Pedosphere

The pedosphere is the soil related sphere, where soil can be (and will be for the purpose of this blog) referred to as the unconsolidated layer above bed rock to ground surface or bio geochemically altered material at the interface of the geosphere and atmosphere. Soil has many variants and different compositions, therefore to better understand this, it is categorised into five basic groups known as soil horizons, which are distinguished by changes in composition and texture (also have distinct colouring).

These different soil horizons form as a result of mechanical and chemical weathering of a parent material. Given time the constituents of each layer begin to separate as the broken down material that they consist of begins to, form the different layers. The lower layers of the soil horizon (B,C,D) have a rock parent material, which allows for a variation between layers as different rocks are made of different minerals, thus there resultant horizons will too

                      

This breakdown of parent rock commonly takes place at the surface or in the C horizon as this is where water is most abundant and the two places in which rocks will be exposed to altering mechanisms. Water is an essential part in the breakdown of minerals, as it is a universal solvent which easily dissolves compounds due to its polarity. Therefore it easily breaks down the minerals that make up rocks and contuse to do so right down to clay formation (http://geophysics.ou.edu/geol1114/notes/weathering/wx_soil.html 25/9/2015).

Water achieves this by combining with carbon dioxide, to form carbonic acid. Using Granite as an example, the carbonic acid dissolves the alkali feldspar mineral within the rock, breaking it down into clay, silicates and remaining cations (Fig 2) and these are what form soil.

H₂O + CO₂ → H₂CO₃   H₂CO₃ + 2KAlSi₃O₈ → Al₂Si₂O₅(OH)₄ + 2SiO₂ + 2HCO₃^‾ + 2K⁺

This is a very simplistic example of soil formation (i.e. covers only one mineral in one rock type) however it highlights how minerals can be chemically broken down to form soils.

This breakdown process of parent material is influenced by four factors:

Climate- Climate controls the amount of mechanical weathering, temperature and chemistry of contributing variables. This leads into different soil types at different latitudes.

Biota- Organisms such as worms and microorganisms help breakdown plant material and ingest the nutrients within the soils. This can produce varying effects on the composition and texture of the soil.

Time- Greater amount of time allows for greater erosion and so greater production of          soil.

Parent Material- Different rocks are comprised of different minerals (see above paragraph), creating different rates at which, they will erode and what the minerals will erode into.

Topography- Area of high orogeny (mountainous areas) are more susceptible to erosion than basin areas, meaning mountainous areas will have a higher soil formation factor than basin areas which will have a high deposition factor.

As soil is formed from above and below the surface, horizons occur as different processes occur to create different layers in which the soil separates. The above factors contribute to the formation of soil horizons in their respective ways.

What does all this mean to us?

Soil is essential for our existence as it provides us with the nutrients for agriculture and horticulture. Without soil harvesting food would be a very difficult process and so it is imperative that issues regarding the pedosphere are dealt with accordingly.

The first such issue is the production of soil. The population of earth is increasing, so too is the demand for food, this drives an increase in nutrients required to produce food which in turn drives the need for more soil.

That is where the issue in lies;

Extensive agriculture in the past century has depleted the soil of nutrients, generated mass erosion and hard pans. This happens by wind and rain removing the lose fine grained sand/silt. Not only is the soil removed so too are the nutrients that were held within them.

This leads to extensive tilling at the same height to try and bring up some more nutrients. Eventually this leads to creating an impermeable layer for water as a result a salt layer forms which inhibits plant growth.

In addition to soil removal and growth inhibition, contamination has become a prevalent issue. To recharge the soil with nutrients for the continuation of agriculture, nitrogen has been added in vast quantities. This has imbalanced the nitrogen cycle (Fig 2) and caused an oversaturation of nitrate, as creating nitrate (using the Haber-Bosch cycle) for use within a system is significantly easier than removing it from a system and turning it back into nitrogen (denitrification process).

So what do we do?

Long term there is nothing we can do, as we are dependent on the formation of new rocks so that they can be eroded. Therefore we need to sustainably use what there is now. This includes multi crops which can cope with a change in climate, reduced tilling or rotational tilling (refrain from ploughing paddocks for a cycle period) and monitoring the amount of nitrate that is used for fertiliser.