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Compost is the ideal fertilizer.

Every gardener knows: the compost is full of microorganisms. They turn green waste into fine, nutrient-rich soil. However, germs that are not welcome in the garden can also lurk in them. Viruses and fungal spores, for example. When our plants come in contact with it, they can get sick. The growth quickly leaves a lot to be desired and some specimens even die. So that you really only experience the benefits of this natural fertilizer, you should sterilize compost beforehand.

sterilize compost

Microorganisms also need good living conditions in order to survive and multiply. This includes, among other things, a certain comfort temperature. If the actually prevailing temperature deviates greatly from the ideal, it becomes critical for them.

  • thermophilic germs also do not like heat
  • from about 75 °C it gets too hot for them
  • their number is rapidly declining
  • at 100 °C they have little chance of survival
  • the duration of the heat exposure is crucial

Raising the temperature to unbearable levels for a period of time is therefore a common way to kill harmful organisms effectively and without chemicals.

Commercial compost

You do not have to sterilize the commercially bought compost, as it is already offered (almost) without harmful germs. In professional composting plants, the temperature is sufficiently high over a long period of time to kill most harmful germs.
However, this heat not only kills the harmful microorganisms, but also the beneficial ones. However, these useful tiny creatures would do the soil life in the garden bed good. Sterilized soil can therefore be called dead material in this sense. It has its advantages as seed soil and potting soil for young plants that are not yet resistant.

Own compost heap

There will always be germs lurking on the specially prepared compost heap. Even with careful disposal of all diseased plant parts in the residual waste container, this can never be completely avoided. In addition, the domestic decomposition process rarely reaches temperatures above 60 °C. But that is not enough to sterilize the decomposing material. If you need germ-free substrate for your plants, you should at least prepare this required amount accordingly. You don't have to buy any special equipment for this, a standard household oven is sufficient. The instructions in this text will guide you through the work process step by step.

tip: You should also sterilize purchased compost that you have stored in an open package for a long time.

Germination test with cress

Before you undertake this tedious work, you can easily test whether this is even necessary. Germs are difficult to spot with the naked eye. Maybe your garden compost or the soil you bought is not as heavily contaminated as you suspected. The so-called cress test is particularly worthwhile if you want to treat larger quantities of compost.

  • fill a pot with compost
  • moisten slightly
  • sow cress on it
  • bright and warm

If the cress germinates within three days and the seeds form white, healthy-looking roots, the germ load is low or acceptable. Otherwise, you should sterilize the soil.

Sterilizing compost: instructions

If you want to sterilize the soil from your compost, you can easily do it at home in just a few steps.


With its heat, the oven is ideal for removing harmful organisms from compost. However, its capacity is limited. If you only need small amounts of compost to sow seeds in a few pots or to repot a flower, its capacity is more than enough. Of course, you can also sterilize a larger quantity in several passes. The instructions below include 6 steps that end with germ-free soil.

  1. Preheat the oven to 200°C top and bottom heat.
  2. Spread the compost flat in a container that must be fireproof.
  3. Moisten the soil a little.
  4. Place the container in the preheated oven. It should remain in it for at least 30 minutes at the same temperature.
  5. Stir the soil a few times in between.
  6. After heating, allow the contents to cool completely before using on your plants.


Because a microwave works at high speed, it often has to serve as a substitute for an oven. This also applies to the sterilization process. The instructions for this are very short: The moistened earth is heated in a microwave-safe container for about 10 minutes at the highest level. Here, too, the contents should be stirred a few times in between.

White dots after heat treatment

After pitch-black compost soil has been heated in the oven or microwave, it is often streaked with white dots. The cause is to be found in protein-containing insect larvae and worms. In the heat, the protein changes, becomes white and only then becomes visible.

  • the white dots are harmless
  • the insects and worms are killed by the heat
  • Earth can be used without restriction

Heat very large amounts

In a garden where all plant matter is consistently composted, a lot of compost will accumulate over time. The desire to sterilize the entire quantity before using it as a fertilizer or soil admixture is understandable, but not always sensible. The living compost fertilizer sustainably improves soil life. Healthy plants that are optimally cared for usually survive the harmful germs that may be present.
However, if a large amount of compost has to be freed from harmful germs, the oven is far too small for this, and the microwave even more so. If you would like to regularly reduce harmful germs in the compost, it may be worth purchasing a so-called earth steamer.

  • suitable for the private sector
  • about the size of a wheelbarrow
  • can also be used for larger amounts of compost

The all electrically operated models are offered with different capacities.

tip: Some large nurseries have their own soil damping equipment. Ask if they would sterilize your compost in it. Although there is a price for this, it is an alternative to owning your own device if you only need it occasionally.

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