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Chemistry of soil potassium in Atlantic coastal plain soils: A review
D. L. Sparks a
a
Department of Plant Science, University of Delaware, Newark, Delaware
Online Publication Date: 01 January 1980
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COMMUN. IN SOIL SCIENCE AND PLANT ANALYSIS, 11(5), 435-449 (1980)
CHEMISTRY OF SOIL POTASSIUM IN ATLANTIC
COASTAL PLAIN SOILS: A REVIEW1
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KEY WORDS: K k i n e t i c s , forms of s o i l K, crop response, leaching
of K
D. L. Sparks2
Department of Plant Science
University of Delaware
Newark, Delaware 19711
ABSTRACT
Literature dealing with general properties of s o i l K and with
K r e l a t i o n s h i p s i n Atlantic Coastal Plain Soils was discussed.
Potassium, among major and secondary n u t r i e n t elements, i s the
most abundant i n s o i l s .
I t , among mineral cations required by
p l a n t s , i s largest i n non-hydrated s i z e .
Potassium has a
3
p o l a r i z a b i l i t y equal t o .88 Å and a low hydration energy of
34 kcal g -1 i o n - 1 .
The major K forms i n s o i l s are water soluble,
exchangeable, nonexchangeable, and mineral.
Various dynamic
i n t e r r e l a t i o n s h i p s exist between these forms with the reaction
k i n e t i c s between the various phases determining the f a t e of
applied K.
Many Atlantic Coastal Plain s o i l s contain high levels of
t o t a l K. Most of t h e t o t a l K i n these s o i l s i s contained i n
mineral forms such as micas and K-feldspars.
These K forms a r e
slowly released t o solution and exchangeable forms that a r e
available to p l a n t s . Many researchers have noted a lack of crop
response to K f e r t i l i z a t i o n on Atlantic Coastal Plain s o i l s .
This
lack of response has been ascribed t o the high indigenous levels
435
Copyright © 1980 by Marcel Dekker, Inc.
436
SPARKS
of m i n e r a l and non-exchangeable
a v a i l a b l e to crops.
K i n the s o i l s which would become
Some researchers have also a t t r i b u t e d the
lack of response t o K accumulations i n subsoil from leaching of
applied K. I f t h e physical and chemical conditions were favorable i n the subsoil horizons, e. g., no pan formation and no
severe Al t o x i c i t y , plant roots could absorb K from t h e subsoil
horizons.
INTRODUCTION
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The r o l e of K i n s o i l s i s prodigious.
I t s complex behavior
in s o i l s and p l a n t s has been epitomized by Albrecht (3): "Because of the prevalence of i t s minerals i n the l i t h o s p h è r e , of
i t s r e a d i l y soluble n a t u r e , of i t s readiness t o become insoluble
and inexchangeable from the c o l l o i d , of i t s movement from
vegetation t o t h e s o i l through leaching from the tops or exchange
from the r o o t s , and of i t s reserve i n the s i l t and sand minerals
to buffer the clay; K i s so nomadic t h a t i t s performances i n any
particular situation are difficult to interpret."
Many reports
have appeared in the literature concerning the K status and crop
response to K fertilization on Atlantic Coastal Plain soils.
Before discussing the latter reports, some general
characteristics
of soil K will be given.
General Characteristics of K
Of the major and secondary nutrient elements, K i s usually
the most abundant in soils.
The lithosphère contains an average
of 2.8% K while soil contains 1.7% K (4).
Some chemical charac-
teristics of K are given in Table 1. Potassium, among mineral
cations required by plants, i s the largest in nonhydrated size
o
(r = 1.33 A) and the number of oxygen ions surrounding i t in
mineral structures i s high (8 or 14) which suggests that the
strength of each K-0 bond i s relatively weak. Potassium has a
o3
-Hpolarizability equal to 0.88 A which i s higher than for Ca ,
Li , Mg
Rb
, and Na ions but lower than for Ba , Cs , NH4 , and
(5). Ions with higher .polarizability
would be preferred in
ion
POTASSIUM IN ATLANTIC COASTAL SOILS
437
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SPARKS
ion exchange r e a c t i o n s .
Potassium has a h y d r a t i o n energy t>of
34 k c a l g~ i o n " which would i n d i c a t e l i t t l e a b i l i t y t o cause
s o i l swelling ( 1 1 ) .
Forms of S o i l K
Most r e s e a r c h e r s concur t h a t s o i l K e x i s t s i n water s o l u b l e ,
exchangeable, non-exchangeable and mineral phases.
