1. CHEMICAL PROPERTIES OF SOILS
1.1 ADSORPTION OF IONS (CATIONS & ANIONS)





adsorption of ions affects their availability to plants and mobility thereby influencing
both soil fertility and environmental quality.
ions do not form a bond with charged sites but are bridged by layers of water molecules
They are held by weak electrostatic attraction
The ions are easily replaced by others of similar charge and are considered to be in the
outer-sphere
complex
of
the
colloid
The inner-sphere complex does not involve intervening of water molecules and
therefore more direct bonds form between the adsorbed ion and the atoms on the
surface of colloids e.g. K+ is tightly held between silica tetrahedral sheet in mica; Cu2+
and Ni2+ bond with oxygen in silica tetrahedral sheets; H2PO4- is directly bonded by
sharing electrons with octahedral Al
1.2 CATION EXCHANGE REACTIONS IN SOILS
There is continuous exchange of ions adsorbed on the clay surface with those in the soil
solution
e.g. If a soil particle saturated with Ca (i.e. all negative sites are satisfied by calcium) is exposed
to a soil solution high in K, exchange takes place (see below)
Colloids, capable of holding exchangeable cations or anions are termed the cation or anion
exchange complex respectively.
1|Page
Principles governing cation reactions in soils
1. Reversibility i.e. the products of a reaction may react to form original ingredients
2. Charge equivalence i.e. cation exchange reactions take place on charge-for-charge basis
e.g. one Ca2+ replaces 2Na+ (i.e. 2 positive replaces 2 positive)
3. Ratio
Law
States that at equilibrium, the ratio e.g. Ca to Mg on the soil colloid will be the same as their
ratio in soil solution and both ratios will be the same as in the overall soil system
2|Page
NB: if the 2 cations have different charges e.g. Mg and K a more complicated modified
version of the law applies
4. Anion
effects
on
Mass
Action
An exchange reaction will be more likely to proceed to the right if the released ion is
prevented from reacting in the reverse direction (i.e. precipitates, released as gas,
volatilizes,
strongly
associates
with
an
anion)
Also the more the concentration of an ion in the soil solution, the more chances that it will
be
picked
up
for
exchange.
NB: this explains why lime is more effective in neutralizing acid soils than CaCl2
5. Cation
Selectivity
Some cations are held much more tightly than others and so some are less likely to be
displaced
from
the
colloid
The higher the charge and the smaller the hydrated radius of the cation, the stronger its
adsorption to the colloids at similar concentrations
Al3+ > Ca2+ > Mg2+ > K+ > NH4+ > Na+
e.g. Na has a small radius but carries a large shell of water upon hydration (i.e. large
hydrated radius) and is therefore adsorbed less strongly than e.g. Al with a charge of +3 and
a smaller hydrated radius.
3|Page
The order may change in certain colloids that favour particular cations e.g. high affinity for
K+ and similar sized NH4+ and Cs+ by vermiculite and fine grained micas (illite and
glauconite); Cu, Hg and Pb have affinity for sites on humus and sesquioxides making such
soil efficient in removing pollutants.
6 Complementarily of cations
The likelihood that a given adsorbed cation will be displaced from a colloid is influenced by
how strongly its neighbouring cations (i.e. complementary ions) are held. Loosely held ions
are easily displaced. This affects availability of plant nutrients.
1.3 ATION EXCHANGE CAPACITY (CEC)
It is the sum total of the exchangeable cations that the soil can adsorb per its unit weight e.g.
If the soil contains the following cations on the exchange sites:
Ca2+
Mg2+
K+
Na+
H+
CEC
25 m.e. / 100g soil (milliequivalent per 100 g of soil = m.e. %)
20m.e.
/
100g
soil
2m.e.
/
100g
soil
1m.e.
/
100g
soil
12m.e. / 100g soil
= 25 + 20 + 2 + 1 + 12 = 60 m.e. / 100 g soil
m.e.
=
1/1000
of
an
equivalent
weight
equivalent weight = atomic weight of cation/ valency of cation (see examples tabulated below)
Cation
4|Page
Atomic Wt. (g)
Valency
Equivalent Wt. (g)
m.e. weight (g)
Ca2+
K+
Mg+
H+
NH4+
Na+
40
39
24
1
18
23
2
1
2
1
1
1
20
39
12
1
18
23
0.020
0.039
