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Received 15 October 2008Received in revised form 5 March 2009Accepted 24 March 2009Available online 2 May 2009

Keywords:PotassiumClay minerals

Geoderma 151 (2009) 109120

Contents lists available at ScienceDirect

Geode

l se1. Introduction

Potassium (K) is a macro element in plant, animal and humannutrition. Understanding the mechanisms that involve release andxation of K in soil is important because soils may contain widelyvariable pools of K that are potentially mobilized by chemicalweathering of soil minerals. The K issue is especially interesting inthe context of organic farming and other production systems where arestricted input of mineral K fertilizers may limit long-term soilfertility (born et al., 2005). Predicting K release from natural sources

and phyllosilicates is the long-term natural source of K (e.g., Bertschand Thomas, 1985; Malavolta, 1985; McLean and Watson, 1985).Release of xed or structural K from phyllosilicates is the dominatingmechanism of K release in moderately weathered agricultural soils(Sparks and Huang, 1985). The reaction may proceed via a slowdissolution of the entire mineral structure, and via selective exchangeof interlayer K (Huang, 2005). The latter takes place when the Kconcentration in the soil solution is inferior to a mineral-speciccritical value (Scott and Smith, 1966; Fanning et al., 1989).

The release of K from clay minerals is inuenced by particle size

is also crucial when attempting to avoidthe diet of cattle where grazing and roughcomponents (Whitehead, 2000). In general, tK in soils corresponds to crop demand durintense cropping and the release of K from w

Corresponding author.E-mail address: magnus.simonsson@mark.slu.se (M.

0016-7061/$ see front matter 2009 Elsevier B.V. Adoi:10.1016/j.geoderma.2009.03.018 2009 Elsevier B.V. All rights reserved.Non-exchangeableFixationReleaseX-ray diffractionXRDWe investigated to what extent soil K balances, resulting from plant off-take and K fertilizer application rates,had changed the content of illitic (10 ) materials (as discrete illite and as illitic layers in mixed-layerminerals) in soil clay fractions. We also assessed to what extent different particle size fractions were involvedin the processes. The study was performed on soil samples from ve long-term agricultural experimentslocated on Mollisols and Inceptisols in South and Central Sweden, each having a range of K fertilizerapplication rates. We analysed X-ray diffraction (XRD) patterns from parallel orientated samples of thefollowing particle size fractions: total clay (b2 m), ne clay (b0.2 m), and (at two of the sites) nemedium silt (220 m). In addition, K xation capacity in the total ne earth (b2 mm) was measured inorder to assess changes in the number of xation sites due to previous release or xation of K in the eld. Atall sites but one, K management had measurably affected the content of the illitic components in the totalclay fraction, as expressed by an illite layer ratio. Although the content of illitic materials in phyllosilicatesdiffered between particle size fractions, they were all altered to a similar extent by the soil K balance; i.e., thenet off-take or input of K in the different fertilizer treatments. Soil K balances also affected the K xationcapacity of the soil. Overall, 86 to 4030% of accumulated net inputs or outputs of K in the eld wererecovered as a change in K xation capacity. This means that K release was reversible by K fertilization to ahighly variable extent. The release of K appeared reversible in (i) two silty clays and (ii) a sandy loamdeveloped in a parent material characterized by the occurrence of calcite. Two other soils showed evidenceof irreversible loss of interlayer K from 2:1-minerals. XRD patterns and the slightly lower pH at the lattersites suggest that the occurrence of hydroxyaluminium interlayers in 2:1-minerals may have hamperedK xation in these soils. To generalize, a reversible nature of K release and xation may be promoted if easilyweatherable soil minerals are abundant.Article history:a r t i c l e i n f o a b s t r a c tChanges in clay minerals and potassiumxation of potassium in long-term eld e

Magnus Simonsson a,, Stephen Hillier b, Ingrid bora Department of Soil and Environment, Swedish University of Agricultural Sciences (SLU),b The Macaulay Institute, Craigiebuckler, Aberdeen, AB15 8QH, United Kingdomc Department of Crop Production Ecology, SLU, Box 7043, SE-750 07 Uppsala, Sweden

j ourna l homepage: www.enutrient imbalances inage are important feedhe pool of exchangeableing only a few years ofeathering of K feldspars

Simonsson).

ll rights reserved.ation capacity as a result of release anderiments,c

7014, SE-750 07 UPPSALA, Sweden

rma

v ie r.com/ locate /geodermaand chemical composition (Huang, 2005). It is generally accepted thattrioctahedral micas, such as biotite and phlogopite, release K morereadily than dioctahedral ones, such as muscovite (Fanning et al.,1989), although the completeness to which this occurs depends onexperimental conditions (cf. Scott and Smith, 1966; Feigenbaum et al.,1981). As for particle size, it is well established that the nest illiteparticles may undergo so called layer weathering implying a rapidinitial release, which, however, results in mixed-layer clays that

strongly retain the remaining K; on the other hand, larger particlesmay release K through edge weathering, albeit slowly, to eventuallybecome completely vermiculitized. Different K release patterns arediscussed in, e.g., McBride (1994) and Huang (2005).

