In certain cases, some metal deer fence posts should be put in cement footings (see video). Determining which ones need this requires a bit of common sense. In New England, where snow typically falls before the ground is fully frozen, gate posts as well as all braced corner and end posts should be put in cement footings.
Beyond that, there is always some risk that multiple heavy snow loads will be able to tilt the fence posts enough to require major repairs or even replacement of the fence. To reduce this risk, it’s wise not to allow heavy snow loads to remain on the fence.
You do not need to put your corner posts in cement footings if these posts are not braced; but all the posts that are braced, or to which earth anchors are attached, should be placed in cement footings.
Note that if your fence never needs to deal with heavy, wet snow at a time when the ground is unfrozen, that relieves its ground-gripping parts of an immense potential weight burden. Similarly, if you are putting up light polypropylene fencing weighing say a tenth of a pound per foot, that requires less ground-gripping power than metal hexagrid fencing weighing about a pound per foot. And if you are putting up a short fence with no run longer than, say 40 feet, that also reduces the potential stress. So if you a protecting a small California vegetable garden with 40-foot sides using polypropylene deer fencing, you may reasonably decide to forego the joy of installing any cement footings.
To create a cement footing, start by digging a 10 to 12-inch diameter hole with an auger or post-hole digger. This hole should be below the frost line if there is deep winter ground frost or 2 feet deep if frost is not a problem. If the hole is, say, 3 feet deep, fill the bottom 12 inches with large rocks (hardball to softball size), place the post in the hole, and measure to ensure that the top is 7 feet above the ground, adding or removing rocks as necessary to obtain the proper height. If the hole is 2 feet deep follow the same procedure without initially placing rocks but adding rocks if necessary.
Plan on using a high-strength concrete mix (cement mixed with crushed rock) that should be available locally in 60 or 80 pound bags. Do not use the quick-setting type unless that is the only kind available. Mix cement and water to create a batch of cement with a jelly-like consistency. Use this soupy mixture to fill the hole to a few inches above the rocks if the hole is 3 feet deep, or to a depth of a few inches if the hole is 2 feet deep.
Then return the pipe to the hole and use moderate-size rocks to secure it firmly in place until it can stand on its own, keeping the post as close as possible to the center of the hole. Take a carpenter’s level and place it against the side of the post to make sure the post is straight up and down; adjust as necessary until the post is vertical. Then fill the hole to the top with the cement mixture and take another level measure to ensure that the post has not shifted. Let the cement set for 12-24 hours if the temperature is above 60ºF, or for 24-48 hours if the temperature is below 60ºF (it is not advisable to do this work if the temperature is below freezing).