0.012
0.001
0.018
0.023
If the soil holds 25 m.e. % of Ca2+, it means it holds 25 × 0.02 g = 0.5 g per 100 g of soil.
In modern publications CEC is expressed in centimoles of charge per kilogram of soil (cmol/kg)
e.g.
60cmol/kg
means
the
soil
can
hold
60cmol/kg
of
H+
ions
2+
+
If 25cmol of adsorbed Ca is replaced by H , how many grams would be replaced?
Since Ca2+ is divalent, the mass needed to provide 1 mole charge (1 mol) is half the atomic
weight of Ca→ 20g. The mass providing 1 centimole (i.e. 1/100 mole) is 20/100 = 0.2g
Therefore 25cmol/kg is 0.2 × 25 = 5g/kg of soil, which is the same as 0.5 g/100 g of soil
milliequivalent/100 g soil (m.e.%) = cmol/kg soil
1.3.1 CEC and type of colloid
Name of colloid
Type of colloid
CEC (m.e. /100 g clay
Comments
Sesquioxide
Non-silicate
Up to 3
Restricted
to
intense
weathering
environments like eastern highlands
Found in high & middle veld & eastern
highlands
Kaolinite
Crystalline silicate
1:1
non-expanding
Illite (hydrous Crystalline silicate
mica)
2:1
non-expansive
Chlorite
Crystalline silicate
2:1:1
non-expansive
Motmorillonite Crystalline silicate
2:1 expansive
5|Page
3 - 15
25 - 40
Associated with geologies rich in mica e.g.
micaceous schists
10 - 40
An octahedral sheet is wedged in the
interlayer space reducing sites for cation
exchange hence 2:1:1 strucrure
80 - 130
Associated with vertisols in Chisumbanje,
Nyamandlovu, tsholotsho, jotsholo or lower
Vermuculite
Humus
Crystalline silicate 100 - 150
2:1 expansive
organic
100 - 300
catenal members of soils formed from
epidiorite in high rainfall areas
Result from alteration of mica. Found in
areas with micaceous rocks
Low
lying
areas
mineralization is slow
(swales)
where
E/C value = CEC per 100 g of clay
S/C value = CEC per 100 g of clay
The above values can indicate the dominant clay type in a soil, hence the degree of weathering
a soil has been subjected to. These values are used to classify soils in the Zimbabwean Soil
Classification System and can be obtained using the clay amount in the soil (i.e. texture),
Example : A soil has the following particle size distribution: clay = 50 %, silt = 8 %, sand = 42 %.
Given CEC = 6.5 m.e. %
E/C value will be: CEC*100/clay content of soil = 6.5*100/50 = 13 m.e. %. The dominant clay
mineral is kaolinite as the E/C value falls within the CEC range of this clay mineral (i.e. 3 – 15
m.e. %). This is simple proportion as follows: If 50 g of clay has a CEC of 6.5 m.e.% (we know
that the CEC is due to clay and not organic matter since subsoil samples are used) therefore 100
g of clay will have a CEC of 6.5*2 = 13 m.e. %
1.3.2 CEC and soil texture
Light textured soils have low amounts of colloids compared to heavy soils and therefore
have low CEC
1.3.3 CEC and soil organic matter content
Soils with high organic matter content have high CEC as indicated in the CEC for humus in
the table above (1.3.1). The CEC of light textured soils may be improved by addition of
organic fertilizers.
This is the reason why CEC is determined from subsoil horizons to avoid interference of
organic matter
1.3.4 CEC and time span of weathering
6|Page
With time, clay minerals weather from 2:1 to 1:1 to sesquioxides. As indicated in table
above (1.3.1) CEC will decrease with time
1.3.5 CEC and soil pH
For pH dependent charges, CEC increased with increase in pH and vice versa (see pH
variable charges under the topic on SOIL COLLOIDS)
1.3.6 CEC and nutrient availability
Most exchangeable cations are also plant nutrients and high CEC prevents them from
leaching out of the soil system (and therefore making them available to plants).
1.3.7 CEC and…
1.4 TOTAL EXCHANGEABLE BASES (TEB)
There are two types of cations in the soil system: the basic (Ca2+, Mg2+, K +, NH4, Na+) and
acidic (Al3+, H+). TEB is the sum of basic cations ONLY.
In our example in 1.3 (CEC), the total exchangeable bases would be: CEC less H+(not a basic
cation) i.e. 60-12 = 48 m.e. % (= 25+20+2+1)
1.5 BASE SATURATION PERCENTAGE (BS %)
It is the amount of basic cations that occupy the exchange sites of the soil colloids
expressed as a percentage of CEC (or TEB as a % of CEC)
In our example: BS % = 48*100/60 = 80 %


