The most vulnerable section of a post extends from a short distance above the soil surface to some distance below it. This groundline zone usually has a sustained supply of moisture and oxygen that favors growth of destructive fungi. Although tops of posts also may deteriorate, that deterioration normally is slower.
From 1949 to 1985, the following causes of deterioration and failure of the posts at the test site were identified:
Primary agent of failed posts Number (%) Fungi 985 (75.2) Fungi and termites 206 (15.7) Fungi and insects other than termites 92 (7.0) Termites 22 (1.7) Other insects 5 (0.4)
Decay-producing fungi or fungi in combination with subterranean termites do the most damage. Discarded wings of damp-wood termites have been found at bases of some posts, and entry holes have been detected at or below ground line. However, termites alone have been the primary cause of failure in only a few instances. Carpenter ants and wood-boring beetles also contribute to de- terioration.Oregon Post Farm
Chemically treated lumber has been proven to last longer than untreated wood. The methods used over the years have changed, as have the laws regulating them. The best pressure treatments are generally the most toxic. The EPA has banned some of the oldest methods, and, within the last two decades, some of the most effective.