Our plants have their feet in a layer of mixture, and are therefor in close contact with a number of bacteria, insects and fungi. Some of these can be rather harmfull to the orchid and a pre-emptive strike can solve this. It's naive to think that all of them can be killed: as soon as the compost comes into contact with air, pots or our fingers we contaminate it again, but these are generally speaking organisms that belong in the soil - they're in the environment we create.
Not all soilmixes are candidates for sterilisation. The higher the content of inorganic materials (perlite, clay pebbles, vermiculite, …) the less you should worry about infestation. The component that harbours and feeds pathogens is traditionally the organic part of the soil. In most cases we get the organic material from garden centres, a compost heap in the yard or from a trip to the forest. All three of them can contain problematic organisms, so you'll have to use some insight:
- Fresh garden centre bags are low in risk, contamination normally occurs when the bag is a number of months old (it was stored in the warehouse), if you left the bag open in the shed or when it has been stored under exposure of rain.
- Sphagnum peat is a potential origin for bad organisms (notably Botrytis) but is difficult to sterilise. Always buy/store pure peat as dry as possible.
- soil from a compost heap can be used unsterilised for garden terrorchids
- soil collected from a forest should always be sterilised

The basic tool of every kitchen. Effectiveness depends on the humidity content and depth of the soil. To achieve best sterilisation, spray the soil two days before sterilisation with a spray bottle. Bacterial spores are very resistant to dry heat, by spraying the soil they'll germinate and become more sensitive to heat. Spread the soil evenly on an oven tray to a thickness of maximum 2 cm, spray with rainwater to moisten it and cover with tinfoil or plastic. Leave the tray at roomtemperature for 2 days, but keep it damp - spray when necessary. Place the tray(s) in the oven and cook for 3 hours at 200°C. Let it cool inside the oven and it's ready for use. The downside is that the oven needs to be thouroughly cleaned afterwards.

A microwave oven is in reality a radiowave-emitter that broadcasts at 2450 MHz. The water molecules absorb the energy causing everything that has water in it to heat up. The first microwave ovens didn't have a turntable and only heated local spots on the dinner plate. This is the reason why for this technique we also need to dampen the mix: dry pockets can escape radiation. Contrary to the oven, there's not a restriction to the thickness of the layer of soil. Mix you organic component while spraying it with water until it's damp. Leave the mix for a day to let the bacterial spores germinate. Then pour the soil in a vessal (which can be put in the microwave) and put a cover on top. Don't create an airtight volume, the vapour will expand and needs to escape. Put in the microwave oven at half the capacity for 10 minutes, take it out, mix with a spoon and repeat this 3 times. The smell is awfull but everything will be killed. Clean the microwave afterwards.

Place the damp soil inside small containers and place them in a layer of water in the pressure cooker. Pressure cook for 20 minutes, allow everything to cool before taking the soil out. This technique is fastest but leaves the most amount of washing up.

The techniques described above will kill 99.99% of the organisms in the soil, but the volume is very low and only fit for the organic components of a mix. The soil in the garden is a huge volume but a number of infections can occur - most notebly that of nematodes and weeds. This technique is very low-budget but gives you the chance of getting rid of insects and pesky weeds in your soil without having to dig out and refill everything. It uses the energy of the sun, so apply or plan for it in the hottest of Summer.

1. remove the largest vegetation and debris
2. water heavily during the night, try to get water down to 30 cm depth (which is very deep)
3. cover the entire surface with clear plastic sheets, hold everything down with bricks and seal the overlapping edges with duct tape
4. wait 4 weeks. You'll see weeds pop up, and most important: you'll see them die. The combination of heat and zero airflow will kill everything.
5. after 4 weeks remove the plastic and turn the soil to a depth of 30 cm.
6. water again as in step 2
7. put the cover back as in step 3
8. wait 4 weeks

after this treatment, everything on the plot will be killed: nematodes, earthworms, plants and most of the micro-organisms.