o Soils are a fertile, natural resource. o Soils develop / form from the weathering of rocks in one place and from re-deposited weathered materials.
o Soil is composed (made-up) of: o Mineral Particles (pieces) weathered or eroded from the soil’s parent rock (original) over a long period of time. o Mineral particles make up about 45% of a soil. o Examples of mineral particles: calcium from limestone (original rock).
o Air is found in spaces (pores) between soil particles. o Air supplies oxygen and nitrogen that help plants to grow. o Air makes up 25% of a soil
Water is found also in spaces (pores) between soil particles. Water dissolves soluble minerals and moves fertile minerals to roots in plants. Water generally makes up 25% of a soil. – in deserts there is so little water that no vegetation can grow.
The remains of dead plants and animals. Dead plants are broken down into humus by micro-organisms (earthworms, fungi). Humus darkens the soil and increases fertility and is found near the surface. Organic matter makes up about 5% of a soil usually.
There are 6 major factors taken into account when describing the characteristics (qualities) of soil. 1 - Colour 2 - Structure 3 - Texture 4 - Organic content (Humus) 5 - PH Value (acidity) 6 - Water (content and retention)
3 major characteristics: 1 - Texture 2 - Structure 3 - Water content and retention.
The texture refers to the smoothness or coarseness (roughness) of soil. The texture depends on the size of the soil particles, which affects the pore spaces (gaps) between the soil particles. The different pore spaces mean that soils will have different: Aeration (amount of air) Drainage (water passing through)
Rough / coarse and loose textures. Large pores between particles so air and water can pass through them. Little water logging so the particles don’t stick together. Leaching of nutrients can occur and during a dry spell there may be a shortage of water.
Smooth with small soil particles and are packed closely together (tight-fitting particles). Very tiny pores so they prevent water and air passing through them. Waterlogged in winter due to heavy rain (heavy / sticky) and hard, dry cracked surfaces in summer. High in nutrients as they don’t allow leaching.
Form from roughly equal amounts of sand and clay particles. Moderate pore sizes allow air and drainage. They usually don’t become waterlogged in winter or too dry in summer. Light soils and high in nutrients so ideal for agriculture.
Soil structure describes how soil grains are lumped (cemented) together by humus and water. The grains of soil are in small lumps (cling together) called Peds. The structure of the soil depends on the shape of the peds. The spaces (pores) between the peds allow the soil to: Hold air and water or Let air or water through.
The main soil structures are: Crumb (granular) – allow air and water to pass through. Platy – where the peds are flat and overlap and prevent water from passing through (water logging occurs).
The amount of water in a soil or the amount of water a soil can retain (hold in) depends mainly on: Texture Structure Humus.
Coarse sandy soils have large pores so they allow water to pass through (little retention). Small grained clay soils don’t allow water to pass through so they are poorly drained.
Crumb (granular) structure allow water to pass through, so they can dry up quickly. Platy structure restrict water movement so they have a large amount of water.
Soils rich in humus can hold more water than those with a low humus content.
Soils vary – no two soils are identical in texture or appearance. Processes occurring within soils produce differences in texture, structure, colour etc. The major processes are: 1 - Leaching and podzolisation 2 - Laterisation 3 - Humification 4 - Weathering.
Leaching occurs when heavy rainfall percolates (soaks) down through soils. The rainwater: Washes soluble substances down through the soils Can dissolve minerals and humus from the upper layers. Though leaching is needed to bring humus to lower layers, it is often so great that the nutrients are brought so far down in soils that plant roots can’t reach them – making the soils infertile.
This can happen in sandy soils (permeable, loosely packed grain structure) as rainwater can pass through very quickly. Podzolisation is leaching that occurs in regions where there is: Heavy rainfall e.g. Irish mountains. A large amount of humic acids e.g. Under bogs or coniferous forests. The humic acids cause the rainwater to become so acidic that it dissolves most of the soil nutrients.
Laterisation is an extreme form of leaching that occurs in equatorial and tropical regions due to a combination of: Heavy rainfall High temperature. The heavy rainfall is the major factor as it washes the nutrients down through the soil. The high temperatures: Speed up the chemical reactions of water on the minerals Break down iron into iron oxide (rust). The iron oxide gives the soil a red, rusty colour and the soils are sometimes called tropical red soils.
There is dead organic matter (remains of dead plants and animals) in most soils. Micro-organisms (bacteria, fungi) cause the dead organic matter to decay slowly into a black gel. Oxygen (from air in the soil) also helps in the breakdown of organic matter in humus. Rain washes the humus into the soil and it is used by plant roots as a nutrient.
Weathering helps to break down the parent (original) rock into the mineral part of the soil: Physical / mechanical weathering breaks the rock into minerals e.g. Freeze-thaw in Irish mountains. Chemical weathering causes rocks to decay / decompose e.g. Carbonation causes calcium carbonate in limestone to dissolve.