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On the behavior of nonexchangeable potassium in soils
H. W. Martin a; D. L. Sparks a
a
Department of Plant Science, University of Delaware, Newark, Delaware
Online Publication Date: 01 February 1985
To cite this Article Martin, H. W. and Sparks, D. L.(1985)'On the behavior of nonexchangeable potassium in soils',Communications in
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COMMUN. IN SOIL SCI. PLANTANAL., 16(2), 133-162 (1985)
ON THE BEHAVIOR OF NONEXCHANGEABLE POTASSIUM IN SOILS1
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KEY WORDS: Chemistry of Soil Κ, Κ k i n e t i c s
H. W. Martin and D. L. Sparks 2
Department of Plant Science
University of Delaware
Newark, Delaware 19717
ABSTRACT
A comprehensive review on the chemistry and mineralogy of
nonexchangeable potassium i s presented. Forms of s o i l K, t h e
e f f e c t of mineralogy on release of nonexchangeable K, methods
of determining nonexchangeable K, and the k i n e t i c s of nonexchangeable Κ release are f u l l y discussed.
INTRODUCTION
Equilibrium reactions e x i s t i n g between the solution and
nonexchangeable phases of s o i l potassium (K) profoundly
influence Κ chemistry. The r a t e and d i r e c t i o n of these
reactions determines whether applied Κ w i l l be leached
into lower horizons, taken up by p l a n t s , converted i n t o
unavailable forms or released i n t o available forms. A
knowledge of the r a p i d i t y of the reactions between solution
and nonexchangeable phases of s o i l Κ i s necessary t o predict
the r a t e of added Κ f e r t i l i z e r s i n s o i l s , and t o properly
make Κ f e r t i l i z e r recommendations.
A voluminous amount of research has appeared i n the
l i t e r a t u r e on the chemistry and mineralogy of nonexchangeable
K, but these data are widely s c a t t e r e d i n numerous s c i e n t i f i c
133
Copyright ©1985 by Marcel Dekker, Inc.
,
0010-3624/85/1602-0133$3.50/0
!34
MARTIN AND SPARKS
journals.
The purpose of t h i s paper i s t o synthesize the above
data i n t o a comprehensive review a r t i c l e .
Forms of S o i l Κ
Soil contains an average of 1.7% Κ (3).
The forms of Κ i n
the order of t h e i r a v a i l a b i l i t y t o p l a n t s and microbes a r e s o i l
s o l u t i o n , exchangeable, nonexchangeable and mineral.
All of
these forms a r e quantified as shown i n Table 1. The equilibrium
r e l a t i o n s h i p s between them are shown i n Fig. ( 1 ) .
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Soil solution Κ i s the form taken up d i r e c t l y by p l a n t s
and microbes ( 4 ) , and i s a l s o subject t o leaching (5).
usually found i n low q u a n t i t i e s .
soil solution is enigmatic.
I ti s
The concentration of Κ i n t h e
I t fluctuates greatly and is
difficult to measure. Because the soil solution is polyionic
and i s often fairly concentrated, the thermodynamic activity
rather than just molar or molal concentration of Κ should be
determined if a picture of what the plant root "sees" i s
desired (6). Levels of soil solution Κ are determined by the
equilibria and kinetic reactions between the other forms of
soil K, soil moisture content, and divalent ion content in
solution and on the exchanger phase (6,7).
Exchangeable Κ is held by the negative charges of organic
matter and clay minerals.
I t is easily exchanged with other
cations and is readily available to plants (8). The release
of exchangeable Κ to the soil solution is called desorption
while the reverse reaction is termed adsorption.
Nonexchangeable Κ is distinct from mineral Κ in that i t is
not bonded covalently within the crystal structures of soil
mineral particles.
Rather, i t is held between adjacent tetra-
hedral layers of dioctahedral and trioctahedral micas, vermicul i t e s , and intergrade clay minerals (3,5,9,10,11,12,13,14). If
nonexchangeable Κ is equated to "fixed" K, then i t can also
occur in random gaps in the structure of x-ray amorphous
clay-sized minerals (15). Nonexchangeable K. ions held in these
lnterlayers and gaps are bound coulombically to the negatively
charged interlayer surface sites.
This binding force exceeds
NONEXCHANGEABLE POTASSIUM IN SOILS
135
Table 1. Forms of s o i l Κ and e x t r a c t i o n methods t h a t a r e
commonly used i n Κ analysis of clays and s o i l s (adapted
from Sparks ( 3) ) .
