Bog Earth is the resource needed by a Peat Bog to produce Peat, which can be used as a fuel source. When turning Bog Earth into Peat, it must be within a block of water, diagonals included, in order to mature.
(Note: as of this edit 05/11/2013 in FTB Ultimate (v1.0.1) which has Forestry 18.104.22.168, Bog Earth will mature within 2 blocks of water, diagonals included, i.e. water, even moving or shallow water, will mature up to a 5x5 square of Bog Earth around it. This may be a recent change and may depend on the version of Forestry in your pack. The Forestry wiki does not discuss this and only talks about producing and harvesting Peat by machine. See the Peat entry for screenshots of this.)