Storing potting soil and reusing the next year

Question

Am I being silly for thinking that it's good idea to store the potting soil I've used in my annual pots? As the first frosts are approaching here, we empty our terracotta pots that were filled with summer annuals. In years past we've dumped the soil but I get so tired all the little white flecks surfacing.

This year I'm playing with the idea of storing the soil (in a dry location) over the winter and adding compost / fertilizer to it next spring and reusing. Is this crazy? Should I just purchase new every year?

Answer 1

If you want my two cents... I think it is feasible, but there are some catches that I can think of.

First, you need to know the right mix ratio of compost and old soil. It depends mostly on your compost of course. If you buy new potting soil, you know for sure it is balanced (right ratio of nutrients, organic matter, etc.).

Second, the compost has to be inactivated before using it for new plants. You don't want to much decomposing still going on with new small seedlings in the pot.

I read on internet (which was in Dutch so I won't link) that people have good experience with mixing 10% compost with old potting soil. They test if their compost is suitable (ready) with a simple test using Garden cress. You can do this by putting some of the compost in a (petri)dish and as a control another dish with new potting soil. Then put the same number of seeds in both dishes and see if the number of seedlings match. If the seedlings look bad (for example yellow) compared to the normal soil the compost is not ready yet.

Of course, like Bamboo said, don't re-use soil with disease in it.

Answer 2

You can reuse old potting soil, many people do, even if its left outside in pots all winter, but there are a couple of points to consider. There may be pathogens present in the used potting soil that you can't tell are there, so there is a relatively small risk that your new bedding plants next year might succumb to something. Given you've got more than one pot full of soil to be stored, I wouldn't empty them out and mix the soils together - just leave the soil in the pots, because that way, you're not combining the soils (and possible different pathogens) from different pots. If you grew anything in any of them this year that developed a problem (root aphids, vine weevil, anything where larvae could be in the potting soil), or if any had fungal infections, dispose of the soil from those pots, don't reuse it.

The next point to consider is this - unless your compost is produced using a hot aerobic method, you should not add it to pots. Cold. anaerobically produced compost is suitable for use on open ground and a valuable addition to the soil, but may contain weed seeds and pathogens which could be harmful (or a nuisance from the seed point of view) contained within a pot or tub. Just use a basic fertilizer, a granular or powder formulation,something with an NPK of around 7-7-7, to 'liven' the potting soil. If you haven't emptied the pots out, but leave the soil in them over winter, then give them a good dig around, turning all the soil within over, extracting roots and checking for bugs,then work the fertilizer in when you want to plant next year.