Soils:

1st lecture

•   Describe and categorise the principal weathering reactions that occur in soils.

• Hydrolysis - the breakdown of rock by acidic water to produce clays and soluble salts.

• Solution (acid-base)  - removal of rock in solution by acidic rainwater. In particular, limestone is weathered by rainwater containing dissolved CO2 , (this process is sometimes called carbonation)

• Oxidation - the breakdown of rock by oxygen and water, often giving iron-rich rocks a rusty-coloured weathered surface

• Chelation: complexation and transport of ions by chelating agents. Primarily organic ligands derived from decay of plant materials.

•   Discuss how weathering reactions at the Earth's surface are able to modulate atmospheric temperature over geologic timescales.

• So something in the atmosphere causes warming (natural variations in incoming solar radiation as shown by the top squiggly line but also co2 levels) (when there is increased solar radiation there is increased co2 because of more respiration). The result of this is a warmer climate

• This will lead to an increased temperature which results in more precipitation and in turn more vegetation growth. The plants respire by taking in co2 so that can increase the amount of chemical weathering that occurs which increase co2 transport (if we take it in and incorporate into rocks it can be buried and taken out of the system)

• This then leads to cooling due to removal of co2

• Therefore, the earth is able to modulate climate at a global scale, but happens over a long period of time. This is an example of negative feedback

•   Predict the effect of pH and specific adsorption on the surface charge of soil minerals of different types.

• Surface charges are the main driver for attraction between molecules and in soil it's no different

• surface charge of a particle can vary depending on its pH

• Surface of the colloid has a negative charge. If you put it in solution it gets surrounded by cations and anions. The electrical double layer refers to how the layer of electrical layers surround the particle

• pH is not the only variable affecting charge. - Specifically-adsorbed species (e.g. phosphate increases negative charge, CEC and pH0 )

•   Describe the principal features of gley, podzol and allophanic soils.

NZ Gley soils

gley soils form where soils are saturated for long periods due to high or perched water tables, or slow drainage. they have a distinctive pale blue-grey colouring due to reducing conditions (Fe2+/Mn2+) and Mottles form where cations oxidise -> along root channels/profile texture changes (Fe3+/Mn4+)

NZ Podzol soils

podzol soils occur in areas of high rainfall and are usually associated with forest trees with an acid litter. they have a Pale “E” horizon beneath topsoil -> Leaching. Fe, Al & Si-bearing minerals moved down the profile. They have an organic-rich, red Fe-rich layer underlying leached horizon and are extremely acidic (pH ~4)

NZ Allophanic soils

allophanic soils are rich in allophane clay minerals formed from the weathering of volcanic glass in tephra. They strongly adsorb anions such as phosphorus (P), sulphur (S) & carbon and have Excellent texture for plant root growth. But requires careful management as natural fertility is low. They are pH dependent and their negative charge increases with pH

2nd lecture

•   Explain the difference between pH and alkalinity and describe and categorise the principal reactions that contribute to alkalinity.

• Alkalinity: a measure of the ability of a water body to neutralise acidity whereas pH is a measure if acidity [H+] ions

• If we have a solution with alkalinity to buffer that acidity, adding acid keeps the pH stable and will do that up until a point where all the buffering capacity is gone, so all of the alkalinity ions have accepted protons and

• Dissolving co2 in water gives us the acidity CO2 (g) + H2O ( H2CO3 ) → H+ + HCO3- and how much of this occurs depends on whether we are in an open or closed system. Open systems allows more exchange with the atmosphere and so can produce more acid than a closed system

• Once in the water, the acid can then dissolve the carbonate CaCO3 (s) + H2CO3 → Ca2+ + 2HCO3 – which provides the alkalinity to those waters (it’s ability to buffer acidity)

• CaCO3 + H3O+ ↔ HCO3 - + Ca2+ + H2O (​​This reaction is the most important contributor to the alkalinity of a water body)

•   Describe how waters evolve along groundwater flow paths.

• water may run straight across the land and into the ocean or it may go underground and move into the groundwater. It can then stay in the shallow regime or can percolate further into the deep regime where it encounters crystalline rock. Because this groundwater moves slowly its in contact with the rock for long periods of time meaning that the groundwater can evolve chemically while it moves

•   Explain why soils may be more or less enriched in different mineral fractions.

• Mineral fractions are the different components that make up the mineral content of the soil.

• Primary minerals then dissolve into secondary minerals. All of these reactions require acidity and to release cations which then become available for plants or organisms

• They not only release cations for other uses but also consume the acidity

• However, they don’t all weather at the same rate - it is a dependent on their resistance to chemical weathering

• Weathering resistance is determined by the degree of Si-O-Si bonding and ligand exchange kinetics of the mineral-derived cations

• so soils can become more enriched it weathering resistance is low

• Therefore, minerals with isolated Si tetrahedra that are bonded through other cations have low molar Si-O ratios, low numbers of bridging oxygens per Si tetrahedron, and weather most rapidly.

• The more complicated the chemical formula/ the more addition of cations to the mineral formulas mean they have lower molar silicon oxygen ratios and so bonds not as strong

Redox Ladder: energy sources for microorganisms

• Microbes will always use the one further up the list as it is easier to get energy from them 

• E.g they won't use manganese if there is nitrate available

• The redox ‘ladder’ is defined by the energy available to microorganisms associated with specific electron acceptors

overall: Primary minerals then dissolve into secondary minerals. All of these reactions require acidity and to release cations which then become available for plants or organisms. acidity, land use, weathering resistance (Therefore, minerals with isolated Si tetrahedra that are bonded through other cations have low molar Si-O ratios,low numbers of bridging oxygens per Si tetrahedron, and weather most rapidly.), and the presence of microorganisms can lead to the enrichment of soil fractions

•   Discuss the causes and chemical mechanisms underlying the arsenic problem in SE Asia.

• Arsenic is naturally produced in geothermal systems and is a byproduct of mining operations so there are areas more prevalent with it than others

• This is because of reduction

• When reducing iron from iron 3 to iron 2 it kicks out the arsenic that is adsorbed to the iron oxides and the arsenic is in a soluble form and so can go into water and be carried around as a result of the iron being reduced and as a result can no longer hold onto the arsenic ions which then go into solution

• This is a particular problem when there are high levels of ion in groundwater. In some countries the cost of treating the water is too expensive

• You dont have to have just reduction to have this effect

• If you are in an oxic environment, the iron state depends on the function of pH so once at alkaline conditions the arsenate is no longer adsorbed to the iron even though it is oxidised, so it can then go into solution.

• In some cities, the groundwater they rely on is heavily reduced so are at risk of high quantities of soluble arsenic in the water. This is more a problem for poorer cities who cannot afford to treat the water