Diploma of Environmental Monitoring & Technology
Study module 3
Soil chemical properties
Sampling & testing of
soils
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STS Study module 3
Soil chemical properties
INTRODUCTION
2
Clay minerals
2
ION EXCHANGE IN SOILS
4
Cation exchange
Significance of CEC
Anion exchange
4
5
5
SOIL PH
6
Trends in soil pH
Soil buffering capacity
Significance of soil pH
Soil pH management
Soil redox potential
6
6
7
8
8
ASSESSMENT TASK
9
Assessment & submission rules
Problems?
References & resources
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Introduction
Knowledge of the chemical composition of a soil is less useful than knowledge of its
component minerals and organic materials. These dictate the reactions that occur in the
soil, and the availability of nutrients, the elements of which, though maybe present from the
analysis, are not necessarily free for plant uptake.
Clay minerals
Minerals are naturally occurring inorganic compounds, with defined chemical and physical
properties. Their parent materials form in the crystallisation of molten rock material: these
are known as primary minerals, and include olivine, quartz, feldspar and hornblende.
Primary minerals are not stable when exposed to water, wind and extremes of temperature.
They weather, which means they break down physically and chemically. Some of the
elements that are released during weathering reform and crystallise in a different structure:
these are the secondary minerals, and include vermiculite, montmorillonite and kaolinite.
Secondary minerals tend to be much smaller in particle size than primary minerals, and are
most commonly found in the clay fraction of soils. All but the youngest and unweathered of
soils will contain mainly secondary minerals. The major elements in the earth’s crust are
shown in Figure 3.1.
Ca
Al
Fe
K
Si
Mg
Na
Others
O
Figure 3.1 - Elemental composition of the earth’s crust
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The dominant element, oxygen is negatively charged, while the other major elements are
positively charged: thus oxygen bonds with one or more of the cations, producing a
chemistry of oxides. Silicon oxides (silicates) and aluminium oxides (aluminates), generally in
combination as aluminosilicates, dominate the minerals. Small concentrations of other
elements account for the differences in minerals.
In silicate, silicon binds to four oxygens in a tetrahedron while aluminate has six oxygens
(often as OH) surrounding the central aluminium ion in an octahedron. However, in each
case, it is not a matter of individual SiO4 or Al(OH)6 units. A number of the oxygens are
shared between the silicate or aluminate units, giving rise to a two-, or some cases three-,
dimensional structure.
The most common structure in clay minerals is the formation of sheets, which are “flat”
layers of silicate tetrahedra or aluminate octahedra. In clay minerals, these sheets stack on
top of each other, and are held together by hydrogen bonding or electrostatic attraction.
The common structures are classified by the proportion of each type of sheet as shown in
Figure 3.2.
Figure 3.2 - Sheet arrangements in clay minerals (silicate sheet in grey)
Real clay crystals are not pure silicates or aluminates: some Si or Al atoms are substituted
during the crystallisation process, creating spare charges which give the overall crystal a
charge which must be balanced by loose cations or anions. For example, a silicon atom (4+
charge) is substituted by an aluminium ion (3+). This creates a 1- charge, which is then
counterbalanced by a cation, such as potassium, as shown in Figure 3.3.
Figure 3.3 - Generation of cation-exchange sites in clay minerals
These cations generally are held on the surface of the clay, and are not strongly held. They
can be exchanged for other cations in an equilibrium process. The extent to which this
process can occur is known as the cation exchange capacity (CEC). This will be discussed
more later in this chapter. The majority of clay minerals have exchangeable cations, and the
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soil pH has no effect on the exchange capacity of the mineral matter.
As minerals weather, they lose silicon (as soluble silicic acid), leading to increasing
proportions of aluminate in weathered clays, such as kaolinite. Aluminium hydroxide species
are amphiprotic, meaning that they can accept or lose protons. As a consequence, soils
dominated by oxides of aluminium (and other metals) can have positive sites, allowing anion
exchange, as shown in Equation3.1.
