Pressure Treated Fence Pickets – The southern yellow pine treate with pressure is one of the most popular woods use in the construction of fences. It is available, inexpensive, and has a long life if properly install. Most southern yellow pine trees treate with Quaternary Copper Alkaline. The process of “treating stress” involves soaking wood in chemicals and placing it in a pressure chamber. Pressure causes chemicals to penetrate wood. The retention rate only means how much chemicals store by wood. The higher the retention rate, the better the resistance to the element. Different external projects require different retention rates.