These forms
can be a n a l y t i c a l l y determined using e x t r a c t i o n methods given
in Table 2. Water s o l u b l e K i s found i n low c o n c e n t r a t i o n s and
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i s very mobile. Anderson e t a l , (12) found t h a t H.O-soluble K
TABLE 2
Methods f o r E x t r a c t i n g Forms of S o i l K.
Form
Water Soluble
Location
Extractant
Soil Solution
H20
Exchangeable
Exchange Phase
NH.OAc and H„SO.
4
2 4
Nonexchangeable Vermiculites and
2:1 intergrade
clay minerals
Micas and
feldspars
Mineral
Hot HNO„
Selective dissolution
using Na-pyrosulfate
fusion
H F digestion
Total
ranged from 0.83 to 83 ppm of K on a soil basis in some humid
soils.
When water content was increased to a soil:
of 1:10, extracted K increased by 3- to 27-fold.
water ratio
They attributed
the increase in solution K to hydrolysis of exchangeable K by
divalent ions, or to dissolution of K-bearing minerals. Exchangeable K, that held by the negative charges of soil organic
matter and clay, i s easily exchanged with other cations such as
i
Ca
i
it
and Mg , and i s readily available to plants (15). Non-
exchangeable K i s moderately available to plants (5, 13, 14, 15)
POTASSIUM IN ATLANTIC COASTAL SOILS
439
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and, like NH^ , i s held between the p l a t e l e t s of clay p a r t i c l e s
of s o i l mica and vermiculite (4, 5, 13, 16, 17, 18, 19, 20).
Potassium'in primary minerals, such as micas and feldspars, i s
very slowly available to plants (5, 13, 15).
Interrelationships exist between the various forms of s o i l
K as shown in Fig. 1. Fixation of soil-solution and exchangeable
K occurs between the p l a t e l e t s of ' i l l i t e ' or of hydrated mica and
vermiculite, and between frayed edges of mica p l a t e l e t s p a r t i c ularly in environments with high concentrations of these two
readily available K forms (21). This fixation occurs in clayey
surface soils when K levels are increased from leaching following
f e r t i l i z a t i o n (13, 14, 21). Release of fixed K to exchangeable
and water soluble forms increases the levels of these readily
available forms (21). Doll and Lucas (22) found that nonexchangeable K in soil mica, vermiculite, and c h l o r i t e was released as
soluble or exchangeable K when levels of the l a t t e r were decreased.
Levels of exchangeable and s o i l solution K are commonly decreased
by crop removal and by leaching (13, 14, 23). A r e l a t i v e l y small
amount of mineral K i s released by weathering during a growing
season (13, 14).
The r a t e kinetics between the various forms of s o i l K
determine the magnitude of leaching, release, and fixation in
s o i l s . The kinetic reactions between the exchangeable and nonexchangeable phases of K are slow (15, 16, 24). Cooke and
Hutcheson (21) investigated the rate of transformation and
release of K from b i o t i t e and " i l l i t e " . Potassium release from
these minerals was extremely slow as compared with reactions
between soluble and exchangeable forms.
The r a t e of reaction between the soil-solution and exchangeable phases of K i s generally proposed to be almost instantaneous
(16, 25, 26, 27, 28, 29). However, the kinetics of K exchange in
s o i l s depend on the type of clay minerals present. Barshad (30)
reported a low r a t e of K exchange i n vermiculite. Sparks e t a l ,
440
SPARKS
Micas and F e l d s p a r s
I
Release
Nonexchangeable K
Exchangeable K
Fixation
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Applied K
>
Water Soluble
N
Plant Uptake
FIG. 1
Chemical Relationships Between Phases of Soil Potassium 13
(31) found that the kinetics of K exchange in two Dothan soils
from the Coastal Plain of Virginia and high in chloritized
vermiculite and vermiculite had low reaction kinetics (Fig. 2 ) .
Equilibrium was reached in these soils in two hours when the
soils were equilibrated with a 5-25 ug/ml K solution but in 1
to 2 days with a 100 yg/ml K solution. These low r a t e kinetics
would suggest that K should remain in solution for an extended
period where i t could be leached or absorbed by plants. Selim
et a l (15) found K exchange to be f a i r l y rapid in some Florida
Coastal Plain s o i l s where the mineralogy was dominated by
kaolinite. Kaolinite has been shown to exhibit a rapid rate
of K exchange (29).
