Soil and its profile

Soil and its profile

Concept:

• Soil comprises 50 per cent solid matter (5 per cent organic matter, 45 per cent minerals)and 50 per cent of what are called “voids”— gasses and water equally divided.

Soil profile:

Soil horizons are the layers in the soil as you move down the soil profile. A soil profile may have soil horizons that are easy or difficult to distinguish. The properties of horizons are used to distinguish between soils and determine land-use potential.

There are two types of soil horizons: 

• Organic horizons: It is  marked with the capital letter O, lie over mineral horizons and are formed from plant and animal matter. The upper Oi horizon contains decomposing organic matter that you can easily recognize by eyes, such as leaves or twigs. The lower Oa horizon contains humus, which has broken down beyond recognition.
• Mineral horizons: There are four main mineral horizons: A, E, B, C

Soil Profile

O Horizon

• The surface horizon of most soils is the A horizon.
• It is dominated by organic material.
• This horizon results essentially from litter derived from dead plants and animals. It is common in forests and generally absent in grasslands

A Horizon or Surface soil

• It is the top soil.
• In this layer, organic matter is mixed with mineral matter.
• It is a mineral horizon that also contains considerable organic matter.
• It is formed either at the surface or immediately below an O horizon. A horizons generally contain enough partially decomposed organic matter to give the soil a darker colour than underlying horizons.
• They are also normally coarser in texture, having lost some of the finer materials by erosion and eluviation. Seeds germinate mostly in the A horizon.
• Nutrients like iron, aluminium, clay, and organic matter are sometimes dissolved and carried out in this layer.
• This layer is depleted of (eluviated of) iron, clay, aluminum, organic compounds, and other soluble constituents.

E horizon

• It is a light-colored eluviated layer, that is eroded of its nutrients.
• It is the horizon that has been significantly leached of clay, iron, and aluminum oxides, which leaves a concentration of resistant minerals, such as quartz, in the sand and silt sizes.
• These are present only in older, well-developed soils, and generally occur between the A and B horizons.

B Horizon or Subsoil

• It is a subsurface layer reflecting chemical or physical alteration of the parent material.
• This layer accumulates all the leached minerals from the A and E horizon.
• Thus iron, clay, aluminum, and organic compounds accumulate in this horizon [illuviation (opposite of eluviation)].

C Horizon or Parent rock

• It  is bedrock, with little evidence of weathering.
• True soil, which is called solum, only extends down through the B horizon.
• It is a layer of large unbroken rocks.
• This layer may accumulate more soluble compounds (inorganic material).
• R Horizon or Bedrock
• This layer denotes the layer of partially weathered bedrock at the base of the soil profile.
• Unlike the above layers, R horizons largely comprise continuous masses of hard rock.
• Soils formed in situ will exhibit strong similarities to this bedrock layer.
• These areas of bedrock are under 50 feet of the other profiles.