Kinetics of Potassium Exchange in a Paleudult from the Coastal Plain of Virginia1
D. L. SPARKS, L. W. ZELA/NY, AND D. C. MARTENSZ
Kinetic reactions are thought to exist between the
various phases of K. The reaction between the soilsolution and exchangeable phases of K is generally
proposed to be almost instantaneous (Way, 1850;
Wood and DeTurk, 1940; Malcom and Kennedy,
1969). Barium-potassium exchange on pure montmorillonite, "illite", and kaolinite was found to be rapid,
with 75% of the total exchange occurring within 3
sec. However, the rate of Ba-K exchange on vermiculitic materials was slower with 50 and 97% of the
exchange reached after 10 and 720 sec, respectively
(Malcom and Kennedy, 1969). This slower rate of
exchange in vermiculite was attributed to slow diffusion into interlayers. Barshad (1954) reported that
diffusion-controlled exchange is characterized by a
linear relationship between percent exchange and
(time)172 for sand and gravel-sized mica and for vermiculite. Malcom and Kennedy (1970) observed that
>75% exchange was completed within 3 sec and 100%
in 5 to 10 min in the fine and coarse clay and fine silt
fractions of stream sediments. Complete exchange required 20 sec to 1 hour in the fine- and medium-sand
fractions, while 1 to 2 days were required for complete
exchange in the coarse-sand fractions. Similar results
were reported by others for sand fractions (Joffe and
Kunin, 1943; McAloose and Mitchell, 1958).
Selim et al. (1976) proposed that a kinetic type reaction existed between soil solution and exchangeable
K with an adsorption rate coefficient (ka) governing
the forward reaction and a resorption rate coefficient
(kd) governing the reverse reaction. They proposed
that the adsorption reaction (ka) was the nth order
while the desorption reaction (kd) was first order.
The purpose of this study was to investigate the
kinetics of reactions occurring between the solution
and exchangeable phases of K in two Dothan soils
from Virginia. Potassium adsorption reactions were
conducted with concentration held constant and adsorption rate coefficients determined using a modified
Freundlich equation.
ABSTRACT
The kinetics of K adsorption from solution to exchangeable
phases were investigated on the Ap, A2, B21t, and B22t horizons
of Dothan soil (Plinthic Paleudult) from two locations in Virginia. These soils are loamy sands in the upper horizons with
clay content increasing with depth, are slightly acidic in the
surface with pH decreasing with depth, have CEC's ranging
from 3.4 to 8.6 meq/100 g in the four horizons and contain
considerable quantities of chloritized vernuculite and kaolinite
in all horizons. Potassium adsorption with time was evaluated
on Al- and Ca-saturated samples from each horizon using 5,
25, and 100 /ig/ml K solutions equilibrated for 0, 1, 2, 24, 96,
and 192 hours. Equilibrium in K exchange was reached hi 2
hours with the 5 and 25 /tg/ml solutions and in about 24 hours
with the 100 /tg/ml solution. This slow rate of K exchange
was attributed to diffusion-controlled exchange, which reflects
the relatively high amount of vermiculitic material in these
soils. Adsorption rate coefficients (/£„) were calculated from
reaction time vs. quantity of K sorbed using a modified form
of the Freundlich equation. The magnitude of the h, values
decreased with increasing ionic strength, which conforms to
Bronsted's activity rate theory. The similar magnitude of the
k, values from horizon to horizon suggests that similar exchange
reactions were taking place in all horizons.
Additional Index Words:
chemistry.
ion exchange, ion adsorption, K
Sparks, D. L., L. W. Zelazny, and D. C. Martens. 1980. Kinetics
of potassium exchange in a Paleudult from the Coastal Plain
of Virginia. Soil Sci. Soc. Am. J. 44:37-40.
