ABSTRACT
Cation exchange equilibira involving K , Ca and Mg were studied in two
representative savanna soils. The study examined Ca-K and Mg–K exchange
equilibria with a view to establishing their preferential retention of the cations in
savanna soils. Soil samples were collected at two depth ranges (0-15cm) and (15-
30cm) from Zaria and Mokwa respectively. A solution of constant chloride salts
concentration (0.1M) containing variable concentrations of K, Ca and Mg was
prepared. The K concentration of the solution was prepared to vary from 0 to 200
mmol L-1 while the Ca and Mg solution were made to vary from 100 to 0 mmol L-1 in
each case. The Ca and Mg equivalents of the Ca-K and Mg-K mixtures were made to
increase from 0 to 1 while the equivalent fractions of K were made to decrease from 1
to 0 in the mixtures. For the Mokwa soil, the total adsorbed metal charge (Qi) for Ca –
K exchange was 22% greater than the total adsorbed metal charge (Qi) for Mg-K
exchange, thus indicating some preference for CaCl+ over MgCl+ by the surface
exchange sites. This was, however, in contrast to the relative consistent total
adsorbed metal charge (Qi) for both Ca-K and Mg-K exchange in Zaria soil. Several
cation exchange models were used to describe the Ca-K and Mg-K exchange
equilibira. The exchange models tested in the study were the Kerr selectivity
coefficient (Kk), Vanselow selectivity coefficient (Kv), Gapon selectivity coefficient
(KG), Gaines and Thomas selectivity coefficient (KGT), Davies selectivity coefficient
(KD) and the thermodynamic exchange constant (Kex).The trends in selectivity
coefficients as a function of exchanger phase composition for Ca-K exchange were
similar to Mg-K exchange. All selectivity coefficients determined for Mg-K and Ca-K for
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the soils were far greater than 1 at all exchanger phase composition, suggesting a
preference for K to either Ca or Mg by the soils. The Ca-K and Mg-K exchange
isotherms were thus compared with the thermodynamic non-preference isotherm
(TNI) line by plotting the charge fractions of adsorbed K versus the charge fraction of
K in soil solution in the soil solution. The result indicated that the experimental points
were below the TNI line for both Zaria and Mokwa, clearly suggesting non-preference
for either Ca or Mg in the presence of K. The K preference by the soil can be
explained by the relatively high hydration shell of Ca and Mg as compared to K that
tends to limit the rate of diffusion of Ca and Mg from the soil solution to the surface
exchange sites as compared to K. The thermodynamic exchange constant Kex for the
surface soil fron Zaria was 7.98 while the subsurface soil was 7.81, but Kex decreased
sharply to 3.12 and 4.60 for the surface and subsurface soil respectively for Mokwa.
The negative values of ΔGex determined for Ca-K and Mg-K exchange seemed to lend
additional credence to the notion that K was strongly preferred to Ca and Mg by the
soils. The selectivity for K in the Zaria soil was stronger than the Mokwa soil
apparently because of differences in mineralogical composition.
