O Horizon

A soil profile layer, often at the surface, which is dominated by organic matter. The organic layer might be of uniform consistency, especially where the pH is around 7.0 and this is termed mull, whilst in contrast in very acidic soils there could be up to three distinct organic layers, a surface litter layer, then fermentation layer and then a lower humus layer, primarily due to the relatively slow breakdown of organic matter in these soil conditions. This is called mor and might be similar to some thatch layers on very acidic golf fairways. Organic horizons that exhibit intermediate stages between mor and mull are called moder. Organic layers help to retain moisture and provide for a gradual supply of nutrients to plants following decomposition, although an extensive organic layer would essentially be a peat layer or bog.

An O Horizon in an amenity or sports turf situation will typically be a thatch layer. This can influence the playability of a sports surface by producing a softer surface, increased divoting, a slower game, lower ball bounce, slower speed and distance of ball roll, which all leads to a reduction in the quality of the playing experience.