VOLUMINOUS AMOUNT OF RESEARCH has been perA
formed on various aspects of ionic exchange
with K, but a meager amount has appeared in the
literature on the rate of K exchange or the kinetics of
K adsorption in soil. While research has been conducted on the kinetics in pure clay systems, little has
been conducted on soil systems where complex mixtures of clay minerals and organic matter are present.
Furthermore, the kinetics of ion-exchange reactions
are not understood in either system (Thomas, 1977).
A further understanding .of K chemistry requires research on the rate of K exchange in soil systems.
Most researchers concur that soil K exists in soilsolution, exchangeable, nonexchangeable, and mineral
phases. The soil-solution and exchangeable phases are
regarded as readily available forms of K (Reitemeier,
1951). The nonexchangeable form of K is generally
considered as a slowly available form of K occurring
in "illitic" clay and other 2:1 types of intergrade minerals (Wood and DeTurk, 1940; Reitemeier, 1951;
Rich, 1968). The mineral phase of K is relatively unavailable and is a constituent of primary minerals such
as micas and feldspars (Reitemeier, 1951; Rich, 1968).
MATERIALS AND METHODS
Studies were conducted on two Dothan soils (fine-loamy, siliceous, thermic Plinthic Paleudults) located in Greensville and
Nottoway Counties, Virginia. The Greensville County soil had
been cultivated for over a century, whereas the Nottoway County soil had been under forest for 3 decades prior to this study.
Bulk samples were selected from the Ap, A2, B2H, and B22t
horizons at the two locations. The samples were air-dried and
crushed to pass a 2-mm sieve. Particle size analyses, were determined by the pipette method (Kilmer and Alexander, 1949).
Mineralogical analyses, consisting of X-ray diffraction and
differential scanning calorimetry, were performed on the <2jum clay fraction of the soils. Prior to soil mineral particlesize fractionation, subsamples were treated with 30% H2O2 to
remove organic matter (Kunze, 1965) and with Na-dithionitecitrate to remove Fe oxides (Mehra and Jackson, 1960). Sand
was separated from silt and clay by wet sieving and clay was
separated from silt by centrifugation and decantation. X-ray
diffractograms were obtained with a Diano XRD 8300 AD instrument employing a CuKa radiation source and a graphite
monochromator from oriented clay slides prepared according
to the procedures of Rich and Barnhisel (1977), and differential
thermograms with a DuPont 990 differential scanning calorimeter.
1
Contribution from the Dep. of Agronomy, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061.
Presented to Div. S-2, Soil Sci. Soc. Am., 7 Dec. 1978, in Chicago,
111.2 Received 11 Jan. 1979. Approved 27 Sept. 1979.
Former Graduate Research Assistant, Associate Professor,
and Professor of Agronomy, respectively. The senior author is
presently Assistant Professor of Soil Chem., Dep. of Plant Sci.,
Univ. of Delaware, Newark, DE 19711.
37
38
SOIL SCI. SOC. AM. J., VOL. 44,
Organic matter was determined by the Walkley-Black (1934)
method and cation exchange capacity by a MgCl2 saturation
with subsequent displacement by CaCl3 (Okazaki et al., 1963;
Rich, 1962). Exchangeable Ca, Mg, K, and Na were extracted
with IN NH 4 OAc (Jackson, 1958) and determined by atomic
absorption spectrophotometry. The pH measurements were obtained from a 1:1 soil/water mixture.
Prior to initiation of the kinetic sorption studies subsamples
from each soil horizon from the two locations were Al- or Casaturated, using either IN A1C13 or IN CaCL. The soil was
subsequently washed with deionized water, followed by a 50%
acetone-H2O mixture until a negative test for Cl" was obtained
with AgNO3. The saturated samples were air-dried and crushed
to pass a 2-mm sieve.
Soil pH was measured on the Al- and Ca-saturated samples
from each horizon using a 1:1 soil/water mixture. Cation exchange capacity of the Al-saturated samples from each horizon
was determined by displacement with IN KC1. The quantity
of Al in the leachate was analyzed by titrating with standardized
base. The CEC of the Ca-saturated samples was ascertained by
displacement with IN MgCl2. The quantity of Ca in solution
was measured using atomic absorption spectrophotometry.