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TABLE OF CONTENTS
Title page……………………………………………………………. i
Declaration …………………………………………………………. ii
Certification page…………………………………………………… iii
Acknowledgement ……………………………………………….. iv
Dedication…………………………………………………………… v
Abstract……………………………………………………………… vi
Table of contents…………………………………………………… viii
List of Tables ……………………………………………………….. x
List of figures ………………………………………………………. xi
1. Introduction………………………………………………….. 1
2. Literature Review…………………………………………… 3
2.1 Distribution of major soil classes in the savanna 3
2.2 Clay mineralogy of soils…………………………… 6
2.3 Origin of charge in soil…………………………….. 8
2.4 Cation Exchange Equilibra in soils………………. 10
2.4.1 Selectivity of Cation in soil Exchange Equilibria … 13
2.4.2 Modelling cation exchange Equilibria …………… 15
3. MATERIALS AND METHODS…………………………….. 20
3.1 Site information ……………………………………….. 20
3.2 Routine soil analysis …………………………………. 22
3.2.1 Particle size analysis…………………………………. 22
3.2.2 Organic Carbon………………………………………. 22
3.2.3 Exchangeable cations ………………………………. 23
3.2.4 Soil pH ………………………………………………… 24
3.2.5 Cation Exchange Capacity…………………………… 24
3.2.6 Exchangeable Acidity ………………………………… 25
3.3 Determination of Ca-K and Mg-K exchange equilibria 25
3.4 Modelling the Ca-K and Mg –K exchange equilibria 27
3.4.1 Kerr Selectivity Coefficient (KK) ……………………… 28
3.4.2 Vanselow Selectivity coefficient (Kv)…………………. 29
3.4.3 Gapon Selectivity coefficient (KG)…………………… 30
3.4.4 Gaines and Thomas selectivity Coefficient (KGT)…… 32
3.4.5 Davies selectivity coefficient (KD)…………………….. 32
3.4.6 Thermodynamic exchange constant (Kex)………….. 33
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3.5 Cation adsorption isotherms …………………………. 36
4. RESULTS AND DISCUSSION ……………………………… 38
4.1 Adsorbed Metal Charge ………………………………. 38
TABLE OF CONTENTS
Page
4.2 Cation Selectivity Coefficients ………………………. 41
4.3 Cation Adsorption isotherms………………………….. 43
4.4 Thermodynamic Exchange constant (Kex) and
the change in free Energy (ΔGex) of Exchange…… 51
4.5 Implications for K-fixation and plant Nutrition…………. 62
5 SUMMARY AND CONCLUSIONS…………………………….. 67
6 REFERENCES……………………………………………. 72
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CHAPTER ONE
INTRODUCTION
The knowledge of the quantitative relationship between ions in solution
and those on the exchange complex is of paramount interest to soil and
environmental scientists. Such a relationship allows the prediction and control of
cation release by soils into soil solution and aquatic environment (Sposito, 1989;
Essington, 2004; Sparks, 2003).
Soil colloids exhibit a net negative charge that attracts and retains cations
that are readily exchangeable with other cations. The amount of this negative
charge in a soil is called the cation exchange capacity (CEC) and is often
expressed as centimole charge per kilogram of soil.
Savanna soils are known to be acid to slightly acid with pH ranging from
5.0 to 6.5. The predominant soils are Alfisols/Ultisols, inceptisols and localized
vertisols. Alfisols have high base saturation and are less leached as compared
to Ultisols. They are both low in organic matter and cation exchange capacity
(Jones and Wild, 1975). The low cation exchange capacity of savanna soil is
attributed to the dominance of kaolinitic clays and low organic matter content
(Jones, 1973).
The low exchange capacities of savanna soils have implications for soil
management. Nutrients applied as cations such as ammonium (NH4
+),
potassium (K+), magnesium (Mg2+) and calcium (Ca2+) are loosely held and are
readily leached (Jones and Wild, 1975). Furthermore, the low CEC of the soil
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and its dependence on pH will assume increasing importance as demands for
nutrient cations become intense with the introduction of improved crop varieties,
and with increasing application of nitrogenous and phosphatic fertilizers (Jones
and Wild, 1975). Consequently, managing the soil CEC is a task and challenge
to the resourcefulness of soil scientists in the region. At low pH, nutrient cation
applications are not likely to have any beneficial effect because of rapid leaching
in the absence of cation exchange sites to retain or adsorb them.
In a soil testing programme cation exchange capacity is one of the most
important soil chemical parameters determined. Regretfully, very little attention
has been paid to cation exchange equilibria and the preferential retention of
certain cations in savanna soils. Understanding preferential retention of cations
by soils is extremely important in formulating guidelines for fertilizer applications
to avoid leaching and displacement of certain nutrient cations, and the excessive
retention of others, thus creating cationic imbalance in soil
The objectives of this study were to:
(i) determine K-Ca and K-Mg exchange equilibria in two savanna soils
of Nigeria.
(ii) evaluate competing models for describing cation exchange
equilibria in soils.
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