What is soil?

There are many elements that contribute to the formation of Soil and they are: climate, time, organisms, parent material, relief. Climate is the most important with determining soil characteristics. Soils are a valuable non renewable source. 

Horizons are the different levels in the soil’s profile, in each level something different is happening within the soil. It is the downward movement of material by leaching and eluviations that creates the layers within the soil profile. These horizons show the major inputs and transformations from the soil. Eluviation is the downward movement of particles in the soil.

H – is the top layer of organic material which is saturated with water

O – decomposed litter which is mixed with minerals

A – this is the mineral horizon

E – this layer is depleted of material and is mainly silt or sand particles, therefore resulting in a lighter colour

B – it is the subsurface, the zone of accumulation

C – the layer of unconsolidated parent material

R – this parent material is hard bedrock eg limestone

H – Humus = organic matter. Organic matter is dead organisms and plant remains. There are three stages to a humus layer. From top to bottom layer one is Mor – twigs, leaves etc, middle layer is Moder – which is the partly humified remains and the bottom layer is Mull, which is well decomposed organic matter. Humus is very stable yet very variable, it is known as a chemical junkyard. 

Soil composition

Air 20-30%

Water 20-30%

Organic Matter 5%

Mineral 45%

Clay has the smallest soil particles.


A Platy soil structure is produced after a lot of freezing and thawing in the soil. 

Gravitational Water – removes nutrients from the soil as it moves down macropores, it takes nutrients lower, moving it away from the plants, making growth harder.

Capillary Water – it’s found in the micropores of the soil and therefore is found higher as the plant roots can find this water. 

Hygroscopic Water – is different as it is useless for plants as it is in vapour form.

Water in the soil; www.growflow.com.au

Gleying (mottling) is due to waterlogged soil.

Podzol – have a Ea layer which is bleached by the leaching of humic oxides and also has a Bh layer, this layer is blackened by humic oxides. (Chapman J. 2002) it is founds in a cool, humid and temperate environment. Rainfall is a major input. 

IronPan – has a really thick dark humus layer. The soil below has a glacial origin and is called induration. 

Rendzina – bright white soil, its chalky and good for grasses.

Rendzina. credit K. Rilling, CC BY-SA 3.0 DE

Gleysol – water logged environments, it provides anaerobic conditions. 

Gleysol © ISRIC, http://www.isric.nl

Ferralsol – an old soil, it has been thought high levels of weathering, it is a bright red colour due to mineralization. 

Histosol – a soil which has a lack of O2. It is very dark due to rapid accumulation of organic matter. It has a very hight water and organic matter input. 

Spodosol – lower layers are stained with aluminium and iron oxides, due to the acidic soil formed by the amount of needles from coniferous trees which decompose and it is these needles that form a weak acid during decomposing.

The calcification process occurs when the soil has a high organic matter amount and there is calcium carbonate precipitated from water. This produces a bit of leaching leading the A horizon to become dry, yet organic rich. The vegetation is mainly grassland.

Peds are small particles in the soil that form clusters and therefore making bigger pore spaces, for water and air. 

Temperature regulates the rate of decomposition. Therefore showing that Evapotranspiration and rainfall affect the movement of water and leaching. 

Soil structure – the proportions of sand, silt and clay and the bonds which form aggregates 

Soil Colloids – are clay minerals that have a large surface area and a positive charge, they include hydrous oxides of iron and aluminium. 

The cation exchange capacity (CEC) is the soils ability to absorb the cations. A soil with high CEC has a high change of particle collision. A clay rich, high humus soil will have the most collision. (Ashman, 2002)

Rainfall leads to leaching, and when certain types of rock are decomposed by this process a process called lateralization occurs, it deposits large amounts of ferric hydroxides and aluminium. (http://www.bibarch.com/concepts/Theory/Matrix.htm)

Photo by Zbynek Burival

Aluminium is toxic to plants as it inhibits cell division, for example shorter roots are made, reaching less horizons, making it easier for them to die in a drought or during soil erosion. 