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Location
Extractants
Column displacement
Pressure membrane
Immiscible displacement and
centrifugation
Exchangeable
Colloidal
II NH4OAC
exchange s i t e s - N. NH4CI,
clay and organic
Dilute H2SO4 and HC1,
matter
N, CaCl2 or MgCl2
Dilute CaCl2 or MgCl2
Electrodialysis
Electroultrafiltration
Silver thiourea
Exhaustive cropping
Nonexchange able Vermiculite
Exhaustive leaching
Trioctahedral
with 0.01N HC1
mica
Equilibration with
Dioctahedral
mica
0.5N HC1
Strong HC1 at 373K
Hydrous mica
Boiling 23% HC1
(Illite)
ChloriteExhaustive leaching
vermiculite
with 0.1N NaCl
Sodium Cobaltinitrate
intergrades
Interstratified
Hot MgCl2
mica-smectites
Successive moist
incubations and salt
X-ray amorphous
leachings
minerals
Equilibration with
sodium-tetraphenylboron
(NaBPh4)
Serial extractions with
4
Boiling HNO3
Electrodialysis
Electroultrafiltration
Serial extractions with
Ca - saturated cation
exchange resin
Equilibration with H saturated cation exchange
resin
Trioctahedral mica Selective dissolution
Mineral
Dioctahedral mica with Na-pyrosulfate fusion
Orthoclase(K-feldspar)
HF digestion
Total
Form
Water soluble
Soil Solution
136
MARTIN AND SPARKS
Soil
solution
Κ
Exchangeable Κ
k
k
Nonexchangeable Κ
Mineral
Κ
d
k
= Adsorption rate coefficient
a
k,
α = Desorptlon rate coefficient
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k. = Fixation rate coefficient
k_ = Release rate coefficient
k. = Weathering and dissolution rate coefficient
k, = Crystallization rate coefficient
FIG. 1. Equilibrium and kinetic relationships between the various
forms of K.
the hydration forces between individual Κ ions resulting in a
p a r t i a l collapse of the crystal structure. Thus, the Κ ions
are physically trapped to varying degrees making diffusion the
rate-limiting step. Barshad (16) a t t r i b u t e s the good f i t of Κ
ions in interlayers to holes in adjacent oxygen layers of the
tetrahedral sheet of 2:1 clay minerals.
Nonexchangeable Κ can also be found in "wedge zones" of
weathered micas and vermiculites (10,11). These "wedge zones"
2+
2+
are too narrow for exchanging Ca or Mg ions to enter;
however, NH, and H_0 ions, due to their similar hydrated
r a d i i , can enter these zones (10,16,17,18).
Nonexchangeable Κ i s moderately to difficulty available to
plants, depending on various s o i l parameters (5,8,9,12,19,20,21).
Release of nonexchangeable Κ to the exchangeable form occurs
when levels of exchangeable and s o i l solution Κ are decreased
(5,22) by crop removal and/or leaching (9,14,20) and perhaps by
large increases i n microbial a c t i v i t y .
NONEXCHANGEABLE POTASSIUM IN SOILS
137
As much as 94% or more of the t o t a l Κ In s o i l i s in the
mineral form (14,23,24).
Mineral Κ i s only very slowly available
to p l a n t s (5,8,9,12,24).
Common s o i l Κ bearing minerals in the
order of availability of their Κ to plants are biotite (trioctahedral mica), muscovite (dioctahedral mica), orthoclase (Kfeldspar) and microcline (25,26,27,28,29). Small amounts of
mineral Κ are released by weathering during a growing season
(9,20) but over long periods Κ release could be substantial.
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This is particularly true where erosion i s important (30) or
where rapid soil genesis is taking place and unweathered
material i s abundant. The release of mineral Κ to more available forms i s referred to as weathering or in severe cases,
dissolution.
The reverse of this reaction i s immobilization or
precipitation.
Effect of Mineralogy on Release of Nonexchangeable Κ
The release of nonexchangeable Κ i s not thought to be the
result of dissolution of primary Κ bearing minerals but actually
a diffusion controlled exchange reaction.
This exchange is too
slow to be measured with normal methods of determining exchangeable K. When this slow exchange occurs in the interlayers of
clay minerals such as mica, the replacing ion without i t s
hydration shell must f i r s t enter the unexpended interlayer.
Then (31) or simultaneously (32), the interlayer will expand
upon hydration of these ions (31), allowing fixed or trapped Κ
ions to hydrate and slowly diffuse to exchange sites on outer
parts of the clay particle. Evidence also exists for very slow
solid state diffusion of urthydrated nonexchangeable Κ ions out
of these interlayers and inward diffusion of exchanging cations.
This diffusion occurs in 1.0 nm areas of interlayers that are
near expanded 1.4 nm areas (32).
Much work has been done on the release of interlayer Κ from
trioctahedral micas (33,34,35,36,37,38,39,40,41,42,43). I t i s
convenient to work with trioctahedral micas since their interlayer Κ i s more easily removed than the interlayer Κ in dioctahedral micas (38).
The trioctahedral micas are also much more
138
MARTIN AND SPARKS
subject t o acid d i s s o l u t i o n than t h e i r dioctahedral counterparts
(41).
