I have been researching in to Victorian and Wartime garden practices when I stumbled across methods they used for sterilising soil/ spent compost to re-use for seed sowing compost . One of the biggest expenses of having an allotment seems to be the price of compost for seed sowing .
Cutting the story short basically you have to boil the soil for 30 minutes . I use a 20 brick ‘rocket stove’ at the allotment powered by dried weeds , wood and old newspapers but you can (and I have ) use one of those portable gas stoves that take gas cartridges and an old non-stick pan and lid.
The method is to fill the pan with soil / spent compost up to an inch from the rim . Boil some water and add around 500ml of boiling water (it needs roughly to be the amount that would fill the empty pan up to a depth of one inch ) . Add the boiling water to the soil/spent compost . Put the lid on the pan and simmer for 30 minutes. Once cooled the sterilised mixture is ready to use for sowing. Please note this is a seed sowing mixture and will contain no nutrients as seeds do not require nutrients to germinate . Use your sieved home-made compost for potting on. Also please don’t try this indoors – the mixture has an unpleasant odour that your family wont thank you for.