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There are two types of aeration devices, the spike aerator, which punches holes in the sod without removing any soil, and the plug aerator, which removes small plugs of soil (about 3/8" thick and 1-1/4" long) and lays them across the lawn. The plug aerator is preferred, but the spike aerator is better than nothing if aeration is needed. If aeration is not needed, the spike aerator can actually make compaction worse because the holes are created by displacement and forces soil to the side rather than removing it. If you use a spike aerator, the aeration needs to be followed immediately by a topdressing of compost or peat moss. If you use a plugger, you need not topdress, though it does help.