Kinetics of Nonexchangeable Potassium Release from Two Coastal Plain Soils1

H. W. MARTIN AND D. L. SPARKS2

ABSTRACTThe kinetics of nonexchangeable-K release using H-saturated resin

were investigated on Kalmia (fine-loamy, siliceous, thermic TypicHapludults) and Kennansville (loamy, siliceous, thermic Arenic Ha-pludults) soil profiles from the Coastal Plain of Delaware. Calcium-saturated soil samples were equilibrated with H-saturated resin from0.5 to 960 h. Equilibrium in K release in both soil profiles wasattained in about 960 h. The kinetics of K release were evaluatedusing the Elovich, parabolic diffusion law, first-order diffusion, andzero-order equations. The first-order diffusion equation describedthe K-release kinetics best as evidenced by the highest correlationcoefficient (r) and the lowest value of the standard error of the es-timate (SE). The parabolic diffusion law also described the datasatisfactorily indicating diffusion-controlled exchange. The zero-or-der and Elovich equations did not describe the data well as shownby higher SE values than those found with the first-order diffusionand parabolic diffusion law equations. Nonexchangeable-K releaserate coefficients (k2) ranged from 1.20 to 2.2 X 10~3 h~' in theKalmia soil and from 1.5 to 2.9 X 10 3 h ' in the Kennansville soil.The magnitude of the /c2 values suggested low rates of nonexchange-able-K release from the two soils.

Additional Index Words: first-order diffusion kinetics, diffusion-controlled exchange, Elovich equation.

Martin, H.W., and D.L. Sparks. 1983. Kinetics of npnexchangeablepotassium release from two Coastal Plain soils. Soil Sci. Soc. Am.J. 47:883-887.

NONEXCHANGEABLE K is released to the exchange-able form when levels of exchangeable and soil

solution K are decreased by plant uptake and leaching(Jackson, 1964; Sparks et al, 1979; Sparks, 1980).Schmitz and Pratt (1953) found that while 47% of thevariation in corn (Zea mays L.) yield percentage wasdue to exchangeable soil K levels, 88% was ascribedto the quantity of nonexchangeable and exchangeableK.

1 Published with the approval of the Director of the DelawareAgric. Exp. Stn. as Misc. Paper no. 1018. Contribution no. 150 ofthe Dep. of Plant Science, Univ. of Delaware, Newark, DE 19711.Received 11 Oct. 1982. Approved 12 May 1983.2 Former Graduate Research Assistant and Associate Professor ofSoil Chemistry, respectively. The address of the Senior Author isSoil Science Dep., Univ. of Florida, Gainesville, FL 32611.

The release rate of nonexchangeable K from micas(Mortland, 1961; Reed and Scott, 1962; Scott, 1968;Feigenbaum et al., 1981) and from vermiculite (Mort-land and Ellis, 1959) is diffusion controlled. Mortland(1958) investigated the kinetics of K release from bio-tite and found that the release rate was first-order us-ing a batch technique and zero-order with a miscibledisplacement method. Mortland and Ellis (1959), us-ing 0.1./V NaCl as an extractant, found the release ofnonexchangeable K from vermiculite was first order.Using HNO3 extraction at 301 and 31 IK, Huang etal. (1968) showed that K release was first order forbiotite, microcline, muscovite, and phlogopite. Therate coefficients were in the order biotite > phlogopite> muscovite > microcline. Using a H-saturated resin,Feigenbaum et al. (1981) found that relative rate coef-ficients based on the parabolic diffusion equation av-eraged 0.40 for muscovite particles.

While there are data on the kinetic reactions be-tween solution and exchangeable forms of K in soils(Sparks et al., 1980a and 1980b; Sparks and Jardine,1981; Sparks and Rechcigl, 1982), there are few re-ports in the soil chemistry literature on the kinetics ofnonexchangeable-K release from soils. Accordingly, theobjectives of this study were: to investigate the kinet-ics of nonexchangeable-K release using a resin tech-nique, and to employ various kinetic equations to de-scribe nonexchangeable-K release from two AtlanticCoastal Plain soils.

MATERIALS AND METHODSSoil samples were taken at 0.15-m depth increments from

0 to 0.90 m of a Kalmia sandy loam (fine-loamy, siliceous,thermic Typic Hapludults) and a Kennansville loamy sand(loamy, siliceous, thermic Arenic Hapludults) from Dela-ware. These soils were chosen because anomalous crop re-sponses to applied K were previously noted (Sparks andLiebhardt, 1982). The soil samples were air-dried and gentlyground to pass through a 2-mm sieve in preparation forlaboratory analyses.

