How deep should the holes be?
As a "rule-of-thumb", corner posts should be set at least 42 inches deep, as these types of posts carry the weight of the fence panels. The posts that hold the panels (commonly known as line posts) should be dug in to at least a depth of 30 inches. The depth of the hole will not independently ensure a successful set, but the materials used when setting the posts will also be a major factor.
Once you have dug your hole, you need to take precautions in regards to water intrusion. To ensure that your post does not experience water damage in the future, shovel a few spades of gravel down the bottom of the hole before placing the post. This will ensure that any water that gets down the hole will be drained away from the wood, consequently avoiding wood-rot.
Once the post is it the hole, place around 10 inches of local stones down the hole. wooden fence post in Piedmont, SD. wooden fence post in Piedmont, SD. You do not need to buy these as you will probably find them in the nearby vicinity.
Flatten the local rocks into place. Continue to do this every 10 inches until the hole is filled. If the rocks are firmly in place, the wooden post should be secure and level.
Fill the hole up to one third of its depth using concrete. Once you have done this, ensure that the post is still level. Continue to do this in 1/3 depth increments until the hole is completely filled. Once completed, check the post level.
Finally, to take extra precaution over wood-rot, you should pour a few inches of extra concrete around the base of the post (above ground level). This will ensure that when it rains, the water will drain away from the wood and not down into the foundations of your wooden fence post.