7|Page
It is useful in evaluating soil fertility as BS % < 50 % indicates poor/low fertility, acidic
conditions (saturation of exchange sites with acidic cations) and possible toxicity due
to excess Al or heavy metals (Cr, Ni, Pb, etc.)
It is a better indicator than just CEC as it shows the composition of the cations
occupying the exchange sites e.g. two soils with the same CEC could have different
base status (say 40% and 80%) something that CEC alone cannot reveal.
1.6 EXCHANGEABLE SODIUM PERCENTAGE(ESP)
It is the amount of Na cations that occupy the exchange sites of the soil colloids
expressed as a percentage of CEC (or Exchangeable as a % of CEC)
Na is of interest for the following reasons:
 When Ex. Na > Ex. (Ca + Mg), there is poor soil structure (remember columnar!)
 High amount of Na is the soil is not favourable for most crops
 High amount of Na may indicate presence of soluble salts (as in saline sodic
soils). Most crops under perform on such soils
 Soils with high amount of Na (sodic soils) are susceptible to erosion
This goes further than BS % in that it indicates the proportions of bases (singling out Na
for above reasons) in the exchange complex. Here also, if two soil with similar base
status, it is important to check if Na is one of the main basic cations or not. This is
important in decision making or in evaluating the productive potential of soils.
In Zimbabwe a soil is considered sodic if its ESP >9 % within 80 cm depth. This is the
depth to which most crops put their roots.
1.7 ANION EXCHANGE






anions from complexes with oxides or hydroxides in the inner-sphere complex
equivalent quantities are exchanged as done in cations
released nutrients are absorbed by plant
generally anion exchange decreases as pH increases and vice versa (see pH dependent
charges)
retards leaching and supplies nutrients for plants movement (e.g. leaching of nitrates to
underground water and pollutants in the environment)
the equations below show how phosphorus is fixed in acid and highly weathered soils
>Al-OH (colloid) + H+  >Al-OH2+ (protonation under extreme acid conditions)………….. (i)
>Al-OH2+ (colloid) + H2PO4- (anion)
 >Al-H2PO4 (anion fixed to colloid) + H2O……….. (ii)
1.8 SALT AFFECTED SOILS
Apart from shortage of water, presence of salts in the soil is one of the major limitations to
crop production in arid and semi-arid environments. Soil with high concentration of soluble
8|Page
salts (i.e. saline soils) and/or with high amounts of exchangeable Na on the exchange
complex (i.e. sodic soils) are considered to be salt affected.
Salinity = high concentration of soluble salts in the soil solution. These are mainly sulphates,
chlorides, nitrates, bicarbonates of basic cations.
Sodicity = high ESP (i.e. > 9% within 80 cm depth for Zimbabwe, ESP >15 in other systems).
The exchange complex is saturated with Na at the expense of other basic cations.
Alkalinity = concentration of hydroxyl ions in the soil solution i.e. high pH. Alkalinity is
associated with dry environments where salt-affected soils are common.
1.8.1 Sources of soil alkalinity and salts