𝐴𝑙 − 𝑂𝐻 + 𝐻 + ⇌ 𝐴𝑙 − 𝑂𝐻2+ + 𝑋 −
Eqn 3.1
Ion exchange in soils
Ion exchange occurs 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, and is necessary for soil fertility. As soils weather, they lose cation
exchange capacity and lose fertility.
Cation exchange
As described above, clay minerals have negative charge due to substitution of aluminium or
silicon in the crystal lattice. Humus – the stable organic matter of soil – also contributes
negative charge, due to the presence of dissociated organic acids, as shown in Equation 3.2.
The negative charge is balanced by the presence of adsorbed positive ions (cations), which
are held on the surface of the clay mineral or humus particle.
ℎ𝑢𝑚𝑢𝑠𝐶𝑂𝑂𝐻 ⇌ ℎ𝑢𝑚𝑢𝑠𝐶𝑂𝑂− + 𝐻 +
Eqn 3.2
The process of cation exchange occurs when a cation in solution replaces an adsorbed
cation on the soil particle, as shown in Equation 3.3, in which initially a sodium ion is held to
the soil, but is replaced by a potassium ion in solution.
+
+
𝑠𝑜𝑖𝑙𝑁𝑎 + 𝐾(𝑎𝑞)
⇌ 𝑠𝑜𝑖𝑙𝐾 + 𝑁𝑎(𝑎𝑞)
Eqn 3.3
You should also be aware that it is charges that are balanced, not number of charged
species.
The exchange reaction is an equilibrium one, which means that it is reversible and
dependent on the levels of each of the species, particularly the solution species. For
example, if a soil solution becomes depleted in calcium, then some calcium will desorb from
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an exchange site into solution. This is known as buffering, and means that in all but the most
leached and infertile of soils, there will be a balance between adsorbed and dissolved ions.
The cation exchange capacity (CEC) is defined as the moles of exchangeable positive charge
per unit mass 100 g of dry soil (this can be mmole/100g or cmole/kg, giving the same value).
Calcium and magnesium ions contribute twice as much to the CEC as an equivalent number
of sodium and potassium ions because of their 2+ charges. Table 3.1 gives typical CEC values
for some soil texture classes. The size and charge of the ion affects the strength of attraction
to the soil particle. Smaller ions and those with 2+ or 3+ charges are more strongly
adsorbed. Those that are more strongly adsorbed are less likely to be exchanged.
Soil Class
CEC
Sand
2-4
Sandy loam
2-12
Loam
7-16
Silt loam
9-26
Clay, clay loam
4-60
Table 3.1 - Typical CEC values
Significance of CEC
During periods of high rainfall, the relatively pure water passing down through the soil will
tend to cause the adsorbed ions to be removed from the exchange sites, to be replaced by
H+ which is more common in rainwater than normal soil solution.
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. Once enough are removed
from soil solution to cause a disturbance to the exchange equilibrium, some of that ion will
desorb from the soil particles and be replaced by another ion, possibly the H+ released by
the plant. However, if the nutrient is a weakly adsorbed one, such as potassium, there may
not be enough adsorbed to replenish the soil, presenting a fertility problem. Of the three
most important cations for plant growth (K, Ca, Mg), potassium is the one most likely to be
in short supply.
Anion exchange
The important soil anions (e.g. nitrate and phosphate) behave very different at exchange
sites. Nitrate (and chloride) are only weakly held at positive sites on the clays, and are more
likely to be found in soil solution. Phosphate (and sulfate to a lesser extent), on the other
hand, are very strongly bound to the exchange sites, to the point where phosphate becomes
covalently and irreversibly bound.
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Soil pH
The pH of a soil is one of its most important properties, because it affects so many other soil
properties, such as ion exchange and nutrient availability. In this section, we will look at how
soils develop different pH levels. The soil pH comes about from a balance between acidic
and alkaline species in the soil. The soil pH reflects mainly the levels of dissolved H+ and OH-,
but also the adsorbed H+ on cation exchange sites. Soil pH values under normal
circumstances range from 4-9.
