Soil Chemistry

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Soil Chemistry

• Chapter 5

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5.1 Introduction

• basic chemical composition of a soil is less
  useful than a knowledge of its component minerals
  and organic materials.
• these dictate
• reactions that occur in the soil
• availability of nutrients

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Exercise 5.1

• Decrease
• uptake by plants
• leaching
• conversion into insoluble forms

• Increase
• addition of fertiliser
• decomposition of plants
• animal poo
• dissolving of rock

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5.2 Clay Minerals

• naturally occurring inorganic compounds
• form initially in the crystallisation of molten
  rock material
• known as primary minerals
• eg olivine, quartz, feldspar and hornblende
• not stable when exposed to water, wind and
  extremes of temperature
• break down physically and chemically
• reform and crystallise in a different structure

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Clay minerals

• called secondary minerals
• eg vermiculite, montmorillonite and kaolinite
• tend to be much smaller in particle size than
  primary minerals
• most commonly found in the clay fraction of soils
• only the youngest and unweathered of soils will
  not contain mainly secondary minerals

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The Earths crust
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• oxygen is negatively charged
• the other major elements are positively charged
• oxygen bonds with one or more of the cations,
  producing a chemistry of oxides
• silicon oxides (silicates)
• aluminium oxides (aluminates)
• generally in combination as aluminosilicates
• these dominate the minerals
• low levels of other elements account for the
  differences in minerals

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• Si binds to four oxygens in a tetrahedron
• Al has six oxygens (often as OH) in an octahedron
• not a matter of individual SiO4 or Al(OH)6 units
• some Os are shared between the silicate or
  aluminate units
• most common structure in clay minerals is the
  formation of sheets
• flat layers of silicate tetrahedra or aluminate
  octahedra
• these sheets stack on top of each other
• held together by hydrogen bonding or
  electrostatic attraction

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Common sheet arrangement in clay minerals
(tetrahedrons in grey)
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• real clay crystals are not pure silicates or
  aluminates
• some Si or Al atoms are substituted during the
  crystallisation process
• creates spare charges which give the overall
  crystal a charge
• balanced by loose cations or anions

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• these cations generally are held on the surface
  of the clay
• are not strongly held
• can be exchanged for other cations in an
  equilibrium process
• measured as the cation exchange capacity (CEC)
• soil pH has no effect on the exchange capacity
  from the clay minerals

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• as minerals weather, they lose silicon
• this leads to increasing proportions of aluminate
  in weathered clays
• Al-OH species are amphiprotic
• soils dominated by oxides of aluminium (and other
  metals) can have positive sites in acidic soils
• this allows anion exchange
• Al-OH H ltgt Al-OH2 X-

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5.3 Ion exchange in soils

• when the loosely held cations or anions on the
  mineral surfaces are replaced by ions of the same
  charge (sign and magnitude) in solution
• cation exchange is by far the most common
• necessary for soil fertility
• as soils weather, they lose cation exchange
  capacity and lose fertility

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Cation Exchange

• clay minerals have negative charge due to
  substitution of aluminium or silicon in the
  crystal lattice
• humus also contributes negative charge, due to
  the presence of dissociated organic acids
• humus-COOH ? humus-COO- H
• Exercise 5.2
• What effect would soil pH have on the amount of
  cation sites from humus?
• low pH, less dissociated acid, less sites

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• a cation in solution replaces an adsorbed cation
  on the soil particle
• eg soil-Na K (aq) ? soil-K Na (aq)
• charges that are balanced, not number of charged
  species.
• Class Exercise 5.3
• Write an equation for the exchange of adsorbed
  sodium with solution calcium.
• soil-Na soil-Na Ca2 (aq) ? soilCa
  2Na (aq)

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• exchange is equilibrium
• reversible and dependent on the levels of each of
  the species, particularly the solution species
• eg if a soil solution becomes depleted in
  calcium, then some calcium will desorb from an
  exchange site into solution
• known as buffering
• in all but the most leached and infertile of
  soils, there will be a balance between adsorbed
  and dissolved ions

