Compaction is a process in which soil layers lose some or most of their characteristic pore space and beneficial structure, and as a result become more impermeable. Compaction can lead to a number of undesirable soil characteristics including reduction in pore space, increase in bulk density, reduction in the amount of water held in the soil at field capacity, restriction to root and water penetration, and soil erosion. It can occur naturally and as a result of human activity.
Most soils have had a long history of development with most dating back over several thousand years. During this period soils may have experienced heavy loading pressures such as in the case of the ice sheets during the Ice Age. Where ice sheets have covered layers of soil and where there have been freezing conditions generally these have often compressed the soils to form dense soil horizons (termed fragipans). Imagine the weight of a glacier on the ground below! These dense compacted soil layers have been preserved in the lower parts of some post-glacial soils. Such layers often restrict the movement of water downwards and also hinder roots from exploiting the whole soil depth.
In other soils the lower soil horizons have sometimes become 'cemented' by the washing down (leaching) of dissolved chemicals over many years which then are deposited in the lower soil horizons where they cement the soil. This cementation, for example, can be by silica (forming duripans), or calcium carbonate (as in petrocalcic soil horizons). In some tropical conditions also, plinthite horizons occur in which iron enriched parts of the soil are soft when permanently moist but if allowed to dry out harden irreversibly. It is clearly important to know about these conditions when soils are to be used for specific purposes, otherwise difficult situations can arise. This is why it is important to have accurate maps of the soils of an area.