The more unstable t r i o c t a h e d r a l mica - ferruginous
b i o t i t e , can be completely broken down by Η-saturated r e s i n i n
about 10 days, r e l e a s i n g a l l of i t s Κ (36).
This explains t h e
much higher r a t e c o e f f i c i e n t s f o r Κ r e l e a s e from these micas
than has been observed f o r dioctahedral micas (37).
Bassett (37)
a t t r i b u t e s t h i s difference t o the angle of the 0-H bond i n the
t e t r a h e d r a l layer of the micas.
I n t r i o c t a h e d r a l micas, t h i s
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angle i s perpendicular t o t h e t e t r a h e d r a l sheet whereas i n t h e
dioctahedral micas, i t i s oblique t o t h e plane.
The oblique
angle allows a closer approach by a K ion t o the negatively
charged oxygen, r e s u l t i n g i n a stronger e l e c t r o s t a t i c a t t r a c t i o n .
Because of t h e i r i n s t a b i l i t y , t r i o c t a h e d r a l micas generally occur
only i n s o i l s where l i t t l e weathering has taken place and thus
such s o i l s contain large q u a n t i t i e s of nonexchangeable Κ (44).
Highly weathered s o i l s of temperate, subtropical and t r o p i c a l
regions usually contain none of these minerals and a r e even very
low or lacking i n dioctahedral micas.
This i s why most minera-
l o g i c a l s t u d i e s of nonexchangeable Κ r e l e a s e i n t h e l i t e r a t u r e
are of limited a p p l i c a b i l i t y t o these weathered s o i l s .
L i t t l e i s known about how nonexchangeable Κ i s held and
released by the intergrade clays and x-ray amorphous minerals
t h a t occur i n such s o i l s .
Fields (45) a s s e r t s t h a t there i s no
possible mechanism f o r Κ f i x a t i o n by allophanes.
Schuffeien and
van der Marel (46), Martini and Suarez (47), and Barber (15)
however, present evidence f o r such f i x a t i o n . Martini and Suarez
(47) a t t r i b u t e t h i s f i x a t i o n t o changes i n the degree of
c r y s t a l l i n i t y and hydration of these minerals, e s p e c i a l l y when
subject t o wetting and drying cycles.
Many A t l a n t i c Coastal Plain s o i l s of t h e United States do
contain some hydrous mica and weathered vermiculite (10,24,48,
49,50,51).
(54).
Hydrous mica can f i x Κ (52,53) as can vermiculite
Coastal Plain and t r o p i c a l s o i l s , e s p e c i a l l y U l t i s o l s
and Oxisols, tend to be high i n k a o l i n i t e , which while r e l e a s i n g
exchangeable Κ quite rapidly (8,55), do not f i x Κ (56,57,58).
NONEXCHANGEABLE POTASSIUM IN SOILS
139
With a l l these minerals, i t i s l o g i c a l to assume t h a t i f f i x a t i o n
takes place, nonexchangeable Κ r e l e a s e also occurs.
The nonexchangeable Κ s t a t u s of c h l o r i t i z e d vermiculite, an
intergrade clay, has not been reported on to any extent, but t h i s
mineral has been i d e n t i f i e d in s o i l s by Cook and Hutcheson (59)>
Garaudeaux and Quemener (60); Sparks e t a l · , (49); and by Martin
and Sparks (24).
I t i s common in Southeastern U.S.
s o i l s (48,49,
50,61).
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Methods of Determining Nonexchangeable Κ
The various methods used to e x t r a c t nonexchangeable Κ include
exhaustive cropping of s o i l in the greenhouse, boiling HNO,, hot
HC1,
leaching with d i l u t e HC1,
e l e c t r o u l t r a f i l t r a t i o n , Na-
tetraphenylboron (NaBPh,) with EDTA, and Η-saturated and Casaturated exchange r e s i n s . Exhaustive cropping techniques are
quite useful in evaluating the a v a i l a b i l i t y and plant uptake of
nonexchangeable K. However, to assess accurately the kinds and
q u a n t i t i e s of nonexchangeable K, s o i l chemical and mineralogical
techniques must be applied since cropping e n t a i l s too many unc o n t r o l l a b l e v a r i a b l e s . Accordingly, e x t r a c t i o n s with s a l t s ,
acids, e l e c t r i c current, and ion exchange r e s i n s are manually
employed.
Exhaustive cropping to determine nonexchangeable Κ
a v a i l a b i l i t y to p l a n t s and to c h a r a c t e r i z e the Κ supplying
power of s o i l s has been used by numerous workers (60,62,63,64,
65,66,67,68,69,70,71,72,73,74,76,77,78,79,80,81,82,83,84,85,86,
87,88,89).
Soils are cropped in the greenhouse to p l a n t s t h a t
are clipped repeatedly f o r many months o r u n t i l the p l a n t s die.
