Clay has a reputation for causing muscle strains and back aches in homeowners who attempt to install fence posts in its rock-hard soil layers. Although you can add gypsum and compost to your clay soil to soften and condition it, both materials require a lengthy amount of time for the soil softening to occur, typically at least 6 months before you'll notice a significant difference in the soil hardness. If you're in a time crunch, you can soften your clay soil in just a few hours, but you'll still need to make a significant investment of manual labor during the process.
Clear the fence line of rocks and excessive vegetation. Mow the grass down to 2 inches or less in height. Mark the post locations for your fence with a can of spray paint.
Use a shovel to excavate the first 6 to 12 inches of clay soil at your first post hole location. Make the diameter of the hole approximately 4 to 6 inches larger than the diameter of the posts you'll be installing. Typically, this should take less than 30 minutes. Pour 1 to 2 gallons of water into the hole and let it sit for approximately 1 to 2 hours to soak into the surrounding soil.
Dig up the first 6 to 12 inches of soil for your second post hole location. Pour 1 to 2 gallons of water in this hole and allow it to sit. Move on to your next post hole and repeat the same process until 1 to 2 hours have passed since you poured the water in your first post hole.
Return to the first post hole and use your shovel to loosen the wet soil in the hole. Insert the shovel straight into the water-logged soil and turn it clockwise as you force the shovel further down into the hole to mix the water further into the clay soil. Repeat this process two to three more times before scooping out the loose, muddy soil with the shovel.
Open the digging blades of a clamshell hand-held post hole digger and insert them forcefully into the post hole. Bring the blades together and scoop out the moist clay soil from the hole in the ground. Repeat this process until your hole is the correct depth for your fence post. Move on to the other post holes one at a time and excavate them in the same fashion.