How are the o an horizons similar

Answer: 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.

Explanation:

Soil consists of different layers that are often called horizons. There are three main soil horizons called A, B and C as well as an organic layer (O) on top of the soil (O) and bedrock (R) below: ... It is only between 5 to 10 inches thick and consists of organic matter and minerals.