Your duration for sterilizing soil in a microwave is much different for whether it is in a container or a bag. If I put two pounds in a bowl, it is 90 seconds. In a bag it, is longer. Why the difference? I will be using a bowl. Also, no thermometer needed?
You can use a meat or candy thermometer to check the temperature.
No matter the container, bag or bowl, start with the lesser time of 90 seconds each time until you reach the temperature of 180 degrees.
Since microwaves vary in wattage, this will give you a safe way to heat the soil.