Soil Characterization AnalysesSoil pH was determined using a 1:1 water-to-soil ratio,

and organic matter was estimated using a modified Walk-

884 SOIL SCI. SOC. AM. J., VOL. 47, 1983

ley-Black procedure (Allison, 1965). Cation exchange ca-pacity (CEC) was measured by a MgCl2 saturation with sub-sequent displacement by CaCl2 (Rich, 1962; Okazaki et al.,1963). Particle-size analysis was determined by the hydrom-eter method (Day, 1965).

Before mineralogical analysis, samples were treated withNaOCl adjusted to pH 9.5 to remove organic matter (An-derson, 1963). Iron oxides were removed using a Na dithion-ite-citrate-bicarbonate procedure (Mehra and Jackson,1960). The clay and silt fractions were passed wet througha 300-mesh sieve and separated by centrifugation. Orientedmounts of the clay fraction were prepared by depositing ~250 mg of clay on a ceramic tile, saturating with K or Mgunder suction, washing free of salts, and glycolating the Mg-saturated samples. X-ray dim-action patterns of Mg glycerol-saturated samples at 298 and 383K and K-saturated samplesat 298, 383, 573, and 823K were obtained using a DianoXRD 8300 AD x-ray diffractometer equipped with a graph-ite monochromator, PDP-8 computer, and a printout. Thesamples were scanned at 2° (26) per min using CuKa radia-tion.

Soil samples from each depth of the two soils were char-acterized as to their K status. Exchangeable K was extractedusing \M NH4C1 and 0.5M CaCl2, nonexchangeable K withboiling \M HNO3 (Pratt, 1965), and total K with HF diges-tion (Bernas, 1968; Buckley and Cranston, 1971). The po-tassium in each of the extracts was measured using a Perkin-Elmer 5000 atomic absorption spectrophotometer. MineralK was estimated by subtracting the sum of CaCl2 and HNO3-extractable K from total K.

Kinetics of Nonexchangeable-K ReleaseBefore initiating the kinetic studies, each of the soil sam-

ples was Ca-saturated with 0.5M CaCl2 to remove nativeexchangeable K. The samples were then washed with deion-ized water until a negative test for Cl~ was obtained withAgNO3. Duplicate 2-g samples of Ca-saturated soil wereadded to 80-mL polypropylene centrifuge tubes with 4 g ofmoist Bio-Rad AG 50WX H-saturated resin and 50 mL ofO.OOlAf HC1. The resin had a CEC of 54.1 mol(H+) kg-'.Homoionic H-resin was prepared by leaching the resin withl.OA/ HC1 solution and washing out the salt with deionized

Table 1—Selected chemical and physical properties andmineralogy of the < 2-/un clay fraction of Kalmia and

Kennansville soils.

Particle-sizeanalysis

Depth Sand Silt Clay pH

Or-ganic

matter CECMineral suite of < 2-fOn

clay fractiont

m ———— % ———— % mol(>/2Mg»)

kg-'xlO-!

Kalmia sandy loam0 -0.1569.221.2 9.6 5.0 2.2 2.3 Ka,J, Cva, Q3, Gi«, Fes0.15-0.3067.521.411.1 5.3 1.4 2.0 Ka,, Cv2, Ve,, Q,, Gis, Fe,0.30-0.450.45-0.600.60-0.750.75-0.90

0 -0.150.15-0.300.30-0.450.45-0.600.60-0.750.75-0.90

70.174.581.684.6

84.682.778.775.181.083.2

14.911.37.26.9

12.012.615.215.08.35.4

15.0 5.2 0.5 1.814.2 4.9 0.3 1.711.2 4.6 0.2 1.48.5 4.7 0.1 1.0

Kennansville loamy sand3.44.76.19.9

10.711.4

5.25.65.85.85.65.5

1.10.80.20.20.20.1

1.51,40.81.11.41.5

KaKaKaKa

Q,,CvCvVeKaKa

„ Q,, Ve,,,,Q,,Cv3,,,Q,,Cv3,. Q,. Fe3

Cv,, Ka,,„ Q,, Ka3,,,Ka!,Q3,„ Ka2, Qs,,,Cv2,Q3,.,Q,.Ve,,

Cv4,Gi.,

Gi.Ve.,Gi.,Gi.,Ve.,Cv.,

Gi.,Mi.,

Gi,,FesFes,Gi..Gi5,

Fe,Fee

Fe.

