All our plants have one thing in common: they need soil to grow. It would be a shame to buy new soil every year, when you can spice up the old one with a few tricks to make it fresh and nutrient-rich again.
In this small but nice article we have a few ideas for you, what is possible with the old soil.
Used soil – ripe for the trash?
First of all, the most important info: If you had big problems with fungal diseases or pests that overwinter (or their larvae) in the planting soil, unfortunately, you have to dispose of the soil. The risk is too high that the same problems will arise again and cause trouble.
Otherwise, however, it would be a waste to simply throw away “healthy” soil, because you can do some great things with it. Even soil that is already very old and completely exhausted, can still be used further.
Old potting soil in bags
Often, in addition to the soil in the tubs and beds, there is some left over in the bag that hasn’t even been used yet. If it was reasonably well protected from wind and weather, you don’t have to worry about it and can just keep using it next year. It’s best to keep it covered during the winter in a sheltered place, such as a shed, under a bench, or some other suitable place.
If it is growing soil, you should be a little more careful. If small insects have crept into the packaging over the winter, they can interfere with seedling emergence. The small seedlings are much more sensitive than already well-established plants and are ideally raised in sterile soil. Fungus gnats, which lay their eggs in the soil, are particularly annoying.
To make the soil germ-free, it must be heated. For growing you usually need only small amounts of soil, so heating in the oven or microwave is feasible. To do this, put the soil in heat-resistant containers or on a baking tray, moisten slightly and heat in the oven at about 120 degrees for 30 minutes, in the microwave about 10 minutes at 600 watts is enough. Stir a little in the soil from time to time, so that the temperature is evenly distributed.
Check beforehand that no larger insects have overwintered in the soil, for example woodlice or earthworms. These would also die due to the heat treatment, which is not so nice and also not very appetizing. If you are unsure, you can also roughly sift the soil beforehand.
If you have a garden or even a greenhouse, you can of course use the old soil generously for mulching or to loosen up the empty beds in the greenhouse.
If you don’t want to use the leftover potting soil from bags, you can also spread it on the compost pile, if you have one available. And if you can neither compost it nor have a garden, you can either dispose of it in the organic or residual waste garbage can or even spread it under a nearby tree or bush with a clear conscience.
Old pot soil
It is quite similar with the soil from the tubs. If it is already very old and sandy (i.e. extremely fine-grained and dry – but it takes a few years to reach this state), you can compost it or use it for mulching. For plants that are sensitive to cold, you can use it to form a mound around the trunk to provide cold protection. Otherwise, the easiest way is to collect the soil from the tubs in a large container and, if necessary, store it protected and covered during the winter. If the pots are frost-proof plastic pots, the soil can also be covered and stored in them over the winter. By the way, you don’t necessarily have to remove old plant and root remains, just see how coarse they are and decide as you like.
In spring you should then check whether a few winter guests have crept in. If everything looks fine, you can use the soil again. To refresh it with new nutrients, mix it with compost or new potting soil. It depends on where the soil will be used. If you want to grow high-yielding vegetables, use less old soil and more fresh soil. Humus from worm bins or a mature bokashi fill are also suitable materials to mix with old soil.
Mixing with compost soil can restore nutrients to old potting soil.
I have 30 years of experience and i started this website to see if i could try and share my knowledge to help you.
With a degree a Horticulture BSc (Hons)
I have worked as a horticulture specialist lead gardener, garden landscaper, and of course i am a hobby gardener at home in my own garden.
Please if you have any questions leave them on the article and i will get back to you personally.