Distribution of Forms of K in Soils
The major portion of t o t a l K in most soils i s nonexchangeable.
An average of 99.6% of the t o t a l K in samples of 20 soils of New
Jersey was nonexchangeable. The remaining 0.4% included both the
exchangeable and water soluble forms (32). Yuan e t a l (33) found
that some Florida Coastal Plain soils had significant portions of
K in feldspar forms. Sparks e t a l (23) found that most of the
POTASSIUM IN ATLANTIC COASTAL SOILS
CD
CE
O
(/>
O
<
2¿
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too
A 5 ppm
• 25 ppm
• 100 ppm
10
0.1
10
100
200
REACTION TIME, hours
FIG. 2
Potassium adsorption a t 250 by Nottoway county Ap soil horizon
(Al-saturated) as a function of time.
t o t a l K i n two Virginia Coastal Plain soils was in the nonexchangeable and mineral forms (Table 3). The exchangeable K
comprised only a small percentage of the t o t a l s o i l K and was
always <0.2 meq/100g. Most of the t o t a l K in both soils occurred
in the feldspar and mica forms. Feldspar K comprised approximately
94% of the t o t a l K in the Ap horizon of the Nottoway County s o i l .
Potassium from the nonexchangeable and mineral forms could be
released with time t o the exchangeable and solution phases that
would be available to plants. Significant portions of the t o t a l
K i n both these s o i l s were present in the s i l t fraction of both
s o i l s and even i n the sand fraction from the Nottoway s i t e . Other
researchers have observed the contributions made by the sand and
s i l t fractions to t o t a l K release (34, 35). Munn e t a l (35),
found that K released from sand and s i l t fractions could reach
a maximum of 56 and 21% of the t o t a l K released, respectively.
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TABLE 3
Forms of K i n Dothan s o i l s i t e s examined from Greensville and Nottoway Counties.
Horizon
Depth
NH.OAc
Ext.
Acid E x t . K
HNO3
H2SO¿,
K-Minerals
Mica
Feldspar
cm
Total
Soil K
Sand
Total K
Silt
Clay
meq/lOUg
Greensville Countv
Ap
A2
B21t
0-20
20-31
31-41
0.11
0.11
0.22
0.11
0.10
0.21
0.17
0.19
0.38
5.4
5.7 .
5.1
0.8
0.9
3.4
6.5
6.9
9.3
0.3
0.4
0.2
3.7
3.4
1.4
2.5
3.1
7.7
12.0
10.8
11.4
2.5
2.0
1.8
4.1
5.5
4.7
5.4
3.3
4.9
Nottoway (bounty
Ap
A2
B21t
0-15
15-33
33-58
0.11
0.09
0.13
0.11
0.11
0.14
0.22
0.19
0.24
11.3
8.2
5.4
0.3
2.3
5.5
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POTASSIUM IN ATLANTIC COASTAL SOILS
443
Leaching of K in Coastal Plain Soils
Soil solution K i s either leached or sorbed by plants or
s o i l s (15). The leaching of K i s of considerable magnitude in
many sandy Atlantic Coastal Plain s o i l s (36). Three main factors
that influence the magnitude of K leaching are K absorption by
plants, the reduction in the volume of s o i l water by t r a n s p i r a t i o n ,
and the composition of f e r t i l i z e r materials and s o i l s (37).
Crops reduce K leaching by assimilating K into their tissue
as well as by reducing water percolation through s o i l (38) . Volk
and Bell (37) suggested that greater K leaching in fallow versus
cropped s o i l was due to movement of more gravitational water.
Rainfall duration and intensity affects K movement in s o i l .
Gammon (39) noted that downward water movement occurs in s o i l
during a rain and that upward movement occurs when water i s lost
at the surface through evapotranspiration. Upward movement of K
s a l t s may accompany this upward movement of water.
Retention of K can be enhanced in Coastal Plain s o i l s a f t e r
application of lime and P (37, 38, 40). Nolan and Pritchett (38)
found that liming to pH 6 to 6.5 caused maximum retentivity of
added K in a Lakeland fine sand. Potassium was replaced by Ca on
the exchange complex at higher levels of limestone application.