Total plant top and root uptake i s measured along with exchangeable s o i l Κ l e v e l s before and a f t e r the cropping.
Simple
ezuations for determining nonexchangeable Κ r e l e a s e by t h i s
method have been described by Reltemeler e t a l . , (72), P r a t t (90)
and Addiscott and Johnston (87).
This method has helped t o
define the Κ supplying power of s o i l s and the Κ depleting
a b i l i t i e s and depletion tolerances of various crop species of
regional i n t e r e s t .
A v a r i a t i o n on t h i s technique used by Burns
140
MARTIN AND SPARKS
and Barber (76) involved exhaustively cropping the s o i l , then
incubating i t i n a moist condition a t high temperatures for
various periods of time.
They extracted exchangeable Κ with
lí NH.OAc a f t e r each incubation and called t h i s Κ nonexchangeable.
The quickest and e a s i e s t way of measuring the amount of
nonexchangeable Κ i n s o i l i s with b o i l i n g HN03
(47,54,77,81,91,
92,93,94,95,96,97,98,99). Most workers b o i l the s o i l i n IN HN03
for 10 minutes over a flame, t r a n s f e r the s l u r r y t o a f i l t e r ,
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leach the s o i l with d i l u t e HNO,, and then, determine the Κ
content of the e x t r a c t .
This method has been described by
P r a t t (100). Huang e t a l . , (29) did not b o i l the s l u r r y but
r a t h e r allowed i t t o stand a t 301 and 311K for various periods
of time.
McLean (77) used overnight soaking i n 0.1N HNO, and
repeated b o i l i n g , e x t r a c t i n g more Κ than the regular procedure
would.
One of the problems with b o i l i n g only 10 minutes over
flame (100) i s t h a t i t i s d i f f i c u l t t o be p r e c i s e about the
correct b o i l i n g time, the time i t takes for b o i l i n g t o occur,
and the vigor of b o i l i n g .
To avoid t h i s problem, P r a t t and
Morse (94), P r a t t (100), and Conyers and McLean (81) have
used a 386K o i l bath for 25 minutes including heating time.
This r e l e a s e s the same amount of Κ as with a flame but i s
more precise and e a s i e r when large numbers of samples must
be handled.
The nain problem with b o i l i n g HNO
and other
strong acids for s o i l s i s t h e i r p o t e n t i a l for dissolution of
mineral forms of Κ (19,24).
Other researchers have used continuous leaching with
d i l u t e acids (36) such as 0.0111 KC1 (77,101), or with e l e c t r o lyte solutions such as 0.1î[ NaCl (102), repeated extractions
with 3, 0.3 and 0.03N NaCl (40), strontium s a l t s (ΑΙ), hot
MgCl, (103), and sodium c o b a l t i n i t r a t e (103).
The use of cation exchange r e s i n s to simulate the uptake
of nonexchangeable Κ by plants was suggested by Wiklander (104).
Hydrogen-saturated
resins have been used for t h i s purpose by
P r a t t (90) , Schmitz and P r a t t (93) , Salomon and Smith (105) ,
Arnold (35), Stahlberg (106), Scott e t a l . , (107), MacLean (77),
NONEXCHANGEABLE POTASSIUM IN SOILS
141
Barber and Mathews (19), Haagsma and Miller (108), Feigenbaum
et a l . , (A3), and Martin and Sparks (24). These r e s i n s have
very high cation exchange c a p a c i t i e s , f a r exceeding those of
soils.
When saturated with an appropriate cation and mixed
with s o i l and with a d i l u t e s o l u t i o n of some s o r t , they w i l l
adsorb and hold a l l of the Κ r e l e a s e d from t h e s o i l .
Calcium- and Na-saturated r e s i n s have been t r i e d and found
unsatisfactory by Arnold (35), Stahlberg (106), Haagsma and
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Miller (108) and Feigenbaum e t a l . , (43) when used with any
s o i l minerals more s t a b l e than t r i o c t a h e d r a l micas.
However,
Talibudeen e t a l . , (109) argued t h a t Η-saturated r e s i n may be
destructive t o s o i l minerals and consequently used Ca-saturated
resin.
After " 100 hours of e q u i l i b r a t i o n t h e r e s i n could not
absorb f u r t h e r Κ and Κ release stopped.
Talibudeen e t a l . , (109)
ameliorated t h i s problem by separating t h e r e s i n and the s o i l
before f u r t h e r Κ r e l e a s e stopped and then adding a new charge of
resin.
The separation process however seemed t o have caused some
e x f o l i a t i o n of clay p a r t i c l e s during dispersion i n deionized
water.
The question of the r o l e of H,0 ions i n nonexchangeable Κ
release and s o i l mineral weathering i s surely important.
Arnold
(35) found muscovite and hydrous mica t o be comparatively
r e s i s t a n t t o H-resin attack.
