for instance through major earthworks or regular deep ploughing, may lack distinct horizons almost completely. When examining soils in the field, attention must be paid to the local geomorphology and the historical uses, to which the land has been put, in order to ensure that the appropriate names are applied to the observed horizons. Examples of soil profiles Horizons according to the Australian Soil and Land Survey Field Handbook (2009) Source: Horizons O horizon The "O" stands for organic matter. It is a surface layer, dominated by the presence of large amounts of organic matter in varying stages of decomposition. In the Australian system, the O horizon should be considered distinct from the layer of leaf litter covering many heavily vegetated areas, which contains no weathered mineral particles and is not part of the soil itself. O horizons may be divided into O1 and O2 categories, whereby O1 horizons contain undecomposed matter whose origin can be spotted on sight (for instance, fragments of leaves), and O2 horizons contain organic debris in various stages of decomposition, the origin of which is not readily visible. O horizons contain ≥ 20% organic carbon. P horizon These horizons are also heavily organic but are distinct from O horizons in that they form under waterlogged conditions. The "P" designation comes from their common name, peats. They may be divided into P1 and P2 in the same way as O horizons. P horizons contain ≥ 12 to 18% organic carbon, depending on the clay content. A horizon The A horizon is the top layer of the mineral soil horizons, often referred to as 'topsoil'. This layer contains dark decomposed organic matter, which is called "humus". The technical definition of an A horizon may vary between the systems, but it is most commonly described in terms relative to deeper layers. "A" horizons may be darker in colour than deeper layers and contain more organic matter, or they may be lighter but contain less clay or pedogenic oxides. The A is a surface horizon, and as such is also known as the zone in which most biological activity occurs. Soil organisms such as earthworms, potworms (enchytraeids), arthropods, nematodes, fungi, and many species of bacteria and archaea are concentrated here, often in close association with plant roots. Thus, the A horizon may be referred to as the biomantle. However, since biological activity extends far deeper into the soil, it