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I grow potatoes and tomatoes and cucumbers in pots and grobags abd potato bags and hanging baskets every year .At the end of the season I am left with half full bags pots etc of compost which I usually put on the garden.I am struggling to find more room for it and wonder if it can be topped up and reused for these crops. i do not have any blight problems todate. There is also the question of cost I grow large amounts and buy lots of bags of compost
Some do what you say-I am reluctant because you don't know what is lurking in the compost that could attack or infect this years crop
Some use it on the bottom and top up with fresh-I would rather use it as a soil conditioner.
The choice is yours-but opinions differ
I agree with Geoff, but if you do want to try it, mix some general fertiliser in with the old compost, something like blood, fish and bone. If you use an inorganic mix, check that it includes trace elements, as there may be few of those left in the old stuff and lack of trace nutrients could cause stunted growth or other similar problems. I've done what you are contemplating before and got reasonably good results, but prefer to put the spent compost on the garden; With clay soil, I never have enough of it, even though I have a relatively small garden and buy 30-40 bags of compost every year. Clay just eats compost!
I have an old box around two boards high next to the compost bins (also wood) this years spent compost from the pots and tubs goes in there after bagging up the stuff that was in the box to use again.Mixing in some of my fresh compost from the heaps and a little granular fertiliser I leave it to weather until next year even put a few strawberries on it, yearlings to grow on, never had any trouble that way.We use the garden soil every year so why not rest and then reuse the potting compost?
I really wouldn't re-use compost for crops in the same family (like tomatoes/potatoes). Not only might there be diseases lurking there, but the compost will have given up most of the nutrients required by those plants. Even in open soil, most people avoid re-planting the same crops in the same place each year, and compost in pots runs out of nutrients faster than soil/compost in garden beds.
But you could try try some different crops or plants in the old compost (salad leaves, herbs, baby carrots, flowers, bulbs).
I grew potatoes a few years ago, in coir compost that is excellent at retaining moisture. I use it to plant up tubs of annuals and herbs, with added fertiliser, or as potting compost, but I would get fresh for any more potatoes or tomatoes.