Soil Chemistry
Chapter 5
5.1 Introduction
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basic chemical composition of a soil is less useful than a
knowledge of its component minerals and organic materials.
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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
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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
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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 Earth’s crust
Ca
Al
Fe
K
Si
Mg
Na
Others
O
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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)
1:1
2:1
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2:2
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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
O
O
Si replaced
O
Si
O
O has only 1 bond,
so has -ve charge;
requires balancing
positive charge from
free cation
O
by Al in crystal
O
Al
O
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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+ <=> Al-OH2+ + X-
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5.3 Ion exchange in soils
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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
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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
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humus-COOH  humus-COO- + H+
Exercise 5.2
 What effect would soil pH have on the amount of cation sites
from humus?
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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)
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charges that are balanced, not number of charged species.
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Class Exercise 5.3
 Write an equation for the exchange of adsorbed sodium with
solution calcium.
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soil-Na + soil-Na + Ca2+ (aq)  soil=Ca + 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
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What do you think would happen to a soil which is treated
with lime (calcium hydroxide), in addition to a pH change?
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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)
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the moles of exchangeable positive charge per unit mass 100
g of dry soil
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usually mmole/100g or cmole/kg (the same value)
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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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Soil
CEC
Sand
2-4
Sandy loam
2-12
Loam
7-16
Silt loam
9-26
Clay, clay loam
4-60
Class Exercise 5.5
 Comment on the trend in CEC
in Table 5.1.
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CEC increases with higher clay
levels
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Significance of CEC
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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
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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
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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
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rain - polluted or fresh will be slightly acidic due to dissolved
gases
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microbial and root respiration – this produces CO2, which is
slightly acidic in solution
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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
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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
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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
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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
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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
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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
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What factors will affect the amount of liming required?
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buffering capacity
pH
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Redox potential (Eh)
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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
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