Setting wood fence post---I've heard to set wood fence posts using quikrete dry, allowing it get moisture from the ground. Is this a good idea?
No. Unless you live in a swamp, there won't be enough moisture, and even if you do it won't be mixed with the cement properly. The concrete will be dry, crumbly, and have no strength. Just mix it like the directions say.
Many vendors do recommend pouring the concrete in dry, but then they advise to pour a specific amount of water on top to immediately set, not to allow rain and ground moisture to cure the concrete. This lets you skip mixing in the wheelbarrow or bucket and then shoveling the wet concrete in with a lot more cleanup of the tools required. For strength, the latter method of mixing first and pouring in wet should give you a stronger result over the dry pour method since the moisture and aggregate will be more thoroughly mixed.
Personally I don't like concreting in wooden posts, because eventually they will rot and have to be replaced. Getting the old concrete out is then difficult.
Ideally, concrete in a short concrete fence post and use coach screws to fix your wooden posts to these. The concrete will never rot, your wooden posts can be set off the ground, so will last almost indefinitely.
If you must concrete in wooden posts, make sure you buy them properly pressure treated and at least 4" square to survive rot longer.
Like someone else said, put a rock in the bottom of the hole. This helps keep everything level. I also jam small rocks either side of the post to get it square, leaving plenty of space for the dry concrete to pass.
In the UK, we have a product called Postcrete. Having looked at Quikrete, it seems almost identical. Both are specifically designed to be used without mixing, but do require water.
Someone else mentioned putting water in the hole first. This is how Postcrete is used.
A video and instructions for Quikrete is at: https://www.quikrete.com/athome/video-setting-posts.asp
Instructions for Postcrete are at: http://www.tarmac.com/media/756324/postcrete-product-data-sheet.pdf
I imagine following either instructions for either product will produce good results. Neither of them are as strong as mixing proper concrete from scratch.
Particularly note the limitations quoted for Postcrete: "for the purposes of fixing domestic posts, for fixing decking posts, rotary washing lines and small/medium sized gateposts (less than 1.0m width gate). Postcrete may not be used for general concreting, screeds, mortar, grout or render. If a particular compressive strength is required, Postcrete should not be used.
Using dry concrete is very helpful in places where a hose is not available and where tool cleanup is not going to occur within the next few hours.
I have set hundreds (maybe thousands) of wooden fence posts with dry concrete with satisfactory results (0 fails). I've only done this in an areas with moist soil though, so this method might not work everywhere (desert?).
I also like to put a large stone on the bottom first, if it's handy; and if I have a little water, I add it to the hole before pouring dry contrete (although it may not make any difference). For larger holes (or for cable anchors) I sometimes add concrete, then water, mix/tamp with a stick, then add more contrete and water.
It is a good idea if heavy mixing and shoveling aren't in your best interest.
Moisture is absorbed and distributed very well by capillary action and other mechanisms, and the concrete will eventually cure as well as if you had mixed it yourself.
It will take a while in some cases, so the drawback is that the post may move in the meantime.
It doesn't take that much effort to mix the concrete in a wheelbarrow before dumping it into the post hole. If you want the result to be the best result possible, then do all that's necessary to make it that way.
I sent my post 10 years ago in dry mix with no water. Concrete set up just fine and the fence is still plumb. The weed eater may eat through the post before they fail
You heard 2 pieces of very bad advice. 1st, concrete is not needed nor beneficial. It's extra work, it delays the installation, it doesn't compact itself into the hole & therefore isn't as solid, it absorbs water, it retains water & it's a chore to remove in future replacements.
2nd, no, your posts will not remain plumb in both directions. You need to pour in at least 1/3rd of the amount of water for the amount of concrete you put in & wait at least 2-hours, to lock the posts quite anemically into position.