Arid climates are a low leaching environment resulting in the dominance of basic
cations at the expense of acidic ones and accumulation of salts. P:Eo ratio is less
than 0.75
 Soluble salts are from weathering of primary minerals in rocks and parent materials
Parent materials that are have high salt content e.g. some parent materials in the
Zambezi valley were formed through deposition in a lacustrine environment and are
naturally high in salt content/ fossil salt deposits of past lacustrine areas
 Irrigation with salty water leads to accumulation of salts in the soil
 Over irrigation may raise the water table of saline water
 Deposition of salts from other sources especially upland positions or inundation
(flooding) of coastal areas with sea water
 Generation of hydroxyl ion (OH-) from anions such as CO32- and HCO3-. These two
anions originate from dissolution of calcite (CaCO3) as illustrated below:
CaCO3 (solid)  Ca2+ (dissolved) + CO32- (dissolved)……………………………………………. (i)
CO32- + H2O  HCO3- + OH- ………………………………………………………………………………. (ii)
HCO3- + H2O  H2CO3 (carbonic acid) + OH- ……………………………………………………. (iii)
H2CO3 (carbonic acid)  H2O + CO2 (gas) …………………………….……………………………. (iv)
NB: the series of reactions above produce OH- ions resulting in alkalinity. Compare these
reactions with aluminium speciation that produces H+ under acid environments.
1.8.2 Role of carbon dioxide and carbonates

CO2 is 0.035 % in air but 0.5 % in the soil due to its release by roots during respiration
9|Page



Release of CO2 shifts the reactions above TO THE LEFT and this reduces pH
Precipitation of CaCO3 removes Ca from the soil solution and moves the reaction to the
LEFT and lowers the soil pH
If other carbonates other than CaCO3 that are soluble (e.g. Na2CO3) are in the soil
solution, the pH will rise. This explains why soils with CaCO3 have pH 7-8.5 while in those
with Na2CO3 ranges 8.5-10.5 with the latter being toxic to plants.
Carbonates of Na+ and Ca2+


It is important to note that it matter which of the two cations is associated with the
CO32- anion
Since carbonates (CO32-) and bicarbonates (HCO3-) of Na are more soluble than those of
Ca, the reactions quickly move to the RIGHT and increasing the pH in the process.
1.8.3 Soluble salt levels in the soil




Salts such as chlorides and sulphates of Na and Ca tend to lower pH by moderating
alkalinization reactions
Increase in Na and Ca ions on the right drives the reaction to the left by common ion
effect
Addition of Na and Ca ions from other sources other than (CO32-) reduces dissolution of
carbonates.
High salt levels result in high total ionic strength in the soil solution and this improves
the physical condition of the soil by increasing the flocculation of clays (aggregation/
formation of soil structure) *tell the Chiredzi experience

1.8.4 Characteristics of Alkaline soils





Nutrient deficiencies: deficient in Zn, Cu, Fe, Mn especially under maize, beans, sorghum
and wheat. These nutrients are not available under high pH conditions
Boron: is tightly held and the strength of adhesion increases with pH
Molybdenum toxicity: available under high pH conditions such that it might be taken up
in toxic quantities to plants and grazing animals
Phosphorous deficiency: P forms insoluble phosphates with Ca and Mg. However
vegetables of the Brassica family (rape, cabbage) can access P under alkaline conditions
by dissolving Ca and Mg around their roots.
CEC is higher than that of acid soils with comparable textures. This is because 2:1 clay
minerals are common in alkaline soils of arid environments. pH dependent charges are
stimulated at high pH especially in humus.
10 | P a g e

