You haven't said whether the stump is dead, or whether its still living. If its still living, you will need to have it taken down to ground level, then drill into it in several places near the outer edges with a medium to large wood bit to make holes a quarter inch deep, then fill the holes with SBK, a brushwood killer. If you want to immediately put soil over the top, you need to put a sheet of plastic or something over the stump to prevent the soil coming into contact with the SBK.

If the stump is dead, you don't need any chemicals - there is nothing chemical that will make it rot down any quicker, and covering what's left immediately with soil will encourage quicker decomposition of the dead wood.