Wet/damp soil will be in full contact with the post and keep the wood wet until the soil dries out. On the other hand, the gravel is not porous and is only in partial contact with the post, so it will only retain a bit of moisture on its surface and will allow water to flow away quickly and the post to remain relatively dry. Unless you continuously have water saturating the soil with no drainage, the gravel will keep the post much drier. Finally, soil will have all kinds of destructive micro-organisms in it, while gravel will support far fewer of these.

Hi
Got to see some results from this as the trellis/shed had been up more than 10 years . Had 8 posts and due to running out of money and time all were set differently though all were set at the same depth. Set in concrete rotted right at ground level, set in pea gravel no rot but severe checking. set in soil rot below groundlevel . All PT pine.4x4's. No method did a thing for 125 MPH hurricane lol The gravel set were as strong as concrete set. In fact concrete were the first to snap.
I replaced the posts with vinyl lol. My aviary made of extruded aluminum with concrete footing suffered only minor damage. gary

OK, it's starting to make some sense now. I was thinking more about saturated soil after a rain rather than clay up against a post being more wet compared to gravel, in part due to how much area is in contact with the post.

garyfla, I'm surprised to hear that the PT posts rotted after ~10 years....were they rated for ground contact?

Alex

Alex
You got me . When i built it I didn't know there were different kinds lol Was interesting to see the results of the various types of decline. It did not prevent either termites or carpenter ants though only in one post.
A local contractor said I should have used steel footing set in concrete the wood is bolted to this so avoids all ground contact. Also I should have allowed the wood to cure 3 weeks as I recall and then applied a wood preservative.. There also seems to be some chemical reactions between fasteners and treatments?? He said SS was the only way to go .
Anyway when replacing I went with vinyl. All things considered the total price wasn't that far apart.
Thinking of replacing the wood in the aviary with extruded aluminum but don't want to use a footing which they tell me I need.
There ought to be some websites on proper methods somewhere?? gary

Over the years, we have tried just about every variation on setting fence posts and the one that has worked out best has been to start with a very deep hole, at least 2 ft. deeper than the bottom of the post.

Fill the hole with med. sized river stone up to the point where the post bottom will set and fill in around the post with screenings. The screenings will pack firmly, almost like concrete, but still drain, so the end effect is the stability of concrete and as close to total drainage as is possible for the application.

That's the connumdrum, really: Once it's been leveled, concrete will set a post perfectly and prevent sagging, but unless it's in a desert area, rain water will also eventually cause any wood post set in concrete to rot at the base.

However, for an arbor with presumably only 4 posts, you can avoid the issue altogether by sinking concrete footers and setting the posts atop them, anchoring the posts with brackets and if a formal look is desired, hiding the brackets with base collars.

If you think about it, a fence needs posts that are set firmly and deeply enough to prevent any from leaning and taking the fence with them. However, an arbor is actually self supporting, so if set on a solid, level base it will never lean- it just needs to be anchored in place.