The replacement of i n t e r l a y e r Κ
has been shown t o be unaffected by pH changes i n the range of
4.6 t o 9.2 (110), 4 t o 8 (111), 3 t o 6.8 (112), and 3 and above
(41).
Haagsma (113) found t h a t l i t t l e acid decomposition of
s o i l minerals took place above pH 2.5. i n a s o i l - r e s i n mixture.
Wells and Norrish (41) showed t h a t the H 3 0 + ion behaves l i k e a
metal cation with regard t o Κ replacement.
Norrish (114) f u r t h e r
s t a t e s t h a t i n very weak acid concentrations (10 N), t h e H,0
ion behaves as any other cation i n replacing interlayer-K and
t h a t only with higher concentrations of acid i s t h e octahedral
sheet attacked and i t s s t r u c t u r e destroyed. Martin and Sparks
(24) found t h a t Η-saturated r e s i n did not cause r e l e a s e of
mineral Κ from two A t l a n t i c Coastal P l a i n s o i l s .
Huang e t a l . ,
142
MARTIN AND SPARKS
(29) j u s t i f y mild acid treatment for measuring Κ r e l e a s e on the
b a s i s of K e l l e r ' s (115) statement t h a t the rhizosphere i o n i c
atmosphere i s dominated by H,0 . The generally accepted notion
t h a t the rhizosphere pH i s lower than t h a t i n the bulk s o i l has
been s e r i o u s l y challenged by Nye (116).
He provides
evidence
for the rhizosphere pH being 1-2 u n i t s higher than the bulk
soil.
This i s s u e remains unresolved.
In order for e l e c t r o l y t e s o l u t i o n s and cation exchange
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r e s i n s t o be e f f e c t i v e , the Κ concentration i n the s o l u t i o n
phase must be kept very low, or Κ r e l e a s e i s i n h i b i t e d
(32,41,A3,110,117,118,119,120). The c r i t i c a l concentration
above which r e l e a s e i s i n h i b i t e d has been reported as 4 wg/ml
(110) for s o i l s i n general, 2.3 t o 16.8 yg/ml f o r t r i o c t a h e d r a l
micas i n d i l u t e s o l u t i o n , and a s low a s <0.1 vg/ml f o r muscovite
and i l l i t e .
Maintenance of a low enough concentration of Κ can
be accomplished with continuous flow of e x t r a c t i n g o r exchanging
s o l u t i o n (32,41), cation exchange r e s i n s (35,43,90) o r with
Na-tetraphenylboron
(121).
The NaBPh, method was developed by Scott e t a l . , (121) and
has been used a l s o by Scott and Reed (39), Reed and Scott (38),
Scott (122), Conyers and McLean (81) and Ross (123).
The
"
anion combines with released Κ i n s o l u t i o n and p r e c i p i t a t e s ,
while the Na a c t s as an exchanger f o r i n t e r l a y e r K.
Some of these methods have been compared on the same s o i l
samples.
P r a t t (90) found t h a t H-resin e x t r a c t e d Κ c o r r e l a t e d
b e t t e r (r=0.96) than b o i l i n g HN0_ extracted Κ (r-0.913)
with
a l f a l f a (MediRago s a t i v a L.) uptake of nonexchangeable K.
Schnitz and P r a t t (93) found exhaustive cropping released 1.2
times as much Κ a s H-resin while HNO, released 2.3 times a s much;
however, both H-resin and HN0, e x t r a c t i o n s c o r r e l a t e d equally
well with cropping.
Conyers and McLean (81) found t h a t NaBPH^
sometimes removed more Κ than HN0_, sometimes l e s s .
Reed and
Scott (38) found NaBPH, a b e t t e r way of evaluating nonexchangeable Κ than the 0.3JI NaCl leaching method of Mortland (36).
MacLean (77) reported " r " values for various methods of e x t r a c t ing nonexchangeable K.
NONEXCHANGEABLE POTASSIUM IN SOILS
143
Schmitz and P r a t t (93) found t h a t while 47% of crop yield
v a r i a t i o n could be a t t r i b u t e d to exchangeable Κ l e v e l s , 88% of
yield v a r i a t i o n was a t t r i b u t e d to HNO e x t r a c t a b l e Κ [including
exchangeable and nonexchangeable K],
P r a t t (90) incorporated Κ
released to Dowex 50 r e s i n i n t o a multiple regression equation
for predicting crop removal by a l f a l f a on Iowa s o i l s .
Barber
and Mathews (19) included exchangeable Κ and H-resin e x t r a c t a b l e
nonexchangeable Κ i n t o simple l i n e a r c o r r e l a t i o n , multiple l i n e a r
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c o r r e l a t i o n , and multiple quadratic regression equations t o
predict f i e l d response of corn (Zea mays L.), wheat
(Triticum
durum Def.), oats (Avena s a t i v a L.), and potatoes (Solanum
tuberosum L.) to K. These three equations accounted for 27, 37,
and 56 percent, r e s p e c t i v e l y , of the y i e l d v a r i a t i o n i n the four
crops.
