On top of the soil horizons there can be a layer of leaf litter, but it is not considered a horizon, so I'm assuming that it's the topmost layer of the horizons.

See the following definitions of the top three soil horizons from http://en.wikipedia.org/wiki/Soil_horizon#O_horizon:

O horizon

The "O" stands for organic. It is a surface layer, dominated by the presence of large amounts of organic material in varying stages of decomposition. 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 decomposed matter whose origin can be spotted on sight (for instance, fragments of rotting leaves), and O2 horizons containing only well-decomposed organic matter, the origin or which is not readily visible.
[edit] 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. This layer accumulates iron, clay, aluminium and organic compounds, a process referred to as illuviation.[2]
[edit] A horizon
Main article: Humus

The A horizon is the top layer of the soil horizons or 'topsoil'. This layer has a layer of dark decomposed organic materials, which is called "humus".The technical definition of an A horizon may vary, but it is most commonly described in terms relative to deeper layers. "A" Horizons may be darker in color than deeper layers and contain more organic material, or they may be lighter but contain less clay or sesquioxides. 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.[3][4] However, since biological activity extends far deeper into the soil, it cannot be used as a chief distinguishing feature of an A horizon.