When areas are totally water logged all the time this dark, thick peat layer is found and it is due to the amount of bacterial activity, this soil is found normally in low lying areas where rainfall is high. Iron is released from the decaying organic matter changing the colour of the soil because of oxidation. This process is called Gleization.

Salt precipitates from the water making saline soils, like the soils you will find in dry, warm conditions, the process where salts accumulate in the soil is called salinization.

salinization – Credit: Antonio Jordán (distributed via imaggeo.egu.eu)

Nutrients are a substance used by an organism for food. They need certain amounts to be at full potential. The nutrients they need are below. The three most important nutrients are Nitrogen, Potassium and Phosphorous. Soil nitrogen is found in the soils organic matter, it is found in the form of ammonium and nitrate. If a soil has too much water it leaves the plants with nitrogen deficiency. 

Macro Nutrients 

C & O = Atmospheric

PO, SO, NO – Anions = Soil Sources

NH, K, Ca, Mg – Cations = Soil Sources

Micro Nutrients 

All are Soil Sources –

Mo, BO – Anions

Cl, Fe, Mn,Zn, Cu – Cations 

The exchange of nutrients within a pool is referred to as turnover. 

Soil Surveys map the distribution by measuring the soil and inspecting it.

Soil degradation 

Catena is…

The sequence of soils that occupy a slope transect, from the topography divide to the bottom of the adjacent valley. (Smithson, 2002) 

Bulk Density = weight of soil per unit volume

Air Capacity = percentage of volume occupied by air

Water Holding Capacity = water held once drained (using gravity)

Soils can become contaminated by many different human influences, they include –

  • Sewage sludge, pathogens
  • Metals from mines,
  • Landfill sites,
  • Fertilizers

Elevate chemical levels in the soil is counted as contamination. These contaminates can be divided into organic and inorganic compounds. Organic include, pesticides through leaching, PHA. Inorganic includes, acid rain, metals and radiation. 

Soil Erosion

It happens in two main steps: 

  1. Is the dislodging of soil particles
  2. The movement of particles to a new location 

Soil erosion increases the pollutant risk, it is the least renewable physical component of an ecosystem. Erosion is important to understand as it reduces plant root depth, leading to less plant species in that area, it is however a natural process but human activity does increase it. 


Soil is formed when the mineral material from rock and organic matter combine. There are many different processes which break up soils.

  • Physical Weathering 

Two main types are Thermal where heat causes expansion and the second is Mechanical, e.g Frost Shattering. 

  • Chemical Weathering 

Hydrolysis – H20 separated into two molecules that attack the mineral bonds within the soil.

Carbonation – this is basically accelerated hydrolysis due to biological activity.

Hydration – the absorption of water e.g like pasta. 

Dissolution – the minerals dissolve from the soils.

Oxidation and Reduction – is the loss and gain of electrons through the contact of air/oxygen.


Ashman M.R, Puri G. 2002.Essential Soil Science, a clear and concise introduction to soil science. Blackwell Science Ltd

Chapman J. L, Reiss M. J, 1999. Ecology Principles and Applications. 2nd edn. Cambridge University Press   

Smithson P. et al, 2002. Fundamentals of the physical environment. 3rd edn. Routledge UK.           Pages 398 – 418

http://www.forestry.gov.uk/pdf/FCIN078.pdf/$FILE/FCIN078.pdf visited: 23/12/2010 

http://www.bibarch.com/concepts/Theory/Matrix.htm visited: 27/12/10

http://www.uwsp.edu/geo/faculty/ritter/glossary/S_U/salinization.html visited: 27/12/10

http://www.uwsp.edu/geo/faculty/ritter/glossary/E_G/gleization.html visited: 27/12/10

http://www.microbiologyprocedure.com/microbes-and-lithosphere/soil-water.html visited: 27/12/10

http://www.uwsp.edu/geo/faculty/ritter/glossary/A_D/calcification.html visited: 27/12/10

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