Their precision was quite low however from year to year
and within each crop.
The highest c o r r e l a t i o n of nonexchange-
able Κ with y i e l d was for s i l t loam s o i l s , while the lowest
c o r r e l a t i o n was for sandy loams.
Another technique t h a t has been used for nonexchangeable Κ
analysis i s e l e c t r o d i a l y s i s . I t has been used by Peech and
Bradfield (124), Gilligan (125), Ayres e t a l . (126), Ayres
(70), and Reitemeler e t a l . ( 3 ) .
to a current, usually 110V,
A s o i l s l u r r y i s subjected
for various lengths of time,
causing various forms of Κ to be relased i n t o solution.
More
recently, e l e c t r o d i a l y s i s equipment has become more sophisticated
(127).
E l e c t r o d i a l y s i s and a new technique,
tion (EUF)
electroultrafiltra-
have been used extensively for s o i l analysis i n
Germany and Austria and has been used i n Malaysia (128) and i n
the P h i l l i p i n e s (129).
I t s use in English speaking countries
has been very limited.
Barber and Mathews (19) warned t h a t
e l e c t r o d i a l y s i s may break down Κ minerals excessively;
but
whether the same i s possible for EUF has not been determined.
Kinetics of Nonexchangeable Potassium Release
The r a t e of r e l e a s e of nonexchangeable Κ from the i n t e r layers of mica (9,38,43,88,122,130) and vermiculite (102) i s a
diffusion controlled process.
A diffusion controlled process
144
MARTIN AND SPARKS
i s characterized by a l i n e a r r e l a t i o n s h i p between the percent
of t o t a l Κ released versus / time (24,43,50,51,131,132).
The
d i f f e r e n c e i n concentration between newly mobile ( j u s t released)
Κ and t h a t i n the e x t e r n a l s o l u t i o n supplies t h e driving force
for t h i s d i f f u s i o n (111).
The general equation f o r t h e d i f f u s i o n of Κ from clay
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interlayers (111) is:
where Κ
Κ
o
D
a
= Κ released at time t
" Κ released at equilibrium
=
diffusion coefficient
=
cylindrical radius of the area through which
the Κ diffuses
Dividing through by t yields
fA(^\
. ΔΛ /.\ Η JA
\»»/
y
m
The D value can be calculated if the value of "a" i s known. In
pure systems, "a" can be determined from mean particle size
diameter by means of N» adsorption surface area measurements
while a width to thickness ratio of the particles must be
assumed (40). In particle size controlled pure mica systems
two or three different diffusion coefficients have been found
(32,133). Each diffusion coefficient corresponded to a
different release mechanism. Rausell-Colom et a l . (32) and
Scott (122) speculate that the small coefficient represents
the slow diffusion of unhydrated ions toward the outer edge
of 1.0 nm interlayers, while the next highest coefficient
represents diffusion of partially or fully hydrated ions out
from interlayers 1.4 nm or thicker.
A third D value was found
by Talibudeen et a l . (109) and Goulding and Talibudeen (21).
According to Crank (133), the linear relationship of ion
release with the square root of time i s not degraded by the
presence of more than one value of D.
Crank (133) and Rausell-
NONEXCHANGEABLE POTASSIUM IN SOILS
145
Coloni e t a l . (32) have also found that not only r e l e a s e , but
also the movement of observed release-exchange weathering fronts
Is l i n e a r l y r e l a t e d t o / time.
In a heterogeneous s o i l with numerous types of clay of
varying particle sizes, a realistic value for "a" is usually
not measurable; thus, D cannot be measured either (134). For
this reason, Eq. [2] must be arbitrarily simplified to:
/κ \
[3]
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Κ
o
where k' is an apparent diffusion rate coefficient.
Diffi-
culties in accurately determining the value of Κ are caused
by an i n i t i a l fast release of Κ which did not obey the parabolic
diffusion equation (40,50), and perhaps the problems inherent in
distinguishing between mineral Κ and slowly released nonexchangeable K.
Using H-resin, Feigenbaum et a l . (43) found k'„ values for
-1
-1
muscovite of 0.44 hour
for 5-20 ym particles and 0.38 hour
for 20-50 ym particles.
The authors used the total Κ content
of the mica as the KQ value.
Corresponding values for triocta-
hedral micas were 7 to 18 times as high, phlogopite releasing Κ
more slowly than biotite.
There i s a paucity of classical kinetic analyses of
nonexchangeable Κ release in the literature.
Mortland (36)
used leaching of biotite with 0.1N NaCl to calculate release
rates.
