretty well in it the first year without additional feed. I'll take a chance and see what happens re-using the old.
It depends on drainage. In my very sandy, very poor soil, I better add something new every year. In clay soil, nutrients may be better retained. Compost is still good in the second or third year, as far as humus and water retention. So a little fertilizer will make it as good as new. Do not overestimate the nutrient content.
Some numbers: compost may have 0.2-0.3% P and K by dry weight if prepared with yard clippings. One ton of wet compost may then have some
2 lbs of K. Those 2lbs are just what I get from 25 tomato plants and maybe 8 squash plants in one year. And that is not including that most K leaches into the subsoil and that in october I throw away the plants to limit disease. Herbivory can deplete your soil faster than you think, specially with K and fruiting veggies. Of course I add much more than one ton of compost, and I also add tens of pounds of wood ash. I add some phosphate (some P is in the ash, and all micronutrients are in there as well except S), but much less because P does not leach. N comes from a variety of sources such as manure and kitchen scraps in the compost, grass clippings used for mulch, beans crops, blood meal to discourage rodents, etc.It depends on drainage. In my very sandy, very poor soil, I better add something new every year. In clay soil, nutrients may be better retained. Compost is still good in the second or third year, as far as humus and water retention. So a little fertilizer will make it as good as new. Do not overestimate the nutrient content.
Some numbers: compost may have 0.2-0.3% P and K by dry weight if prepared with yard clippings. One ton of wet compost may then have some
2 lbs of K. Those 2lbs are just what I get from 25 tomato plants and maybe 8 squash plants in one year. And that is not including that most K leaches into the subsoil and that in october I throw away the plants to limit disease. Herbivory can deplete your soil faster than you think, specially with K and fruiting veggies. Of course I add much more than one ton of compost, and I also add tens of pounds of wood ash. I add some phosphate (some P is in the ash, and all micronutrients are in there as well except S), but much less because P does not leach. N comes from a variety of sources such as manure and kitchen scraps in the compost, grass clippings used for mulch, beans crops, blood meal to discourage rodents, etc.
Stuart,
I re-use my potting mix in planters year after year after year, but it is NOT "compost", and I wonder if we are writing about different growing media. Are you in the US or elswhere?
The potting mix I use is "soilless mix", a mixture of peat, ground bark, and perlite. The "compost" sold here in garden centers is generally heavy, black in color and becomes waterlogged easily in containers, inhibiting root growth and encouraging plantings to sulk.
I planted my window boxes today, just turned over the mix in the planters to areate and fluff, added timed release granules, set my plants. No water needed as everything was quite damp and showers are forecast for the forseeable future.
I do AGRESSIVELY recycle planting mix. Astonishingly, I've cut my purchase of "Pro-Mix" from 2 bales a season ( US 29$ each) to one US 7$ bag. The new mix is specifically for starting seeds, cuttings and divisions when I want to be sure I'm not harboring mold, mildew, disease, weed seed, or pests in the starter mix. All the recycled mix goes in annual container plantings, the tubs, hanging baskets, pots and decorative "stuff".
When I decide the containers are " Overly Contaminated" with too many impatiens, allyssum, pansy/viola or seeds, they get dumped on a perennial garden, worked in as I'm dividing/planting, and I deal with the "volunteers" on a laissez -faire basis. This ensures that my sandy riverine garden soil gets some peat and perlite now and then, and that is a good thing.
I used to throw ALL my used planter mix in my "compost pile" every fall, and start fresh every spring. That got VERY expensive... and now that compost pile hosts the lushest, most pestilent, Blackberry crop from Hell. If it's THAT good for growing, I'm not giving it over to Bramble cultivation.
2 cents worth becomes a novel.Good growing to you.
Sue Western Maine
Over time it turns to muck and smothers the roots. Make sure you have plenty of coarse sand or pearlite to open up some pore space and let air in.
Thanks to everyone for the replies. I'm in UK where "general purpose compost" is described on the bag as "peat reduced". Reading on (and maybe I should have done that before posting the question) it says it contains enough nutrients for the first 4-6 weeks only, after which you should go and spend more money on liquid fertilisers, so it looks like this stuff is "soilless". I'll try recycling with the time release granules. We do get a lot of ants in our window boxes so it may have to be dumped anyway.
Don't worry about what you didn't read. Soilless Mix simply provides a medium where nutrient delivery can happen, where roots can have purchase, and therefore support growth. In the greenhouse environment, watering is done with a soluble fertiliser that nets out to 20-20-20 NPK, kickstarting the plant material to pleasing maturity early enough so you'll pay good money to buy it. There is usually enough nutrient in the mix to carry your purchased plants for much of the planting season, but you'll have to supplement the soilless mix with additional NPK for best performance.
Gardening does test the wallet, by design.
Sue Western Maine
Thanks, Sue. Much appreciated.
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