Geology And Geophysics

How does Geology Affect Soil Composition



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What most people call "dirt" is being eroded at an alarming rate across most of the world's land mass and is not replaceable. Soil holds together the masses of entangled roots of the ever-shrinking Amazon rainforest in Brazil, but is being washed away by the deforestation perpetrated by logging companies.




Soil is something we all take for granted then, but is ultimately essential for most of the life on earth, including the human race. Without soil we would have no crops and therefore no cattle and would thus starve very quickly. The need for soil conservation is of utmost concern to ecological companies and governments, but this can only be achieved by understanding the principles of the geology that affects the composition of the soil itself.




What is soil?




Soil is a mixture of organic and inorganic rock material, formed over thousands of years, which covers most of the land surface of the earth. Soil is formed from bedrock via geological processes such as tectonic plate movement, which gives rise to sedimentary or consolidated rock. Turbidity currents deposit sedimentary rock, where as igneous rocks (unconsolidated) are formed by volcanic activity.




Both tectonic and volcanic geological processes give rise to the Parent Material, which supplies the soil with most of its minerals.

This article seeks to explain how the main soil forming factors influence the formation, structure and chemical properties of soil, partly in relation to geology (the parent material/bedrock). The five soil forming factors are: 1) parent material), 2) climate, 3) topography (relief), 4) biota and 5) time.

In both cases the bedrock is exposed and acted upon by the four other factors. These will interlink to a certain extent and enhance or inhibit the soils' structure, texture, biotic content, pH and mineral content.

1) Parent Material

The parent material itself dictates which minerals the resulting soil will gain and therefore determines the structure, texture and drainage characteristics of a soil. In the example of luvisols (clay soils), the igneous parent material will provide the soil with a lower translocation potential. This being the rate at which materials i.e. minerals/nutrients move within a soil solution (osmosis), and those which can result in a gleying or water logging of the soil, due to the poor drainage qualities of the clay minerals inherited from the parent material (the immediate geology of the area).

As previously mentioned, other factors may influence each other in the formation of soil. Climate may mechanically weather bedrock in the process of synthesis (soil formation); precipitation or aelonian weathering (wind erosion) forms new clay minerals. This involves the removal of silica (which is quite unreactive) from a rock.

The abundance of silica, and the relative ease at which it is chemically weathered (in comparison to igneous rocks), may explain why clay soils predominate throughout the world. This is because silica is found in up to 90% of rocks, and as it is weathered easily. It is also coarse grained meaning clay mineral particles with smaller particle size are left intact to form the majority of soils.

Indeed with texture in relation to parent material, soils containing silica (sandy soils) will be more resistant to mechanical weathering which produces a stonier or coarser textured soil.

This is because as it is chemically unreactive, particles will simply be eroded and form soil consisting of larger particles.

Also the type of clay e.g. Kaolinite clay can be influenced by pH factors. Temperate climates, such as Britain's, usually give the soil a pH of 6.5 and are common around arable and lowland areas. As stated the synthesis of clay minerals, which involves combining silica, alumina (aluminium particles) and cat-ions, creates this absorption of silica.This is mainly due to the fact that aluminium found in the chlorite compounds of a rock is insoluble above pH 5 so after weathering the alumina can absorb the silica in the higher levels of pH.

The resultant Kaolinite clay is an indicator of soils with low silica content; this reveals that geology, (particle size of the minerals) in relation to mechanical weathering, has formed a clay soil because its particles are finer and more conducive for retention of water. This is advantageous in an arable area for crop growth in conjunction with a silty soil.

Also the humus contained within a soil is affected by an-ion absorption and exchange (negatively charged ions) as oppose to cat-ions (positively charged ions). This soil has low humus content because of its clay mineral particles, which have a low specific surface area'. This means the soil has a lower surface area to volume ratio, which inhibits cat-ion charge and promotes anion charge, this increases the alkalinity of the soil (pH increases). Here the nutrient nature of the soil is affected in relation to plant growth, as plants obtain most of their nutrients from cat ion exchange, by exchanging cat-ions for hydrogen ions through the root system.

The relevance to soil as a biotic effect on plants is that the more hydrogen that is present, the more alkaline the soil. Here the geological process of forming parent material has indirectly resulted in the alkalinity of a soil being affected. This is advantageous for species such as birch which thrive in less acidic environments.

Parent material can also affect temperature change as different minerals have varied temperature fluctuations (insulation weathering': N, M. Comber 1960) this is due to "each mineral having a different thermal coefficient". This meaning that igneous crystalline containing Feldspar may be chemically dissolved by CO2 in rainwater in a crevice. In granite, clay particles and potassium will accumulate where it becomes available for plant use. The parent material causes a decrease in temperature, inhibiting the uptake of minerals, i.e. the temperature uptake in the soil.