Cv.Fe.Fe,

water. The samples were equilibrated at 298 K ± 1 for 30min to 40 d on a reciprocating shaker. Forty days was a timewhen an apparent equilibrium in nonexchangeable-K releasewas obtained in all the soil samples. To minimize weath-ering and abrading of the soils, the shaker was turned offevery other hour during the equilibration period. After equi-libration, the soil was separated from the resin on a 60-meshsieve and the resin was leached with 80 mL of \M NH4C1to remove the nonexchangeable K. The leachate was broughtto the 100-mL volume and analyzed for K as before.

Nonexchangeable-K release conformed to first-order ki-netics which for this study is described as follows:

dKJdt = k2(K0 - Kt), [I]where

K, = nonexchangeable K released at time t,K0 = nonexchangeable K released at 40 d,k-i = nonexchangeable-K release rate coefficient,

t = time.Integrating, with appropriate boundary conditions of t =

0; K, = 0, Eq. [1] becomesln(K0-Kt) = lnK0 - k2t. [2]

RESULTS AND DISCUSSIONSelected chemical and physical properties and min-

eralogy of the clay fraction of the Kalmia and Ken-nansville soils are given in Table 1. The CEC andorganic matter contents of the two soils were low,which is typical of Atlantic Coastal Plain soils. TheKalmia soil contained higher amounts of clay at alldepths except for the 0.75- to 0.90-m increment. Con-siderable quantities of kaolinite, chloritized vermic-ulite, and quartz were present throughout the profilesof the two soils. Lesser quantities of vermiculite andfeldspars were also found in both soils.

Potassium Chemistry of the SoilsThe amounts of CaCl2- and NH4Cl-extractable K

were generally higher in the Kalmia than in the Ken-nansville soil (Table 2). At the three lowest incrementsin the two soils the amounts of NH4Cl-extractable Kwere about the same, whereas more CaCl2-extractableK was found in the Kennansville soil than in the Kal-

Table 2—Potassium chemistry of the Kalmia andKennansville soils.

t Ka = kaolinite; Cv = chloritized vermiculite, Q = quartz; Gi = gibbsite;Fe = feldspars; Ve = vermiculite.

t Subscript 1 = most abundant; 6 = least abundant.

Depth

m

NH.C1ext.

CaCUext.

HNO3ext.

H-saturatedresin ext.

<#0)TMineral

KtTotal

K

————————————— mmol (K*) kg~' —————————————Kalmia sandy loam

0 -0.150.15-0.300.30-0.450.45-0.600.60-0.750.75-0.90

2.251.351.301.451.501.65

1.720.951.050.971.231.15

2.201.921.861.751.892.04

2.181.801.642.131.571.60

37.5841.2341.9941.6846.5849.71

41.5044.1044.9044.4049.7052.90

Kennansville loamy sand0 -0.150.15-0.300.30-0.450.45-0.600.60-0.750.75-0.90

2.100.860.951.381.481.60

1.150.690.671.231.311.49

2.092.222.142.792.522.23

1.511.391.121.561.921.86

31.2634.7934.2933.5829.4735.28

34.5037.7037.1037.6033.8039.00

t Represents the quantity of K extracted at 40 d with H-saturated resin.j Mineral K = [(total K) - (CaCl, Ext. K + HNO, Ext. K)].

MARTIN & SPARKS: KINETICS OF NONEXCHANGEABLE K RELEASE FROM TWO COASTAL PLAIN SOILS 885

23.00

17.90

Q- 15.40

7.67

< 5.10Io o KALMIA SANDY LOAM

• KENNANSVILLE LOAMY SAND

0.00 100 200 300 400 500 600 700 800 900 1000 1100 1200TIME, h

Fig. 1—Amount of nonexchangeable K released vs. time in the 0.45-to 0.60-m depth of Kalmia and Kennansville soils.

mia soil. Although clay content is important in theextractable-K status of these soils, the K-containingmineral content may be more important. For exam-ple, at the depth of 0.45 to 0.60 m, the clay contentswere 14.2 and 9.9% in the Kalmia and Kennansvillesoils, respectively, whereas the amounts of CaQ2-ex-tractable K were 0.97 and 1.23 mmol kg"1, respec-tively. This can probably be ascribed to the highercontent of vermiculite in the Kennansville soil thanin the Kalmia soil (Table 1). Ammonium chloride ex-tracted more K than CaCl2 in the two soils. Both soilscontained considerable quantities of vermiculitic clayminerals (Table 1) which usually contain "wedgezones" or specific sites for K adsorption (Rich, 1964;Sparks and Liebhardt, 1981; Sparks and Liebhardt,1982). Ammonium, with a crystalline radius of 0.143nm (Rich, 1968), could displace K+ with a crystallineradius of 0.133 nm (Rich, 1968) from the sites, whereasthe wedge zones would selectively screen out the largerCa2+ ion. This would result in higher levels of extract-able K with NH4C1 than with CaCl2. Thus, in soils ofthis type, NH4C1 or NH4OAc would tend to overpre-dict exchangeable K.

