Posts &
Post Holes
I can't speak for your part of the country but in my area the rain, humidity, bugs, heat, and fungus all add up to short lived wood or iron posts. And if these are installed poorly it may look good for the short term but in the long run something is going to compromise the post whether it be wind or whether it be a ram.
There are commonly two types of wood posts, the standard 4 x 4 square post and the semi-round landscaping posts, and each can be found as treated or untreated. Of course you can also look for 6" diameter and telephone polls but most local stores won't have these available and might be a bit overkill for this project.
Pressure Treated versus Plain ol' Wood :
At one time all "treated" posts were pressure treated with a mixture of Chromium, Copper, and Arsenic (Chromated Copper Arsenate, or, CCA) which were fused deep into the cells of the wood. This stuff is great but they simply don't make it for residential use any longer so it's harder to find. If you find some neighbors with old fences or decking made of dark green treated wood, see if you can salvage it because it lasts for more than 20 years even under the harshest conditions. . .much longer than today's treated lumber. Today this wood is impregnated with a few other ingredients but the longevity is still not as long.
Why is this important? Because if you are going to use a tree you just cut down in your field or you want to purchase the less expensive untreated lumber product then plan on replacing the main posts of your fence about every 2 - 3 years or so. The treatment process is what makes your wood last and there is no way to change your mind once it's in the ground.
A few years back when hurricane Rita came rumbling our way our back fence gave way at my home. Fortunately for me I did all of my neighbors a favor and they let me dispose of the old fence for them. It was a lot of work to salvage what I did but the wood was 18 years old and after all that time only one of the posts were rotten, and the only reason that occured was because it had been underwater a great deal of the time; the rest were perfectly usable, dry, and ready for use. I still use that lumber for a lot of things and it only costs me my time.
Scrap 4 x 4 post leftover put to use
Landscaping posts verses 4 x 4 posts :
Treated landscaping posts (Rounded posts) are anywhere from 30 - 50% less expensive than 4 x 4 lumber for the same length post; typically 8-ft lengths. I like the idea of inexpensive landscape posts because they cost about $3.70 each while the 4 x 4 is somewhere around $7.00 but I occasionally (actually often) run into issues because the pressure treating process often leaves the posts wet. While moisture isn't the real issue, an improper cut of wood and the drying that occurs later once the pole is set can leave you with a warped post (As shown below). Even wet untreated wood will warp which is why kiln dried wood is commonly used in building materials.
The only way I know to detect the severity of the problem lumber is by giving it time to dry before sinking the wood into the ground. If you leave the posts in an upright position, spaced apart so that they can dry on all sides evenly, then the warping will occur before you sink the post. But then you're stuck with posts that are warped and you'll be waiting at least a week or more before you can actually start the project.
4 x 4's are also much stronger than the landscaping posts and since I've hung gates on the smaller posts I now wish I had used the 4x4's since the small landscaping posts easily bend back and forth when they're weighted down with a gate.
For all of the reasons stated above, it's worth the extra money to use ALL treated 4x4's (or heavier). There are simply fewer issues in the long run and you'll have all four sides perfectly square for you instead of only two.
Post Spacing:
Wood posts are the main support for your fence and should not be spaced any further than 40 feet apart; this is the maximum, closer is better. In between the wood posts the fence will be supported using the best quality T-posts you can afford and planted about 10 feet apart.
T-posts should be planted a MAXIMUM of 15 feet apart or else the fence will lean and can be easily pushed over by your flock; 10-ft spacing is most typical for the spacing of these posts.
T-posts are sunk using a Post Driver until the bottom flag is driven under the ground (~25% of the post hight). The fence is hung onto the wood posts using barbed staples while the T-post only supports the fence using a T-post clips.
T-post puller can be helpful if you need to move a fence line.
How to sink a wood post:
There are a couple schools of thought on posts. One is to use concrete and the other is to avoid it. If you use concrete then if later decide to move the post/s, you're in a world of hurt in having to dig up the post and purchase new ones.
But if you decide NOT to use concrete then you'll be exposing the post to additional moisture from the soil as well as subterranean pests. I've been told that treated posts, the residential type sold commonly today, will still only last about 3-5 years at best in very wet & humid climates. If available, use marine grade posts, however the cost is significantly higher.
I've come to prefer the risk of having to move the fence and sink all of my posts in 
concrete since the moisture levels are very high in my pasture, in fact, the pasture can be almost boggy in some areas. I dig a hole NO LESS than 30 inches deep (Wet or dry - makes no difference). If you cheat on this beware as the post may shake loose from the ground and will start to lean in time or not offer the kind of support you need when pulling against the strain of a fence that's attached to it. It's especially important if a gate is mounted to the post but there are other ways to help add stability which is shown on the corner posts page. The added depth helps the stability of the pole tremendously.
An easy way to keep track of the depth is to mark it directly on the post hole digger. Here I've used painters tape but a marker will do fine.
The post hole should be at least twice the width of the post or more to prevent cracking of the concrete by swelling posts. Swelling is natural and simply depends on the moisture levels in the air and rain. If you've dug your post hole correctly then you should need about 1 entire 80 lb bag o
f concrete per 4x4 post installed
I pre-mix my concrete with water even though many people don't; the instruction say it's not necessary. Simply adding water atop dry-mix concrete will leave air pockets or gaps in the cement where moisture/water can later penetrate and rot the timber. Pour
a few inches of pre-mixed concrete first, placing the pole in the hole, and then fill the remainder of the hole. This will completely seal the post from the bottom as well as the sides. If the base is encased in concrete a few inches higher than the soil then the amount of moisture from saturated soil will be reduced and will help extend the life of the post.
To hlep raise the level of the concrete above the soil I use a 4-inch piece of concrete form(See pictures above and to the right); a piece of plastic drainage pipe could work equally as well. The form costs about $7 per 4-ft section and you can cut it using any saw avaiable. After allowing a week of curing time for the concrete, the form can be broken free and removed so that the form itself doesn't collect the water around the post. The form also helps to prevent the concrete from cracking as the post might swell slightly during this process. under normal conditions the concrete is hardened in about 2-days.
After you've added all of the post concrete needed, use a level to ensure the post is perfectly vertical. Simply place it on each side of the post and adjust the post according to the bubble in the level.
Approx. Cost per 8-ft Post:
Post (landscaping post or 4x4) $3.50 - $7.25
Concrete $3.00 - $3.75
Form (~$7 tube / 10 sections) $0.70
Total = $7.20 - $11.70