I have problem with mold in soil. I’m trying to grow a baby lemon tree indoors this winter and I was wondering if you had any advice on how to kill this fungus. Is sodium bicarbonate and any other organic fungicides like it ok to use on this fungus?
If you are starting new plants you should start with a good commercial potting soil.
I don't recommend reusing old potting soil.
You can compost the old soil.
It is possible to sterilize old soil but this isn't an easy or a task you may want to do in your oven.
You also can help prevent mold and fungus growth by making sure your plants have adequate air circulation.
Here are some links with more information.