K Sorption Vs. Time
One-gram aliquots of the Al- or Ca-saturated samples from
each horizon were weighed into 100-ml polypropylene centrifuge
tubes. Fifty milliliters of KC1 containing 5, 25, or 100 /ig K/ml
were added to three samples. The samples were shaken at 25 °C
on a reciprocating shaker for 0, 1, 2, 24, 48, 96, or 192 hours,
centrifuged, and K in the supernatant measured by atomic absorption spectrophotometry. Concentration differences before
and after shaking were assumed to represent the amount of K
that had sorbed on soil material surfaces.
The sorption values (/ig K/g) for each soil treatment were
plotted against reaction time (hours) on a logarithmic scale.
Adsorption rate coefficients (&„) were calculated from a modification of the Freundlich equation as proposed by Kuo and
Lotse (1974):
Greensville and Nottoway Counties are given in Table 1. The CEC of the soil was relatively low and was
highest in the B22t horizon which had the highest
clay content. With the exception of the Ap horizon,
the exchangeable K was higher in the Greensville
County soil. This may reflect K fertilization during
the prolonged period of cultivation. Considerable
quantities of chloritized vermiculite, - kaolinite, and
gibbsite were present throughout the profiles of the
two Dothan soils. The mineral suite in the two soils
at both locations was similar, with chloritized vermiculite being the most abundant mineral in the Ap,
A2, and B21t horizons; and kaolinite in the B22t horizon. The A2 horizon from the Nottoway County location contained some vermiculite and mica. These
minerals were absent from the A2 horizon of the
Greensville County location.
Soil pH of the presaturated samples was slightly
acidic and was lower in the Al-saturated soils in all
horizons for both soils than in the Ca-saturated soils
(Table 2). The pH of the Al-saturated and Ca-saturated systems in the Greensville County soil tended to
decrease with increasing depth. This conformed with
Table 2—Soil pH and cation exchange capacity of the Al- and
Ca-saturated samples used in kinetics studies from
Greensville and Nottoway Counties.
Saturation
treatment
Horizon
=
=
=
=
=
1
adsorption rate coefficient in hours' ,
amount of K adsorbed in /tg/g>
initial K concentration in ppm,
reaction time in hours, and
constant.
5.0
5.3
5.2
5.6
5.2
5.4
4.6
5.0
Ca
Al
Ca
Al
B2H
Ca
B22t
The parameter 1/m was calculated from the slope of the linear
portion of the plots.
CEC, meq/100 g
Greensville County
A2
where
pH
Al
Ap
/£„ = X/C,tV<
£„
x
c0
t
1/m
1980
Al
Ca
Nottoway County
Al
4.5
Ca
6.1
Al
4.6
Ca
5.9
Al
5.0
Ca
6.7
Al
4.6
Ca
4.9
Ap
A2
RESULTS AND DISCUSSION
B21t
Soil Characteristics
B22t
Selected physical and chemical properties, and mineralogy of the clay fraction, of the Dothan soils from
2.3
2.5
2.1
2.3
4.2
4.3
4.7
5.1
3.2
3.6
2.0
2.3
3.8
4.1
4.9
5.1
Table 1—Selected physical and cnemical properties and mineralogy of the < 2-/un clay fraction of Dothan soils from
Greensville and Nottoway Counties.
Particle size analysis
Horizon
Depth
Sand
Silt
Clay
pH
— % —
cm
23.8
17.3
9.0
7.0
10.3
28.1
6.1
5.8
4.8
4.4
15.0
20.1
19.1
15.6
3.4
9.1
11.9
17.5
5.8
5.2
4.7
4.6
Ap
A2
B21t
B22t
0-20
20-31
31-41
41-76
66.6
77.0
65.9
54.6
24.4
16.0
Ap
A2
B21t
B22t
0-15
15-33
33-58
58-84
81.6
70.8
69.1
66.9
Organic
matter
Exchangeable bases
CEC
%
Greensville County
4.2
0.5
0.3
4.0
4.8
0.3
7.2
0.2
Nottoway County
1.2
5.8
3.4
0.2
0.2
4.0
0.2
8.6
Ca
Mg
Na
K
clay fraction!