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Exercise 5.4

• What do you think would happen to a soil which is
  treated with lime (calcium hydroxide), in
  addition to a pH change?
• high concentration of Ca in solution
• this would be partly reduced by exchange with the
  soil cations

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Cation exchange capacity (CEC)

• the moles of exchangeable positive charge per
  unit mass 100 g of dry soil
• usually mmole/100g or cmole/kg (the same value)
• Ca Mg contribute twice as much to the CEC as an
  equivalent number of sodium and potassium ions
  because of their 2 charges

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• Class Exercise 5.5
• Comment on the trend in CEC in Table 5.1.
• CEC increases with higher clay levels

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Significance of CEC

• uptake of nutrient ions from plant roots occurs
  from solution only
• as cations are absorbed into the roots, they are
  replaced in the soil solution by H ions
• when the exchange equilibrium is disturbed, some
  of that ion will desorb from the soil particles
• replaced by another ion
• if the nutrient is a weakly adsorbed one, such as
  K, there may not be enough adsorbed to replenish
  the soil, presenting a fertility problem
• K is the most likely cation to be in short supply

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Anion exchange

• the important soil anions, nitrate and phosphate,
  behave very different at exchange sites
• nitrate and chloride are only weakly held at
  positive sites
• more likely to be found in soil solution
• phosphate and sulfate are very strongly bound to
  the exchange sites
• phosphate can become covalently and irreversibly
  bound

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Soil pH

• one of its most important properties
• it affects so many other soil properties, (eg ion
  exchange and nutrient availability)
• soil pH comes about from a balance between acidic
  and alkaline species
• reflects mainly the levels of dissolved H and
  OH-, but also the adsorbed H on cation exchange
  sites
• normally ranges from 4-9

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Sources of soil acidity

• rain - polluted or fresh will be slightly acidic
  due to dissolved gases
• microbial and root respiration this produces
  CO2, which is slightly acidic in solution
• oxidation of organic matter this produces
  organic acids known as humic acids, together with
  nitric and sulfuric acids

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Sources of soil alkalinity

• carbonate minerals calcium and magnesium
  carbonate are common materials in minerals
• they are slightly soluble in water, and produce
  OH- as they dissolve
• these cations and Na K are known as bases
  because of their association with alkaline soils
• mineral weathering many primary minerals as
  they weather release hydroxide salts of the basic
  cations

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Trends in soil pH

• as soils age by weathering and leaching, they
  tend to become more acidic
• primary minerals that release alkaline materials
  are replaced by neutral or slightly acidic
  secondary minerals
• leaching removes the carbonate minerals
• weathering occurs from the surface downwards so
  that the A and B horizons will tend to be more
  acidic than the C horizon

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Significance of soil pH

• nutrient availability the ability of plants to
  take up nutrients is very much dependent on the
  soil pH

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Significance of soil pH

• effect on soil organisms soil organisms prefer
  different pH levels
• acid-sulfate soils - soils that are rich in
  inorganic sulfide minerals, such as pyrites,
• can lead to the formation of excessive levels of
  sulfuric acid through oxidation
• soil pH dives to very low levels
• causes solubilisation of toxic levels of
  aluminium, manganese and iron from soil minerals
• plant preferences most alkaline soils a few
  which need acidic soils

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Soil pH management

• soils tend towards lower pH values as they age
• the main need for pH management is to making the
  soil more alkaline
• most common method by liming
• agricultural lime is a mixture dominated by
  CaCO3, but also containing MgCO3 and Ca(OH)2
• comes from ground limestone,
• add the nutrients calcium and magnesium to the
  soil
• dolomite lime has a higher proportion of
  magnesium carbonate
• to reduce pH , add Fe, S or peat

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Exercise 5.9

• What factors will affect the amount of liming
  required?
• buffering capacity
• pH

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Redox potential (Eh)

• a measure of its ability to produce oxidation or
  reduction of chemical species in it
• the most important soil property indicated by the
  soil Eh is whether it is aerobic or anaerobic
• aerobic soils give a positive value
• the lower the value the more anaerobic the
  conditions
• a value that is affected by soil pH