According to Build Green: Wood Can Last for Centuries, a common homeowner and contractor mistake following construction projects is to set the finish grade for soil or mulch above the level of wood framing. Soil contact is one of the primary culprits in wood decay.
Where untreated wood is used in a structure, it should be at least eight inches above the finish grade for framing members and six inches above finish grade for siding. Composite products should never be used in contact with soil.
Preservative treatments are designated for above-ground use or in-ground contact (buried in soil or touching soil). When planning your building, it is important that you specify the right treated wood for your specific need and that you insist that the treatment be of certified quality and be labeled accordingly.
A vast array of treated wood is available for the homeowner. Choosing a preservative approved for ground contact, properly grading soil, and avoiding constant wetting will go a long way in protecting your outside wood structure from decay caused by the moisture in soil.