I am building a split-rail fence out of locust rails and posts. The posts
will be set about 8 feet apart. I plan to dig postholes about 18 inches
deep, and set the posts into concrete.

What is the best method, and type of concrete ? My thinking was to use
Quickcrete fast-setting concrete. With similar things in the past, I have
mixed the concrete with water, and poured the mix in and around the post.
But, it seems that I have heard that other folks just pour the dry power in
the hole around the post, and allow the moisture in the surrounding soil to
harden the mix. Others pour in dry, and then just add a modest amount of
water in the hole.

What is the preferred method ??

Thanks for any advice on this !!

James