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
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 sodium and potassium 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
Trends in soil pH
As soils age by weathering and leaching, they tend to become more acidic. The primary
minerals that release alkaline materials are replaced by neutral or slightly acidic secondary
minerals, and 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. Cation exchange sites lose the basic cations and have increasing levels of adsorbed
H+ and Al3+.
Increased levels of aluminium are a problem, and the percentage of aluminium on the
exchange sites is known as the percentage aluminium saturation. This can become as high
as 90% in acidic soils.
Soil buffering capacity
Buffering capacity is the ability to reduce the effects of an added component. In this case, a
soil will have some capacity to resist change in pH as a consequence of addition of extra
acidic or alkaline materials.
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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 (see Figure 5.4)
◗ effect on soil organisms – soil organisms prefer different pH levels; fungi thrive in
acidic soils, while bacteria prefer alkaline ones; worms cannot exist in highly acidic
soils
◗ acid-sulfate soils - soils that are rich in inorganic sulfide minerals, such as pyrites, or
other sources of sulfide (eg relatively anaerobic areas such as swamps) can lead to
the formation of excessive levels of sulfuric acid through oxidation. The soil pH dives
to very low levels, and causes solubilisation of toxic levels of aluminium, manganese
and iron from soil minerals.
◗ plant preferences – most plants prefer alkaline soils, but there are a few which need
acidic soils and will die if placed in an alkaline environment; some of the more
common of these are listed in Table 3.2
Plant group
field crops
Examples
peanuts, rice,
fruit & vegetables
pineapple, blueberry, strawberry
flowers and shrubs
camellias, azaleas, orchids
trees
pines, cedars
Table 3.2 - Common plants requiring acid soils
Figure 3.4 - Nutrient availability at different pH levels (from www.fao.org)
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Soil pH management
Because soils tend towards lower pH values as they age, the main need for pH management
is to reverse that by making the soil more alkaline. Given that most plants prefer alkaline
conditions, this provides another reason.
The most common method of increasing soil pH is by liming. Agricultural lime is a mixture
dominated by calcium carbonate, but also containing magnesium carbonate and calcium
hydroxide. It normally comes from ground limestone, and being a base, will increase soil pH
and add the nutrients calcium and magnesium to the soil. Dolomite lime has a higher
proportion of magnesium carbonate.
If the pH needs to be reduced, because a plant to be grown in it requires low pH, then iron,
sulfur or peat can be added to increase acidity.
Soil redox potential
The redox potential (Eh) of a soil is 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, and the lower the
value the more anaerobic the conditions. It is, however, a value that is affected by soil pH.
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Assessment task
This section provides formative assessment of the theory. Answer all questions by typing
the answer in the boxes provided. Speak to your teacher if you are having technical
problems with this document.
◗ Type brief answers to each of the questions posed below.
◗ All answers should come from the theory found in this document only unless the
question specifies other.
◗ Marks shown next to the question should act as a guide as to the relative length or
complexity of your answer.
1. What does ion exchange do to the level of ions in soil solution? [2mk]
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2. Why is anion exchange less important than cation exchange? [1mk]
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3. How do negatively charged sites occur in clay minerals? [4mk]
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4. For each pair below, identify which has the higher CEC. [3mk]
a. A young or old soil
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b. An acid or alkaline soil
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c. A soil with low or high levels of clay
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5. Why are adsorbed cations on soil minerals important in terms of long term soil fertility?.
[2mk]
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6. Why does heavy rain cause a decrease in soil pH? [2mk]
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7. What effect would soil pH have on the amount of cation sites from humus? [2mk]
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8. Based on equation 3.3, write an equation from the exchange of adsorbed sodium ions
with solution calcium. [2mk]
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9. Other than the obvious pH change, what do you think would happen, in terms of
equilibria, to a soil which was treated with lime (calcium hydroxide)? [2mk]
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10. Review table 3.1. Describe the trend. Explain what is happening. [4mk]
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References & resources
The Physical Geography website given in Study module 1 is a marvellous resource for all
facets of the environment.
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