2) Climate as a factor

Climate as a soil-forming factor may also act upon minerals in bedrock, either chemically or mechanically. The mechanical degradation of rock occurs by precipitation, by rain splash', wind erosion, by ice/glacial erosion, temperature and the growth of plant roots.

Precipitation has greater effects at higher altitudes where rain falls in greater quantity and, therefore physically removes particles, which will eventually become soil. Similarly the melting of snow and ice will erode loose material. The movement of glaciers creates an abrasive action as the ice picks up rock particles, which grind against the rock surface underneath the glacier.

Wind erosion can occur which is greatest in arid areas where vegetation cover is sparse, so the effect is enhanced.

Temperature may also influence the growth of plants, especially on the equator where the climate is humid. Here plant growth is accelerated, and so the decomposition of leaf litter is greater, leading to an increase in the humus content in a soil.

Chemical weathering, which may take place via water infiltrating cracks and crevices in parent material, produces CO2 after hydrolisation. Here the water may expand by up to 9% creating rock fracturing; this relates to insolation weathering as described as a result of the individual mineral properties. These increased temperatures also help to fragment the rock.

The resultant minerals from weathering may affect the fertility of a soil; this is the case in the example of the mineral Apatite. This is the most commonly occurring phosphate mineral, which provides the soil with nutrients.

3) Topography

Another factor in determining the nature of soils is the topographical features of the land, including the aspect (north or south facing). On a rocky outcrop on a cliff face or a scree slope for example, the weathering (determined in part by the aspect of a hill: north or south facing) effect is increased due to the rocks exposure to rain and wind.

Also the slope gradient will effect the drainage and the depth of soils . High slope gradients will result in higher drainage capacity and raindrops will move more freely due to the effect of gravity. Low-angled slopes will produce poor drainage and result in poor humus decomposition.

On hilly regions of land, unconsolidated material residues on the surface will slide down a slope due to gravity (solifluction). The resultant substance at the bottom which become weathered by water and wind will develop as colluvial soil, which has unique properties because of how it was formed in relation to the parent material. This weathered substance will help to constitute soil along with organic or biotic factors.

4) Biotic (living) factors

Biotic factors such as the decaying biomass of detritus will directly affect soil fertility as nitrifying bacteria such as Clestridium psasterium, fix nitrogen from decaying organisms, anaerobically. While these organisms are not using oxygen to respire (break down food) the soil retains aeration; providing efficient transport of minerals which increase the fertility of the soil. The fertility of the soil is also related to the mineral particles present, so in effect geological factors are linked to biotic ones, as plants will grow relative to mineral content, and bacteria will decompose relative to plant/animal matter.

In some soil horizons such as Brown Earth soils (moderate decomposition and pH5.5) the main transfer of nutrients is done by default by burrowing invertebrates. The transfer of nutrients in the soil by elluvial or leaching process here alone is insufficient as this soil has a B' horizon which contains mostly sand so nutrients are lost. Bacteria help fix nitrogen to leguminous plants such as clover and so recycle some of the nutrients provided by the geological parent material.

The humus found within the soil also provides a place for some microorganisms to live and carry out the process of helping to convert minerals into nutrients via plant roots.

Time, as a factor in soil formation is important in conjunction with other factors such as climate, because the thin layer on top of the parent material may take one hundred years to form. This is alongside the correct conditions of humidity and rainfall being met; warmth for plant growth and limited rainfall to decrease the effect of erosion. If any of these factors are not met the soil forming process may take hundreds of years longer.

5) Time

The time taken to form the initial layer is crucial; as once it has been formed the other four factors will take hold and add mineral properties and structure etc. The less erosion that has to take place the quicker a soil can form, for example softer rocks such as calcified limestone, will be more susceptible to chemical weathering and produce CO2 faster. This will erode particles by oxidation and give nutrients to a soil.

In conclusion, geological processes provide the initial parent material, which supports a soil for up to a hundred years. As time becomes an influencing factor, climate weathers the mineral particles in conjunction with topographical features such as slope angle influenced by gravity; ultimately depositing the material. Plants and animals add organic matter, which helps to form the soil by binding mineral particles.

Also, since the Holocene epoch man as a species may have become a sixth soil-forming factor'. This is because humans can manipulate large expanses of land, and influence organic contribution to soils via burning of rain forests; this strips away areas of soil. Also, crop growth and quarrying of minerals may have a detrimental effect upon soil in years to come, as well as population densities increasing which physically disrupts soil formation.

Source(s):

Comber, N.M., 1960. An Introduction to the Scientific Study of Soil.

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