Less leaching of K occurred a t pH 6.0 to 6.5 due t o enhanced
substitution of K for Ca than for Al which i s more abundant a t
low pH. Leaching of s o i l K was observed to vary inversely with
quantities of organic matter and clay (36). *
Several investigations have been conducted to determine the
relationship of crop uptake and rate of K application t o leaching
of K in Coastal Plain s o i l s . Jackson and Thomas (41) applied up
to 524 kgK/ha prior t o planting sweet potatoes (Ipomoea batatas
L.) on a Norfolk sandy loam. At harvest time, s o i l and plant K
exceeded applied K at the 131 and 262 kg K/ha r a t e s . However, a t
the 524 kg K/ha r a t e , 38 kg of K was unaccounted for by s o i l and
plant K. This deficiency of K was attributed to leaching below
sampling depths. During a two year study with corn (Zea mays L.)
444
SPARKS
on two Dothan s o i l s of V i r g i n i a , Sparks e t a l (23) found t h a t 83
and 249 kg K/ha of applied K increased t h e exchangeable K i n t h e
A2 and B21t horizons of t h e two s o i l s .
These i n c r e a s e s were
a s c r i b e d t o leaching of applied K. The magnitude of leaching
v a r i e d d i r e c t l y w i t h r a t e of K a p p l i c a t i o n .
No accumulation of K
was found i n t h e top 76 cm of a Leon sand a f t e r 40 y e a r s of heavy
K f e r t i l i z a t i o n ( 5 2 ) . L u t r i c k (43) found t h a t K l e a c h i n g occurred
on unlimed a r e a s , b u t n o t on limed a r e a s , when 112 t o 224 kg K/ha
was applied on a E u s t i s loamy f i n e sand.
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Crop Response t o Applied K on Coastal P l a i n S o i l s
On many sandy s o i l s of t h e A t l a n t i c C o a s t a l P l a i n r e g i o n K
fertilization has failed to increase corn (Zea mays L.) yield
(23, 33, 44, 45, 46). These soils characteristically have sandy
surface horizons and accumulations of K in clayey subsoil horizons.
Normally, K fertilization recommendations for these soils are
based on samples from the surface 18 to 20 cm. Often these
samples contain low levels of extractable K and yet, there i s no
response to K fertilization.
Black (45) reported high corn yields
on two Coastal Plain soils in Virginia to which no K f e r t i l i z e r
had been applied for 7 years.
He pointed out that when one con-
siders the K content in the soil depths to which corn roots may
penetrate, i t i s necessary that subsurface horizons as well as the
plow layer need consideration when evaluating K availability from
soils.
Hutton and Robertson (44), in a study of the residual
effect of K f e r t i l i z e r in two Ultisols on crop production, showed
some K accumulation in subsoil. Yuan et a l (33) investigated the
lack of crop response to K fertilization for corn, soybeans, and
small grain on some Ultisols in the Lower Coastal Plain. Profile
samples were analyzed for exchangeable, nonexchangeable, mineral,
and total soil K.
A K reserve of 1,500 to 2,800 kg/ha was found
in 15 cm increments of these soil profiles.
A large portion of
this K reserve occurred in the K-feldspar form and was found in
micaceous mineral forms while very l i t t l e of this K reserve was
POTASSIUM IN ATLANTIC COASTAL SOILS
in the exchangeable and fixed forms.
445
Sparks e t a l (13)
a t t r i b u t e d the lack of response of corn t o K f e r t i l i z a t i o n on
two Dothan s o i l s from the Virginia Coastal Plain to large
quantities of K feldspars found in the s o i l s which with time
supplied the s o i l with available K. In Delaware, corn yield was
not s i g n i f i c a n t l y increased by application of K f e r t i l i z e r over
a 4-year period on sandy s o i l s (46).
This lack of K response
was a t t r i b u t e d t o release of K from nonexchangeable and mineral
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forms.
- SUMMARY
Many sandy Atlantic Coastal Plain s o i l s contain large
quantities of t o t a l K. Much of t h i s t o t a l K i s found in mineral
and nonexchangeable forms which can slowly be released with time
to plant available forms.
The lack of crop response t o applied K on many of these s o i l s
has been ascribed to the presence of these K forms.
In addition,
there i s evidence that t h i s lack of crop response could be due to
leaching of applied K into clayey subsoil horizons.
This subsoil
K i s available f o r absorption by plant roots unless
adverse
physical and chemical s o i l properties e. g., pan formation, and
high Al, e x i s t i n these horizons.
The r a t e of K release from the exchangeable, nonexchangeable,
and mineral forms i n these s o i l s needs to be further investigated.
With k i n e t i c data, predictive models could be developed f o r these
s o i l s which would aid i n making sound f e r t i l i z e r recommendations.
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446
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