New Geography in Action Junior Cycle Geography 100 90 80 70 60 50 40 30 20 10
100 90 0
Sandy clay
Clay loam Sandy clay loam
Sandy Loamy loam Sand sand
80 70 60
Medium loam
50 40 Percentage Sand Silty loam 30 20
Silt 10
Clay 0 10 20 30 40
Silty clay
Silty 50 60 clay loam 70 80 90 0 100 ( Figure 16.3 Soil texture chart NUMERACY ACT VITY Take a look at Figure 16.3.
(i) What percentage sand, silt and clay is a medium loam soil?
(ii) Which soil is 30% clay, 10% sand and 60% silt?
Soil profiles
A soil profile is a cross-section of the different layers of soil – from the surface down to the bedrock.
Each layer is called a horizon. You can tell the horizons apart by looking at a number of things, including content, texture and colour.
O horizon
This layer contains organic matter. Leaves and rotting vegetation are called plant litter and this provides the humus for the soil.
A horizon This is the upper layer of the soil and is called the topsoil. This is where most of the organisms in the soil live. It is usually darker than the other layers because it has the highest humus content. It is the most fertile part of the soil.
B horizon
This layer is called the subsoil. It is lighter in colour than the A horizon because there is less humus here. This layer has more stones because it is located closer to the bedrock and because it is protected from weathering and erosion by the A horizon.
C horizon
This is the bedrock or parent material. The lower layer consists of solid rock but the upper section may be broken down into rock particles. In some instances the parent material can actually be a different rock deposited on top of the bedrock by glacial deposition.
: Figure 16.4 Soil profile Go to Chapter 11, pages 184–187, to learn more about glacial deposition. 286 Bedrock Topsoil Subsoil Bedrock:
the bottom layer of solid rock which is usually covered with broken-down rock and soil.