He found the appearance of Κ in solution as a function
of time could be described as:
Κ = klnt + c
where
Κ = mg K/g biotite released at time t
k
= rate constant
c
= integration constant
During depletion of the f i r s t 75% of the Κ in a miscible
displacement system, the rate did not change viz.,
[4]
146
MARTIN AND SPARKS
where
R
=
the release rate
or
[6]
-£-- -k
and the release was thus zero order.
In an equilibrium
experiment, R did change with time, viz.,
dK _ -kt~ 2
[7]
dt
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indicating a f i r s t order process. Differentiating Eq. [4]
dK
dt
and since
[8]
t
dK
dt
R
[9]
then
R =
[10]
Equation [10] indicates that the rate of Κ release i s a function
of the reciprocal of time under equilibrium conditions.
Mortland and Ellis (102) found the release of fixed Κ from
vermiculite to be f i r s t order when they used the 0.1N NaCl
leaching technique.
Using an exhaustive cropping and hot
incubation technique to extract nonexchangeable K, Burns and
Barber (76) found release to be f i r s t order i n i t i a l l y and then
release was zero order.
They reported a f i r s t order rate
constant from a Cherokee clay at 382K of 5.83 X 10~
Using HNO,
hour" .
extraction at 301 and 311K, Huang et al. (29)
found release to be f i r s t order for biotite, muscovite, and
microcline. Where M was the percent of residual mineral Κ at
time t , they showed that release obeyed the equation:
log M
=
t + constant
[11]
-4
-1
The k value for muscovite was 1.39 X 10
hour at 301K.
As
2
303
would be expected the rate constants for microcline were a b i t
lower than for muscovite, while those for phlogopite were almost
one order of magnitude higher and for biotite, two orders of
magnitude higher than for muscovite. The authors, however, did
not remove Κ from solution as i t was released.
NONEXCHANGEABLE POTASSIUM IN SOILS
147
Martin and Sparks (24) determined f i r s t - o r d e r r a t e coe f f i c i e n t s for nonexchangeable Κ release from whole s o i l s a t
298K using a Η-saturated r e s i n .
First-order k i n e t i c s i n the s o i l s were described a s :
k
2
where
Κ,.
(K. - Κ )
o
t
[12]
= nonexchangeable Κ released a t time t
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nonexchangeable Κ released a t equilibrium
•> the amount of nonexchangeable Κ remaining
at time t
f i r s t order nonexchangeable Κ release r a t e
coefficient
Integrating
In (Ko-Kt) = In K ^ t
[13]
Martin and Sparks (24) found that the k„ values ranged
—3
— 1
from 1.1 to 2.2 X 10 hour
(Table 2). The low k„ values
indicated slow rates of Κ release.
The authors found that
the parabolic diffusion law also explained the data well with
apparent diffusion rate coefficients (k*_) ranging from 1.7 to
2.6 X 10~ hour 2. Thus, diffusion appeared to be the major
rate limiting step in the rate of Κ release.
Martin and Sparks (24) used the Elovich, parabolic
diffusion, first-order diffusion, and zero-order kinetic
equations to describe nonexchangeable Κ release (Table 3).
Least square regression analysis was employed to determine
which equation best described the data.
The correlation
coefficient (r) and the standard error of the estimate (SE)
were calculated for each equation. The first-order diffusion
equation was the best of the various kinetic equations studied
to describe the reaction rates of Κ release from the two soils,
as evidenced by the highest value of r and the lowest value of
SE (Table 3).
The parabolic diffusion law also described the
data satisfactorily indicating diffusion-controlled exchange.
This was also found in pure minerals by others (38,43,102,122,
148
MARTIN AND SPARKS
Table 2.
F i r s t - o r d e r nonexchangeable Κ r e l e a s e r a t e c o e f f i c i e n t s
(k.2) of Kalmia and Kennansville s o i l s (Martin and
Sparks (24)).
Depth
k 2 X 10~
m
h" 1
3
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Kalmia sandy loam
0 -
0.15
1.9
0.15 -
0.30
0.30 -
0.45
1.9
2.1
0.45 -
0.60
1.5
0.60 -
0.75
1.8
0.75 -
0.90
2.2
Kennansville loamy sand
130).
0 -
0.15
0.15 -
0.30
1.8
1.6
0.30 -
0.45
1.7
0.45 0.60 -
0.60
0.75
2.3
0.75 -
0.90
2.5
2.9
The r e l a t i o n s h i p showing the good f i t of the data for
the 0.45-0.60 m depth of the two s o i l s t o the f i r s t - o r d e r
equation i s shown in Fig. 2.
The zero-order equation was not
s u i t a b l e to describe the k i n e t i c data as could be seen from
the large values of SE, despite the f a c t t h a t the values of
r were quite high (Table 3 ) .