The total K levels averaged 46.3 and 36.6 mmolkg"1 in the Kalmia and Kennansville soils, respec-tively. These are comparable to total K levels foundin Florida Coastal Plain soils (Yuan et al., 1976) butconsiderably lower than quantities noted for VirginiaCoastal Plain soils (Sparks et al., 1980). Greater than88% of the total K in the two soils was in the mineralphase. The high portion of total K in these primarymineral forms suggests that parent material was theorigin of most of the K at each depth in the two soils.Nitric acid-extractable K, which is used as an indexof nonexchangeable K (Pratt, 1965), averaged 1.94 and2.33 mmol kg"1 in the Kalmia and Kennansville soils,respectively. These relatively low levels would be ex-pected where much of the total K was in the mineralform (Sparks et al., 1980).

Kinetics of Nonexchangeable-K ReleaseBefore initiating the kinetic studies, a number of

measurements were made on the resin-soil suspen-

823K EXTRACTED

823K CONTROL

573K EXTRACTED

573K CONTROL

383K EXTRACTED

383K CONTROL

298K EXTRACTED

298K CONTROL

1.4 1.2 .72 .48 .43 .36 .33d-SPACING. nm

Fig. 2—X-ray diffractogram tracing of K-saturated < 2-ian clay frac-tion of the 0.45- to 0.60-m depth of Kalmia sandy loam soil.

sions. Levels of K in the leachate after washing theseparated resin were measured and found to be ex-tremely low. Samples containing H-resin and the HC1solution without soil were run in quadruplicate foreach of the times investigated. The amount of K re-leased averaged 0.10 X 10~5 mol L"1. This quantitywas subtracted from the total amount of nonexchange-able K released from the soils at each time increment.The level of solution K markedly affects the release ofnonexchangeable K from clays. The K concentrationin the solution phase must be kept very low, or Krelease will be inhibited (Reed and Scott, 1962; Rau-sell-Colom et al., 1965; Fanning and Karamidas, 1979;Feigenbaum et al., 1981). The concentration of solubleK in the soil-resin suspension of this study rangedfrom 1.00 to 1.50 X 10"3 mmol L"1. Rausell-Colomet al. (1965), using a leaching technique, found thatconcentrations of solution K up to 1.0 mmol L"1 didnot retard K release from trioctahedral mica whereasconcentrations above 0.10 mmol L~" inhibited K re-lease from muscovites.

The total amount of nonexchangeable K releasedfrom the 0.45- to 0.60-m depth of the two soils to thehomoionic H-resin is presented in Fig. 1. Equilibriumin K release was attained in about 40 d for the twosoils. Although not shown, a similar trend was ob-served in the other depths of the two soils. With theexception of the two lowest depths of the Kennansvillesoil, more nonexchangeable K was released at 40 d

886 SOIL SCI. SOC. AM. J., VOL. 47, 1983

(Kp) from the Kalmia soil than from the Kennansvillesoil (Table 2). This would be expected since the Kal-mia soil profile generally contained more clay than theKennansville soil profile (Table 1). The higher levelof nonexchangeable K released from the 0.60- to 0.75-m depth of the Kennansville soil can be ascribed tothe large quantity of vermiculite present (Table 1)which is a major source of nonexchangeable K (Sparks,1980).

Cation exchange resins of various saturations havebeen employed to investigate K release from soil clayminerals. Calcium- and Na-saturated resins were foundto be unsatisfactory when used with any minerals morestable than trioctahedral micas (Arnold, 1958; Stahl-berg, 1959; Haagsma and Miller, 1963; Feigenbaumet al., 1981). Arnold (1958) found muscovite and hy-drous mica were comparatively resistant to H-resinattack. Haagsma and Miller (1963) showed that littleacid decomposition of soil minerals took place abovepH 2.5 in a soil-resin mixture. However, Talibudeenet al. (1978) argued that H-saturated resin may be de-structive to soil minerals and they recommended Ca-saturated resin. In this study we found that the H-saturated resin did not appear to cause destruction ofthe soil minerals. The < 2-pm clay fractions of thesoil samples after a 40-d equilibration with the H-sat-urated resin were compared to those without H-resintreatment using x-ray diffraction (Fig. 2). There wasno alteration of the mineral suite after the 40-d equi-libration with H-resin. This is also reflected in the K0values. In each of the depths of the two soils exceptfor the 0.45- to 0.60-m depth of the Kalmia soil, lessK was removed at 40 d with the resin than with theHNO3 extraction which is an index for nonexchange-able K. The higher values for the HNO3-extractableK would suggest some extraction of mineral K as sug-gested by Barber and Matthews (1962).

