Why some posts last longer in the ground than others

Sterling Fence Inc. is constantly contracting to replace the post that have weakened on older wood fences. Why do some posts last longer than others?  Its almost always due to the moisture in the ground being greater in one area than another. At Sterling Fence Inc. we will look at a yard with a 20 year old fence and several broken posts but 90% of the posts are good. While its impractical to test soil moisture in different areas of the yard, it is usually clear which areas are wetter than others.  It may be a garden area, an area where the sump pumps releases water or a low spot in the yard.  Different soils hold moisture differently as well. Clay areas hold moisture more than sandy soils.  Peat soils indicate moisture.  Posts don't deteriorate by themselves it takes moisture.