Soil Layers

eluviation vs. illuviation

Eluviation is the downward transport of particles, especially small clays and colloids, from the upper part of the soil profile. Illuviation is the accumulation of the water and particles in the B or lower horizons.

Bedrock: R horizon

Finally bedrock, the last horizon, is made up of unweathered parent material or unbreakable rock.

C horizon

Of the three main layers the C horizon is the third and final layer of soil (excluding bedrock). This layer is made up of partially altered parent material.

A horizon: Topsoil

The A horizon is the first of the three main layers of soil. Also known as topsoil, this layer is darkest in color (a dark brown or black) and best for plant growth. One of the reasons it is best for plant growth is that it is mineral matter mixed with humus. Plants roots grow in this layer.

B horizon: Subsoil

The B horizon is the second layer of the three main layers. Referred as subsoil sometimes, this horizon is a reddish-brown color which signifies a lack of nutrients. Here illuviation (the accumulation of clay transported from above) occurs. If there is no E horizon bigger plants roots extend till the B horizon.

O horizon: Litter layer

The O horizon is not really a layer of soil. It consists of loose and partly decayed organic material (leaves, living or once living insects, etc.).

E horizon: Eluviation

This layer of soil is only present if leaching (the process where water flows down and takes minerals with it, creating a clay rich B horizon) has occurred. The E horizon is also referred as the zone of leaching and eluviation.

leaching

When water drains through sandy soils, it often dissolves the minerals in the rock and carries the minerals with it. This process is called leaching.