0.74
1.03
0.85
1.08
0.12
0.21
0.27
0.22
0.04
0.03
0.02
0.02
0.09
0.19
0.19
0.22
1.72
0.24
0.26
0.52
0.85
0.13
0.16
0.48
0.01
0.03
0.03
0.03
0.11 VC,,KK,,QZ,,GI4
0.06 VC,,VR,,KK,,MI4 ,QZ S ,GI.
0.08 VC,, KK,, GIS, QZ4
0.16 KK,,VC,,GI,,QZ4
VC1 t,GII ,QZ,,KK4
VC,, KK,, QZS, GI4, MM,
VC,,KK,,GI,,MI 4 ,QZ,
KK,, VC,, GI,, QZ.
t VC = chloritized vermiculite; KK = kaolinite; QZ = quartz; GI = gibbsite; VR = vermiculite; MI = mica; MM = montmorillonite.
i Subscript 1 = most abundant, 6 = least abundant.
SPARKS ET AL.: KINETICS OF K EXCHANGE IN A PALEUDULT FROM THE COASTAL PLAIN OF VIRGINIA
the pH trend of the soil in its natural state (Table 1).
The pH of the Al- and Ca-saturated soils was lower
in the Nottoway County soil than in the Greensville
County soil and generally decreased with depth.
The CEC of the Al-saturated soils was somewhat
lower than the Ca-saturated soils and tended to increase with increasing clay content (Table 1) in both
soils. The lower CEC of the Al-saturated systems is
partly due to a valency effect. The A13+ should be
harder to displace from the exchange phase than would
Ca2+ (Helfferich, 1962). Nye et al. (1961) noted that
it is difficult to displace all of the exchangeable Al
which would result in a lower CEC. The slightly
lower Al CEC could also be due to decomposition of
some exchange sites during Al saturation (Frink,
1964). The greatest differences in CEC between the
Al- and Ca-saturated systems occurred where the pH
differences were greatest. This would suggest some
decomposition of exchange sites when Al-saturated
(Table 2).
K Sorption Vs. Time
Potassium sorption was noninstantaneous for the
Al- and Ca-saturated samples from the Ap and B22t
horizons of the Greensville County location (Fig.
1-4). Although not shown, a similar trend occurred in
the other horizons from this location and in all hori1000
39
zons from the Nottoway County location. The noninstantaneous sorption differs from findings of others
(Way, 1850; Malcom and Kennedy, 1969) with pure
systems.
The sorption process was virtually complete in the
5 and 25 ppm K-treated Al- and Ca-saturated Ap
horizons of the Greensville County site in 2 hours
(Fig. 1 and 2). The 100 ppm K-treated soils sorbed
more K than either the 5 or 25 ppm K-treated soils,
which is expected from a concentration standpoint
(Kelley, 1948), and resulted in a relatively linear relationship when plotted logarithmically. However,
equilibrium was not reached until approximately 24
hours of equilibration time. The Ca-saturated Ap
horizon of the Greensville County soil sorbed considerably more K than the Al-saturated soil (Fig. 2),
which can be expected on the basis of easier displacement of divalent Ca than trivalent Al by K (Helfferich, 1962). Although not reported, both the A2 and
B21t horizons were similar to the Ap horizon for sorption vs. time plots (Fig. 1 and 2), which would be
expected since similar mineralogy and clay contents
were present (Table 1).
Aluminum- and Ca-saturated soils from the B22t
horizon sorbed considerably more K than did those
from other horizons (Fig. 3 and 4). The higher clay
content of this horizon afforded more exchange sites
for sorption of K in the Al- or Ca-saturated samples.
