Joe gave you great information but I thought I would lend just a bit more advice based on my experience.
If you do not provide a place for her to dig a nest she could have problems, egg retention, or she may just pick a spot, lay the eggs, and then scatter them as she will try to bury them and in the process they will wind up all over the place. I have tried using a box with soil before and my females never seemed to figure out that they were supposed to use it as a nest and laid the eggs on the tank floor.
So, I really recommend that you put 4-5" of soil in the bottom of the whole tank. This will allow her to pick her nesting site. I wouldn't mess with any "soil". Instead, if you can't get a brick of coir fiber, bed-a-beast or equivelant, use moistened peat moss or worm bedding. If you use worm bedding make sure it isn't the recycled paper stuff.
And........ if you haven't had her to a vet, make sure you have her checked for parasites. If she is gravid I wouldn't take her in but you should take in one of her poos and have it checked. If she is gravid and carrying parasites chances are real good that she will die.
The site Joe was directing you to is www.froggieb.com/MHDCaresheet.html or you can find it in the kingsnake caresheets.
Good luck, and keep us posted!
Marcia