Answer:
A-Horizon and O-Horizon.
Step-by-step explanation:
O-Horizon is the top layer of the soil made up almost entirely by decomposed organic matter. A-Horizon its found below and is where the seeds germinate, it is made of hummus combined with mineral particles.
The topsoil has the highest concentration of organic matter and is where Earth´s biological activity occurs.
I hope this answer helps you.