1000
100
200
0.1
2
10
100
200
REACTION TIME (hrs)
REACTION TIME (hrs)
Fig. 1—Potassium adsorption by Greensville County Ap soil
horizon (Al-saturated) as a function of time plotted on a
logarithmic scale at 25 °C.
1000
Fig. 3—Potassium adsorption by Greensville County B22t soil
horizon (Al-saturated) as a function of time plotted on a
logarithmic scale at 25°C.
1000
0
LJ
CD
100 -
2
10
100
200
REACTION TIME (hrs)
fig- 2—Potassium adsorption by Greensville County Ap soil
horizon (Ca-saturated) as a function of time plotted on a
logarithmic scale at 25°C.
2
10
100
2OO
REACTION TIME (hrs)
Fig. 4—Potassium adsorption by Greensville County B22t soil
horizon (Ca-saturated) as a function of time plotted on a
logarithmic scale at 25°C.
40
SOIL SCI. SOC. AM. J., VOL. 44, 1980
Table 3—Adsorption rate coefficients for Dothan soil from
Greensville and Nottoway Counties.
Horizon
Treatment
—<j»——————————
K
C,
ppm
Ap
Al-saturated
Ap
Ca-saturated
A2
Al-saturated
5
25
100
5
25
100
5
25
100
A2
Ca-saturated
B21t
Al-saturated
B2H
Ca-saturated
5
25
100
5
25
100
B22t
Al-saturated
B22t
Ca-saturated
5
25
100
5
25
100
5
25
100
*a
Greensville
——— hour
10.23
3.13
1.08
14.23
3.75
1.84
12.79
2.85
1.60
11.74
4.54
2.20
13.22
3.47
1.92
15.75
4.65
2.46
12.26
3.64
2.02
20.01
4.53
3.84
*a
Nottoway
12.07
3.26
0.95
12.93
4.36
2.07
10.00
2.30
0.75
11.12
3.92
2.19
10.85
3.93
1.55
14.26
4.31
2.30
14.38
4.64
2.17
21.91
8.45
5.67
The 5- and 25-ppm K-treated soils reached equilibrium in approximately 1 hour while equilibrium was
not reached in the 100-ppm K-treated soils until about
24 hours. Similar to the other horizons, the Ca-saturated soil sorbed much more K than did the Al-saturated soil.
These data showing noninstaneous ion exchange
for K by Al- and Ca-saturated soils suggest diffusion
controlled exchange. These soils contained vermiculitic clay minerals and mica (Table 1), which others
have shown to exhibit slow diffusion-related exchange
(Barshad, 1955; Malcom and Kennedy, 1970). A practical aspect of this slow rate of sorption is that K could
remain in the soil-solution phase for longer times
where it might be either leached or taken up by
plants.
Adsorption Rate Coefficients
Reaction rates are directly proportional to ka values
(Selim et al., 1976). Measured ka values decreased with
increasing ionic strength (Table 3), which confirms
a faster exchange rate for the lower concentrations of
added K as indicated by Bronsted's activity rate theory
(Moore, 1972). A trend for a faster rate of exchange
in the Ca-saturated system than in the Al-saturated system was also indicated by these ka values. The ka
values o£ the same horizons were similar at both locations, which suggests that similar exchange reactions
were taking place as would be expected because of
similar mineralogy. Apparently, the past cropping
history had no major influence on kinetics.
The ka values of these soils ranged from about 1
to 20 hour"1, which suggest slow rates of reaction
(Fig. 1-4) as compared with values of 81 to 216 hour"1
calculated for Florida soils (Selim et al., 1976). This
can be explained on the basis of the predominance of
kaolinite in the Florida soils as compared with vermiculitic minerals in the Virginia soils.
ACKNOWLEDGMENT
The authors appreciate the partial support for this research
from the Virginia Agricultural Foundation.