Livestock rubbing up against a fence or water erosion are two common causes for loose fence posts. When a large animal, such as a horse or cow, gets an itch, it will often rub its itchy spot on the fence. These animals can weigh over 454kg., so a fence post can be moved back and forth in the soil until it becomes loose. Rushing rain water can also wash away the soil around a fence post, leaving it loose and wobbly. A loose fence post places strain on other adjacent posts in the fence line, because it's not properly supporting the fencing materials. Repair of a loose fence post can usually be done in less than a day.
Dig down around the fence post with a shovel. Dig 6 inches of dirt away from around the post. Toss the dirt away from the work area. Dig all the way down until you get to the bottom of the post. If the base of the post is really deep, you may need to dig the hole bigger than 6 inches out on all sides. Dig down beneath the post, at least 6 inches. If the fencing materials attached to the fence post weigh the post down into the hole, set jack stands under the lowest part of the fence to hold up the bottom of the post.
Dump enough gravel down into the hole to fill the 6-inch space beneath the post. Get on your hands and knees and push the gravel under the post with a free hand or a garden spade. Move the gravel around until the gravel has filled the hole up to the base of the post.
Pour 11.3kg. of a 22.7kg. bag of concrete mix into a 5-gallon bucket. Pour water into the bucket while mixing it. Add enough water to bring the concrete mix to a crunchy peanut-butter texture. Pour the concrete into the hole all the way around the post. Stick a long stick into the concrete to help it fill in the spaces around the post. Some of the concrete will also seep into the gravel beneath the post. Mix another 11.3kg. and pour it in. If 22.7kg. wasn't enough to fill the hole, mix enough concrete until the hole is filled to overflowing the hole by 1 to 2 inches.