The B Horizon has dark layer where minerals, organic matter and bases are being illuviated (washed in/accumulated). Below this is a red/orange layer of iron and aluminium sesquioxides deposit. Some bases remain in the soil, though others may be lost by throughflow. In many podsols, Iron Pans are created. This can cause water logging which may then saturate the A horizon leading to mottling or a gleyed podsol.
Also useful: soil chemistry