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Soil Chemistry
Chapter 5
Soil Analysis Ch5 2
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
Soil Analysis Ch5 3
Exercise 5.1
Decrease uptake by plants leaching conversion into
insoluble forms
Increase addition of fertiliser decomposition of plants animal poo dissolving of rock
Soil Analysis Ch5 4
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
Soil Analysis Ch5 5
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
Soil Analysis Ch5 6
The Earth’s crust
Al
Ca
Fe
K
Mg
Na
O
Others
Si
Soil Analysis Ch5 7
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
Soil Analysis Ch5 8
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
Soil Analysis Ch5 9
Common sheet arrangement in clay minerals (tetrahedrons in grey)
1:1 2:1 2:2
Soil Analysis Ch5 10
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
Si
O
O
OOSi replaced
by Al in crystalAl
O
O
O O-
X+
O has only 1 bond,so has -ve charge;requires balancingpositive charge from free cation
Soil Analysis Ch5 11
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
Soil Analysis Ch5 12
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-
Soil Analysis Ch5 13
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
Soil Analysis Ch5 14
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
Soil Analysis Ch5 15
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) soil=Ca + 2Na+ (aq)
Soil Analysis Ch5 16
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
Soil Analysis Ch5 17
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
Soil Analysis Ch5 18
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
Soil Analysis Ch5 19
Class Exercise 5.5 Comment on the trend in
CEC in Table 5.1.
CEC increases with higher clay levels
Soil CEC
Sand 2-4
Sandy loam 2-12
Loam 7-16
Silt loam 9-26
Clay, clay loam
4-60
Soil Analysis Ch5 20
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
Soil Analysis Ch5 21
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
Soil Analysis Ch5 22
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
Soil Analysis Ch5 23
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
Soil Analysis Ch5 24
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
Soil Analysis Ch5 25
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
Soil Analysis Ch5 26
Significance of soil pH nutrient availability – the ability of plants to take up
nutrients is very much dependent on the soil pH
Soil Analysis Ch5 27
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
Soil Analysis Ch5 28
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
Soil Analysis Ch5 29
Exercise 5.9 What factors will affect the amount of liming
required?
buffering capacity pH
Soil Analysis Ch5 30
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