These show a vertical cross-section through a soil and have a number of different horizons as shown in the idealised diagram below. Each profile is a result of inputs and outputs and three horizons exist:
A - upper layer, here biological activity and humus content is at its maximum. Leaching may occur.
B - zone of accumulation or illuviation, clays etc. taken from A are redeposited. A and B account for the true soil.
C - recently weathered material.
It is possible for a surface horizon to exist. This is when humus is slow to decompose.
Layers between horizons may not always be clear, depth can vary, and humus can mix in the soil or be a separate layer.