JC-Learn Geography Notes Soil 1 P a g e
Soil Soil is the thin covering of loose material that makes up the outermost layer of our planet. It is a living thing and a rich natural resource. Plants need soil to grow and people and animals need plants for food. Soil is made up of varying amounts of organic matter, minerals and nutrients. The formation of soil is a slow process taking thousands of years. The study of soil is called pedology Soil Composition Soil is made up of a number of elements: Mineral matter (rock particles) Humus Air Water Living organisms Mineral matter Mineral matter (rock particles) is pieces of rock resulting from the processes of weathering and erosion. These particles contain minerals, which help plants to grow. The method of breakdown of the rocks will result in different sized particles. Particle size Name Character Large Sandy Space for air and drainage Small Clay Holds moisture Small Loam Mix of clay and sand Humus Humus forms when plants and animals decay and rot. Plant life prospers in soil with high humus content as it makes soil fertile. It also provides food for living organisms in the soil. The colour of the soil is based on the amount of humus in it. The darker the soil the richer the humus content. 2 P a g e
Air There are air spaces between the rock particles within the soil. Air contains oxygen and nitrogen, which are vital to plant life. The air spaces also allow water to travel to the roots of plants. Water Water can make up 25% of soil composition. There are minerals dissolved in the water. Water carries these dissolved minerals or nutrients to the roots of plants. Living organisms Soil contains living organisms such as earthworms etc. It also contains micro-organisms like bacteria and fungi. Micro-organisms help to break down dead plant matter to form humus. Soil Profile The soils of the earth vary in colour and texture. This difference is seen in the soil profile. A soil profile is a cross-section of the soil, which reveals layers called horizons. Horizons Soils usually contain four layers or Horizons: O, A, B and C. O Horizon (organic matter) - This is a non-decomposed layer of vegetation, also called plant litter. A Horizon (topsoil) - This is the upper layer of the soil and is called the topsoil. Most of the organic material can be found here. Topsoil contains humus and so is dark in colour. In wet conditions, minerals can be leached (washed downwards) from this Horizon. A hardpan layer of leached material may form at the bottom of this layer. This is an impermeable (water cannot flow through it) layer and the soil may become waterlogged. 3 P a g e
B Horizon (subsoil) - This is known as the subsoil and is light in colour as it contains less humus. It s also stonier than the A Horizon as it is nearer to the bedrock or parent material of the soil and is protected from weathering and erosion. C Horizon (parent rock) - This is the bedrock or parent material of the soil. Soil Formation Soil formation is based on a number of factors: Climatic conditions Parent material Vegetation Living organisms 4 P a g e
Water Time Climatic Conditions The climate of an area is the main influence on soil type. Rainfall and temperature changes determine the weathering and erosion that the rock experiences. Moisture and heat influence the type of vegetation and microorganisms that can live in an area and therefore the organic material in the soil. When the rainfall is heavy, leaching is the result. Leaching is the washing down of minerals and nutrients to lower horizons. Parent Material This gives the soil its character. Parent rock is the original rock, which has been broken by weathering. The process of weathering breaks down the parent material into tiny mineral particles. The minerals provide nutrients for living organisms. Vegetation When vegetation dies it is broken down by micro-organisms to form humus. The type of vegetation is an important factor in the quality of humus produced and the type of organic matter in the soil. Living Organisms The soil is broken up by the action of living organisms. They eat the organic matter and mix the layers of the soil. When they decompose they provide a natural fertiliser in the form of humus. Micro-organisms like bacteria and fungi break down dead plant and animal remains within the soil. Plant roots bind the soil together and help prevent erosion. Water Plant roots can only take in minerals if they are dissolved in water. The minerals within the soil can also be washed through the soil by leaching. The amount of water depends on the soil texture and parent rock type. Time The development of a soil may take thousands of years. As soil formation is an ongoing process, soil is a renewable resource. 5 P a g e
