An effective method of sterilizing compost (soil) is to use heat. I am not sure what tools the scientist in question has available, so I will describe three methods.
First method: Use steam. This could be done with a pressure cooker or without. Basically, you are putting shallow pans of soil (no more than 4 inches deep) into a pot of water that you plan on boiling. The pan of soil should be located above the water (not in the water). Cover the soil pans with aluminum foil. Begin heating water container with the lid open just enough to vent the steam so pressure doesn't build up. Once steam is exiting, continue for 30 minutes. Leave foil on until ready to use the soil.
Method 2: Use an oven. Place soil in oven safe pan/container. Make sure soil is less than 4 inches deep. Start baking soil with oven temp around 200 degrees. Bake until soil temp reaches 180 degrees. Use a meat thermometer. Remove, let cool, and keep covered.
Method 3. Use a microwave. Place soil in a microwave safe container that has a lid. Poke a few holes in the lid for venting. Microwave on high for 90 seconds per every 2 pounds of soil. Remove from microwave, let cool, and tape over holes to keep sterile.
Aside from those suggested, there are a number of other sterilization techniques that could be done on soil. Those already mentioned are steam, heat and microwave.
Additionally there are a number of chemicals and gasses available, such as ethylene oxide, nitrogen dioxide, and ozone. These are not all-inclusive, but gasses and chemicals are common means of sterilization.
Theheat mentioned earlier can also be provided a number of ways, including, incineration, flaming, microwave (as already noted) and steaming (already noted)
Additionally radiation (both ionizing and non-ionizing) can be used to sterilize an object. The most common non-ionizing radiation method is Ultraviolet light.
I hope this helps.