- Soil is defined as the unconsolidated mineral and organic matter on the immediate surface of the earth that serves as a natural medium for the growth of land plants.
Soil comes from the weathering and ageing of parent materials
Soils are formed by the physical and chemical breakdown of rocks
Physical breakdown is caused by the Freeze – Thaw phenomenon.
Chemical breakdown occurs when soluble components in the rock are dissolved by water (A very strong solvent)
This is further emphasised by various types of dissolved acids in rainwater.
Carbonic acid, Nitric acid and Sulphuric Acid ate common in rainwater.
Soil formation is also greatly affected by the presence of plant life on the surface of rocks.
Early plant life like Lichens (See them on trees and rocks) can breakdown rocks. Lichens get minerals from rocks by secreting an acidic liquid into the rocks. This further breaks down the rocks decays slowly.
When lichens die they also form the first layer of humus (decaying material) that will eventually become the soil.
When the humus becomes a few inches thick, other plants (like grasses and mosses) can begin to grow on it. The roots of these plants then can break the rock further.
The more plants that grow in the humus the more soil material is available.
When the soil gets thicker still, tree and shrubs get a foothold and further break the soil with their roots.
- Parent Materials
Parent materials can be rocks (residual parent materials)
They can be rocks that have been transported to a different location (transported parent materials)
Parent materials can also be the preserved remains of plants (cumulose parent materials - Rock Types
- Igneous –Igneous rocks are made of the following elements:
- Oxygen
- Silicon
- Aluminium
- Iron
- Calcium
- Sodium
- Other metals and non-metals
- Formed by the solidification of molten rock material e.g. granite and basalt. Contains the minerals mica, feldspar and quartz.
Granite is made up of three main components: 1. Feldspar 2. Mica 3. Quartz
Basalt and Granite are formed from the same material and differ only due to how they cooled down.
Granite cooled more slowly than Basalt and therefore gave very large crystals.- Sedimentary – Formed when igneous rocks are broken down into smaller pieces which are then compacted and cemented together e.g. sandstone and limestone.
The material can either be another type of rock (Igneous) or shells on the bottom of the ocean.
Sandstone is formed by sediments of quartz, (broken up by water, heat and frost as well as Glacial forces).
Limestone is formed by the fusing of pieces of shells and other materials containing Calcium carbonate (CaCO3)- Metamorphic – formed by the action of intense heat or pressure on igneous or sedimentary rocks.
These rock arise form sedimentary rocks when they are exposed to pressure and heat.
Examples are Slate (changed from shale) and marble (from Limestone) - Irish Parent Materials
When glaciers move they scrape all the loose soil and rocks and mix them together. They then deposit them as glacial drift as they melt.
It is these deposits that have acted as the parent material for most Irish soils. Because the last Ice Age was only 12000 years ago Irish soils are considered to be very young. - Organic Parent Materials
Organic means to do with life.
Organic parent materials developed in the last 2500 years from partially decomposed plants which built up under very wet conditions.- They are also called peat parent materials and there are two types:-
Blanket Peat
Basin Peat - Blanket Peat are found in mountainous areas where high rainfall is typical (Wicklow, Clare and Kerry to some extent)
Blanket bogs are so called because they form a blanket over a wide area. They are quite shallow (1 - 2 metres deep at most)
Not as much potential for cultivation by large-scale machinery or for farming.
Occur in areas of high rainfall and humidity e.g. west of Ireland and in mountains.
Wet weather and low evaporation leads to water-logged soils where plants can not fully decompose.
Blanket peats are wet, acid, infertile and of limited agricultural use.- Blanket Peat are found in mountainous areas where high rainfall is typical (Wicklow, Clare and Kerry to some extent)
- Basin Peats : The raised bogs of the midlands are being currently exploited for fuel and moss peat.
But these natural resources will not last for ever and research is on going to find out what should be done with the land after all the turf has been harvested.
Occur in landscape depressions e.g. lake basins, hollows and river valleys. Vegetation from around the edge of lake accumulate at the bottom.
Eventually the lake fills up with peat. If the peat rises higher still we can get a raised bog. See pg. 20.
Because basin peats can occur in dry areas they tend to be of greater agricultural value. - Weathering
Parent materials need to be weathered to form soils.
There are two types of weathering: -
1.Physical Weathering
2.Chemical Weathering - Physical Weathering
1.Frost action – water enters cracks in rocks, freezes, expands and cracks rocks further.
1.Heating and Cooling – different minerals in the rocks expand at different rates causing rocks to crack.
1.Grinding – caused by beating of rain, sand washed in streams and rivers.
1.Plant and Animal Action – roots of plants can crack rocks as can digging by animals - Chemical Weathering
1.Solution – minerals in rocks dissolve in rainwater.
2.Hydrolysis – minerals react with water and become more soluble.
3.Carbonation – carbon dioxide from air reacts with water to form carbonic acid which dissolves minerals in rocks
4.Hydration – minerals combine with water.
5.Oxidation/Reduction – adding or taking away oxygen from a mineral decomposing it faster. - Biological Weathering
Micro-organisms such as bacteria, fungi and lichens colonise bare rocks.
Some of these micro-organisms produce acids which dissolve the rock.
These micro-organisms also add organic matter to the rock when they die. - Soil Formation
The factors that affect the type of soil that develop are:-
1.Climate
2.Parent material
3.Living organisms
4.Topography
5.Time - Climate
Most important factor.
Direct influence on weathering and is responsible for the variation in plant and animal life.
In Ireland we have low temperatures and high rainfall so our soils are leached of bases i.e. they are acidic. - Parent Material
The younger the soils the more important this is because a young soil will closely resemble the parent material.
Granite and sandstone parent materials will lead to acid soils with a sandy texture and good drainage.
Limestone parent materials will result in more basic soils because limestone is rich in Ca++ and Mg++.
Shales will lead to more clayey soils with higher mineral contents. - Living Organisms
Living organisms may lead to an organic parent material resulting in a peat soil.
Living organisms contribute to the fertility of the soil e.g. earthworms.
Soils developed under a forest will be more leached of bases than those developed under grassland due to higher levels of humus in the soil under grass.
Soils developed under coniferous forests will be more acid than soils under deciduous trees because deciduous trees recycle bases. - Topography
Topography means the lie of the land.
If the land is sloped then there will be a deep fertile soil at the base of the slope but the sides of the slope will be shallow and infertile.
The type of vegetation growing depends on what direction the land is facing e.g. north facing slopes do not receive any sun. - Time
The older the soil the more weathered it will be and the less it will resemble the parent material underneath it. - Soil Profile
A soil profile is defined as a vertical section of the soil through all its horizons (layers) and extending into the parent material.
A soil may have several horizons or layers that differ in colour, texture, structure, organic matter content and chemical activity:-
–O layer = organic layer.
–A layer (topsoil) = mineral layer with good structure.
–B layer (subsoil) = mineral layer with poor structure.
–C layer = parent material.
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