Kinetic Equations to Describe Nonexchangeable-KRelease

Mathematical equations including the Elovich, par-abolic diffusion, first-order diffusion, and zero-orderwere tested by least square regression analysis for non-exchangeable-K release from the Kalmia and Ken-nansville soils (Table 3) to determine which equationbest described the data. The correlation coefficient (r)

TIME, h100 200 300 400 500 600

o KALMIA SANDY LOAM

• KENNANSVILLE LOAMY SAND

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

-0.8 I-

Fig. 3— First-order kinetics of nonexchangeable-K release from the0.45- to 0.60-m depth of Kalmia and Kennansville soils.

and the standard error of the estimate (SE) were cal-culated for each equation. The standard error of theestimate is defined as

SE = - Kt*)*/(n-2)]W , [3]where Kt and Kt* are the measured and calculated con-centrations of K released at time t and n is the numberof measurements.

The first-order diffusion equation was the best ofthe various kinetic equations studied to describe thereaction rates of K release from the two soils, as evi-denced by the highest value of r and the lowest valueof SE (Table 3). The parabolic diffusion law also de-scribed the data satisfactorily indicating diffusion-con-trolled exchange. This was also found in pure mineralsby others (Mortland and Ellis, 1959; Mortland, 1961;Reed and Scott, 1962; Scott, 1968; Feigenbaum et al.,1981). The relationship showing the good fit of thedata for the 0.45- to 0.60-m depth of the two soils tothe first-order equation is shown in Figure 3. The zero-order equation is not suitable to describe the kinetic

Table 3—Correlation coefficient (r) and standard error ofestimate (SE) of various kinetic equations fornonexchangeable-K release from the Kalmia

and Kennansville soils, t

Kalmiasandy loam

Equation

1. Elovich:Kt = a + b In t

2. Parabolic diffusion law:Kt/K0 = a + bt>"

3. First-order diffusion:\n(K0-Kt) = a-bt

4. Zero order:(K0-Kt) = a-bt

SEJ x 10-4

3.30

5.49

1.35

9.71

r

0.812

0.980

-0.990

-0.985

Kennansvilleloamy sand

SEt

2.30

1.26

1.40

6.63

r

0.871

0.984

-0.986

-0.977

t The r and SE values represent averages for the six depths of each soil.j SE is in mol kg-'.

Table 4—First-order nonexchangeable-K release ratecoefficients (k?) of Kaimia and Kennansville soils.

Depth k, x 10-]

Kalmia sandy loam0 -0.150.15-0.300.30-0.450.45-0.600.60-0.750.75-0.90

Kennansville loamy sand0 -0.150.15-0.300.30-0.450.45-0.600.60-0.750.75-0.90

h-

1.91.92.11.51.82.2

1.81.61.72.32.92.5

MARTIN & SPARKS: KINETICS OF NONEXCHANOEABLE K RELEASE FROM TWO COASTAL PLAIN SOILS 887

data as can be seen from the large values of SE, despitethe fact that the values of r are quite high (Table 3).The Elovich equation satisfactorily described the rateof K exchange between solution and exchangeablephases in soils (Sparks et al., 1980b) and the kineticsof P release and sorption in soils (Chien and Clayton,1980). However, it did not satisfactorily describe thekinetics of nonexchangeable-K release from the soilswe studied as evidenced from the low r values andhigh SE values (Table 3).

Nonexchangeable-K release rate coefficients (k2)were calculated for all depths of the two soils (Table4) using the first-order equation since it best describedthe data. The k2 values ranged from 1.2 to 2.2 X 10~3

hr1 in the Kalmia soil and from 1.5 to 2.9 X 10~3

h~' in the Kennansville soil. The magnitude of the k2values suggests low rates of nonexchangeable-K re-lease from the two soils. The magnitude of the k2 val-ues differed little between depths in the two soils aswould be expected from the similar clay mineral suitesand clay content (Table 1).

ACKNOWLEDGMENTSThe authors wish to express their appreciation to the Po-

tash and Phosphate Institute and to the University of Del-aware Research Foundation for partial funding of this re-search.