Soil Texture Texture refers to how fine or coarse the soil is. Soil can be sandy, clay or loam. 1. Sandy: dry, light and gritty soil. They are well drained, but this can lose nutrients. 2. Clay: have a heavy texture as clay particles prevent water passing through. Waterlogging may result and these soils are difficult to plough. This type of texture is best used for pastoral farming (growing of grassland for animals). 3. Loam: consist of a mixture of sand and clay. They are well drained, retain minerals and are well aerated. They are suitable for crop farming and gardens. Soil Types A number of soil types can be determined based on factors such as texture and humus content. Brown Earth Formed in areas with deciduous forests (lose leaves in winter) and so a lot of rotting vegetation. The level of rain here is relatively low. They are dark brown as they contain rich organic humus. This creates a fertile topsoil, which makes the land fertile for growing crops. Brown earth is the most common type of soil in Ireland. Peat Soils Dark in colour and develop in mountain and low-lying regions with poor drainage. These areas have become waterlogged and so lack oxygen. Micro-organisms cannot exist without oxygen. Peat consists of layers of partially rotted vegetation. The vegetation is usually coniferous trees Peat soil is rich in humus, but it is infertile as it is waterlogged. It is valuable fuel and can be found in two locations: blanket and raised bogs. 6 P a g e
Podzols Developed in cold and wet areas covered by coniferous forests. Heavy rain caused leaching and hardpan developed Podzols are low in humus and so are relatively infertile A Horizon has grey colour as result of development of hardpan Gley Grey in colour Develop where soil or rock beneath is impermeable Gley soils are therefore waterlogged They have limited fertility due to poor drainage The soil is unsuitable for cultivation as it is heavy to work with. It is used for pastoral farming. Latosols (tropical red soils) Found in tropical climatic regions Process of weathering by chemical means is apparent in this climate The conditions here result in decayed organic matter being broken into humus at fast rate Leaching process experienced in the climatic zone. All the minerals are leached except iron. Iron rusts and turns red when exposed to oxygen and this gives the soil its colour Deforestation is causing soil to be starved of nutrients, making it infertile. Irish Soils There are four main soil types in Ireland: brown earths, peat soils, podzol soils and gley soils. Brown earth is the most common of these. Brown Earth Soils After ice age, Ireland was covered with boulder clay. Deciduous forests and grasslands grew on the boulder clay. The forests shed their leaves, providing organic matter for rich organic humus. These soils are fertile and are used for intensive arable farming. Peat Soils Peat soils in Ireland are found in two main locations: 7 P a g e
Bog type Location Deposits Location Blanket Mountains Shallow Wicklow Mountains Raised Low lands Deep Glacial hollows of Central Plains (Bog of Allen) Podzol Soils They are found in poorly-drained upland areas in counties Cork, Wexford and Tyrone. Gley Soils They can be found in the areas of Offaly, Clare and Galway. Human Interference Deforestation Vegetation binds the soil together. It also absorbs the rainfall and controls the surface run off. Decaying vegetation is a source of humus for the soil. Removal of the vegetation by deforestation takes away this source of nutrients and exposes the soil to the elements. The actions of wind and water result in erosion of the soil. With no vegetative cover there can flooding in low-lying regions. Solutions - Manage forest correctly (process of clear feeling, cutting down of trees in area with no replacement, has to be stopped) Reafforestation and controlled felling When trees are felled they should be replaced by new trees Farming Methods Over cropping - This is the planting of the same crop in the same land area time after time. This exhausts the fertility of the soil. Monoculture - 8 P a g e
This is the farming of a single crop, and usually occurs where large areas of land are used for cash crops. Cash crops are defined as crops produced for the export market. The continuous use of the soil by crops like coffee requires good farm management. Overgrazing - Grazing cattle destroy the roots of the vegetation. Without the roots there is no binding agent in the soil. Overgrazing of the land results in the removal of the vegetation. If it occurs in a warm climate the dry conditions can result in wind erosion of the soil. In come cases farmland may turn into desert (desertification). Solutions 1. Use of fertilisers and crop rotation (plant different crops in different fields from year to year) are solutions to monoculture and over cropping. 2. Irrigation (watering) of the soil will reduce desertification. 3. Decreasing number of cattle grazing would reduce overgrazing. 9 P a g e