The Elovich equation s a t i s f a c t o r -
i l y described the r a t e of Κ exchange between solution and
exchangeable phases i n s o i l s (50) and the k i n e t i c s of Ρ
r e l e a s e and sorption i n s o i l s (135). However, i t did not
s a t i s f a c t o r i l y describe the k i n e t i c s of nonexchangeable Κ
NONEXCHANGEABLE POTASSIUM IN SOILS
149
Table 3. Correlation c o e f f i c i e n t s ( r ) and standard e r r o r of
estimate (SE) of various k i n e t i c equations f o r
nonexchangeable potassium r e l e a s e from Kalmia and
Kennansville s o i l s + (Martin and Sparks (24)).
Kalmia sandy loam
Equation
SE~ ,
χ 10~
r
3.30
0.812
2.30
0.871
5.49
0.980
1.26
0.984
3. First-order
diffusion:
In (K0-Kt) = a-bt 1.35
-0.990
1.40
-0.986
4. Zero-order:
(KQ-Kt) = a-bt
-0.985
6.63
-0.977
1. Elovich:
Kfc - a + b i n t
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Kennansville loamy sand
SE™ ,
χ 10"
r
2. Parabolic
d i f f u s i o n law:
9.71
+ The r and SE values represent an average for t h e s i x depths
of each s o i l .
4 SE i s i n mol kg" 1 .
r e l e a s e from the s o i l s studied by Martin and Sparks (24) a s
evidenced by t h e low r values and high SE values (Table 3 ) .
Importance of Nonexchangeable Κ i n Soil-Plant Relationships
The importance of nonexchangeable Κ i n s o i l - p l a n t r e l a t i o n ships has long been recognized. When exchangeable s o i l Κ
l e v e l s a r e low, p l a n t s take up more Κ than was i n i t i a l l y
exchangeable (136). The equilibrium between exchangeable
and nonexchangeable Κ must be b e t t e r understood i f Κ f e r t i l i z e r
use e f f i c i e n c y and economic plant y i e l d s a r e maximized.
Exchangeable Κ l e v e l s c o r r e l a t e well i n many s o i l s with p l a n t
uptake and with the r e l e a s e of nonexchangeable Κ during cropping
150
MARTIN AND SPARKS
1.0
100
200
300
400
TIME, h
500 600
700 800
900 1000
o KALMIA SANDY LOAM
0.8
• KENNANSVILLE LOAMY SAND
0.6
0.4
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0.2
SÉ
c
0.0
-0.2
-0.4
-0.6
FIG. 2. First-order kinetics of nonexchangeable Κ release from
the 0.45- to 0.60-m depth of Kalmia and Kennansville soils (from
Martin and Sparks (24)).
(62,63,70). For other soils, this correlation i s poor (66,92,
137).
Nonexchangeable Κ release can proceed locally in the root
zone even though the exchangeable Κ level in the soil outside the
root zone i s too high for such release (86). The extent to which
root zone Κ depletion occurs i s a function not only of the soil's
Κ status, but of the plant's ability to draw down the available Κ
(86).
Pratt (90) found that in soils that are not highly weath-
ered, exchangeable Κ correlated well with plant uptake. In highly weathered soils, the reverse was true. Abel and Magistad (66)
showed, however, that once the exchangeable Κ had been depleted,
less weathered Hawaiian soils generally release more nonexchangeable Κ than highly weathered soils.
NONEXCHANGEABLE POTASSIUM IN SOILS
151
SUMMARY
In t h i s paper we have reviewed t h e c h e m i s t r y and mineralogy
of nonexchangeable Κ i n s o i l s .
This phase of s o i l K, a l o n g with
the mineral form, comprises the bulk of t o t a l Κ i n most s o i l s .
I t s importance i n supplying Κ t o plant roots cannot be overemphasized.
Perhaps the most important aspect of nonexchangeable s o i l Κ
i s the r a t e a t which i t i s released t o exchangeable and solution
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forms which are readily available for plant uptake. The r a t e and
magnitude of release i s dependent on a number of f a c t o r s . The
level of Κ i n the s o i l solution greatly a f f e c t s the release of
nonexchangeable K.
I f the level i s low, more release w i l l occur
from the nonexchangeable form.
This i s due t o the dynamic equil-
i b r i a l reactions that exist between the phases of s o i l Κ. Ί ί the
s o i l solution Κ level i s high, release from the nonexchangeable Κ
phase w i l l be l e s s .
A second factor controlling the magnitude of
Κ release from the nonexchangeable form i s the type of clay minerals present.
Soils that are high i n k a o l i n i t e and low charge
montmorillonite contain very small q u a n t i t i e s of nonexchangeable
K, while s o i l s containing vermiculitic and micaceous minerals
contain copious q u a n t i t i e s of nonexchangeable and mineral K.
Regrettably,
there are few reports i n the l i t e r a t u r e on the
kinetics of nonexchangesble Κ release from s o i l s .
This informa-
tion i s imperative i n predicting the Κ supplying power of s o i l s .
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NONEXCHANGEABLE POTASSIUM IN SOILS
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