1. Soil Horizons

3. Learner outcomes Define what a soil profile consists of.
Discuss the different profile layers.
Explain the contents of each profile.

4. Student activity
In your group discuss what is a soil horizon
2 mins

5. Soil horizons
They are defined as the layers of soil that lie roughly parallel to the surface
Each has distinct characteristics that are determined by the soil forming process
There are 4 general horizons : organic, topsoil, subsoil and bedrock.

6. Profile diagram of layers

7. The Organic layer
This is referred to as the O horizon and is the uppermost layer
Contains high percentage of organic matter (at least 12% organic carbon)
Typically within a woodland area there are three distinct organic layers: one of leaves, pine needles and twigs ; underlain by a partially decomposed layer ;and then a very dark layer of well decomposed humus .

8. The Topsoil layer
Referred to as the A horizon and situated directly below the organic layer
Composed mostly of mineral material which has no rock structure left, but also containing some accumulated humified organic matter.
Dark in colour due to the mixing of humus form the organic layer with the minerals from below.
Generally no more than 2-3 inches thick

9. The Subsoil layer
Referred to as the B horizon and situated below the topsoil
Composed mostly of mineral matter with little remaining rock structure visible
Contains material (usually iron but also humus, clay, carbonates, etc.) which has moved into it (Illuviation) either from rain water percolating through the soil and leaching material from above or where mineral material may have weathered in place.
In well drained soils, the B horizon is typically a yellowish brown to strong brown colour and is commonly referred to as the subsoil.

10. The Bedrock Referred to as the C horizon and the bottom most layer
The unweathered geologic material from which the soil above formed and therefore solely mineral content
Very little or no sign of soil formation
No colour development – colour is determined by the geologic material

11. Student activity
In your group: From the samples displayed identify and examine. 5mins

12. For next weeks lesson
Please bring a clean jam jar and a small sample of soil from your own garden (about a third of the height of your jar)