They may accumulate layers of calcium salts in sub soils (calcic/petrocalcic;
gypsic/petrogypsic)
1.8.5 Measuring salinity/sodicity
Salinity is measured by an electrical conductivity bridge. It indicates the concentration of
salts in water as conductivity increases with amount of dissolved salts. A saturation paste is
made with a 1:2 ratio of soil: water (weight). EC values > 4 decisiemens per metre (dS/m)
indicate salinity
Sodicity is measured through ESP
Sodium adsorption ratio (SAR) gives information on the comparative concentrations of Na+,
Ca2+ and Mg2+ in the soil solution


It takes into consideration that the adverse effects of n Na + are moderated by Ca2+
and Mg2+
Is used to characterize irrigation water (SAR of 13 = ESP of 15)
1.8.6 Classes of salt-affects soils
Class
ESP
pH
saline
EC
(dS/m)
>4
<15
<8.5
Saline-sodic
>4
>15
sodic
<4
>15
11 | P a g e
8.5-10
Comment
Plant growth affected by excess salts. Good structure as
salts prevent dispersion of colloids
Plant growth affected by excess salts and Na. if salts are
removed, soil structure deteriorates i.e. becomes sodic
pH high due to high solubility of Na2CO3 compared to
CaCO3 and MgCO3. Na is weakly attracted to the colloids
compared to Ca and Mg and therefore colloids are kept
far apart that the forces of cohesion cannot come to play
to attract the colloids to each other resulting in their
dispersion. They move down the profile to form a sodic
horizon.
Plants are affected by toxicity of as well a adverse
physical conditions i.e. low permeability of Bn horizon
1.8.7 Salt-affected soils and plant growth
1. Osmotic effects: this makes it difficult for the roots to take up water and nutrients due to
high concentration of the soil solution compared to that of water in the plant tissue
2. Specific ionic effect: Na+ competes with K+ and this makes it difficult for plants to take up K+.
Presence of Ca2+ helps plants discriminate against Na+ and for K+.
3. Physical effects of sodicity: deterioration of soil structure results in poor aeration, low oxygen
levels and small root reservoir for moisture and nutrients
4. Toxicity: plant growth is affected by toxic levels of Na+, OH- and HCO3- ions.
1.8.8 Reclamation of saline soils
Saline soils may be reclaimed by leaching of excess salts and providing drainage to remove the
salts out of the soil system.
1.8.9 Reclamation of saline sodic soils
In reclaiming these soils, one should first reduce exchangeable Na+ to acceptable levels and
then leach the salts. If this order is reversed, the soils become sodic and deteriorate in
structure.
1. Apply gypsum
Finely ground gypsum (CaSO4.2H2O) is mixed well into the soil. The Ca2+ from gypsum will
replace Na+ from the exchange sites and reduce ESP. This also supplies nutrient S to the soil
and reduces soil pH.
2NaHCO3 + CaSO4  CaCO3 + Na2SO4 (leachable) + H2O + CO2
Na2CO3 + CaSO4  CaCO3 + Na2SO4 (leachable)
12 | P a g e
2. Apply sulfur and sulfuric acid
2NaHCO3 + H2SO4  CaCO3 + Na2SO4 (leachable) + 2H2O + 2CO2
Na2CO3 + H2SO4  CaCO3 + Na2SO4 (leachable) + H2O + CO2

pH is also reduced by the acid
3. Grow tolerant crops e.g. cotton, barley, sorghum. Their roots provide channels for
movement of gypsum down the soil profile.
1.9 FURTHER READING

Abrol, I.P. 1988. Salt affected soils and their management. FAO soils Bulletin 39
(available at LSU library – S595 ABR)
1.10
13 | P a g e
QUESTIONS
 Use appropriate equations to explain the principles that govern cation exchange
reactions
 Explain how salt-affected soils affect plant growth