As for Kxation, themagnitude and location of negative layer chargein 2:1-minerals have a large inuence (Huang, 2005). Thus, negativecharge located in the tetrahedral sheet is believed to promote xationmore readily thanoctahedralnegative chargedoes (Bouabidet al.,1991).Location of charge and the resulting xation capacity of more or lessaltered di- and trioctahedral clay minerals were reviewed by Feng et al.(2003). Other processes thatmayaffect release andxation of interlayerK are redox processes and the formation of hydroxy interlayers. Redoxprocessesmay change layer charge, and hence the readiness bywhich Kxation occurs; e.g., a reduction of structural Fe3+ to Fe2+ promotesxation of interlayer cations (Chen et al., 1987; Stucki et al., 2002;Komadel et al., 2006). Fixation of K is inhibited if hydroxyaluminiuminterlayers are formed in the mineral (Saha and Inoue, 1998; Aide et al.,1999). In vermiculites and smectites of moderately acidic conditions,such interlayers may form and dissolve independently of the rest of themineral (Rich, 1968; Barnhisel and Bertsch, 1989). In an experiment ofHinsinger et al. (1993), plant-induced K release from phlogopite andconcomitant weathering of the 2:1-layers transformed the mineral

Another method to estimate K release is to measure, understandardized conditions, the change in K xation capacity that resultsfromK inputs and outputs in the soil (gaard andKrogstad, 2005). Thisis based on the premise that K release results in vacant sites for Kxation, and that these sitesmaybe quantied by putting K back underexperimental conditions. It is assumed that xation of NH4+, formationof hydroxyaluminium interlayers, and any congruent dissolution ofvermiculitizedmineral residues are all negligible. If this is not true, theK release that has taken place will be underestimated, because thexation accomplished in the laboratory will fail to restore the pool ofinterlayer K in illitic minerals to its pre-weathering level.

Inve long-term (ca 40 years) agricultural experimentswith a rangeof K fertilizer application rates, we have previously analysed temporaltrends in crop K concentrations and in soil K pools (Andersson et al.,2007). Fixation and release rates from mass balance calculations werealso established (Simonsson et al., 2007). In the latter paper, we foundthat K release and xation in the eld was largely a function of soilK balance. Potassium was released from non-exchangeable pools intreatments with a negative soil K balance; i.e., when harvest andleachingexceededapplicationswith fertilizer andmanure. Conversely, Kdisappeared,presumably into axedpool,where the soil K balancewaspositive. We also found that K extracted with 2 M HCl at 100 C (Egnr

HH2O

.17.38.36.46.67.88.16.76.16.15

(190 c) al. (2

AO-FAO198

110 M. Simonsson et al. / Geoderma 151 (2009) 109120directly into hydroxyaluminium interlayered vermiculite. Fixation isenhancedby increases inpH, probablyas an indirect result of a reductionin hydroxyaluminium interlayering of clay minerals (Huang, 2005).In a study by Wiklander and Koutler-Andersson (1959), the K xationcapacity of the soil increased slightly as a result of liming.

Several methods exist to estimate the release of non-exchangeablesoil K. By using X-ray diffraction (XRD) patterns, changes in the 10 component of clay minerals resulting from positive or negative soil Kbalances have been noticed (van der Marel, 1954; Mberg and Nilsen,1983; Liu et al.,1997). The changes observed are often subtle and hencedifcult to measure. This has prompted some authors (e.g., Velde andPeck, 2002; Barr et al., 2007) to use XRD peak decompositionmethods as a means to detect the changes, while more recently Barret al. (2007) used changes in the centre of gravity of a dened area ofthe diffraction patterns as a convienient means to represent subtlechanges that are otherwise difcult to express. Other studies havereported unchanged XRD patterns in the topsoil (Singh and Goulding,1997) and only little changed supplies of exchangeable K (Blake et al.,1999) from suites of eld experimental plots that experienced verydifferent Kmanagement over long periods of time. Extensive uptake ofK from the subsoil was one possible reason given by the authors.

Table 1Basic information on soils.

Site (abbreviationused in text)

Coordinates Classication Texture Depth(cm)

p(

Fjrdingslv (Fj) 5524N; Oxyaquic Hapludollf Sandy loam 025 61314E Haplic Phaeozemf (till) 70110 8

Ekebo (Eke) 5559N; Oxyaquic Hapludollf Sandy loam 025 51252E Haplic Phaeozemf (till) 70110 5

Vreta Kloster (Vre) 5829N; Oxyaquic Haplocryollg Silty clay 025 61530E Haplic Phaeozemg (sorted) 70110 7

Hgsa (Hg) 5830N; Humic Dystrocryeptg Loamy sand 025 61527E Typic Haplocryollg (sorted) 70110 5

Kungsngen (Kun) 5929N; Typic Haplaquepth Silty clay 025 61530E Gleyic Cambisolh (sorted) 70110 5

a Minmax ranges of pH encountered in samples of treatments A and D.b Cation exchange capacity at pH 8.2 of ne earth according to Polemio and Rhoadesc Clay content according to the pipette method performed on the 510 cm and 7011d Concentration of phyllosilicate K in total ne earth (b2 mm) in the topsoil (025 cme Phyllosilicates minerals (weight %, topsoil ne earth) according to Andrist-Rangel et a

minerals; Trioct is trioctahedral phyllosilicates; n.d. means not detected.f Classication of Kirchmann et al. (1999), according to Soil Survey Staff (1998) and Fg Classication of Kirchmann et al. (2005), according to Soil Survey Staff (1998) andh Classication of Kirchmann (1991), according to Soil Survey Staff (1987) and FAO (recent edition (Soil Survey Staff, 2006).et al., 1960), a method commonly used to determine long-term K statusof Nordic soils, was sensitive to positive and negative soil K balances.This pool was demonstrated by Andrist-Rangel et al. (2006) to consistalmost entirely of K in phyllosilicates.

In this paper, we pursued the study of the same sites by focussingon properties of the clayminerals. The overall aimwas to investigate ifK management, in terms of fertilizer application rates and plant off-take, had any detectable effects on soil clay mineral composition andstructure. The specic objectives were to determine (i) to what extent,if at all, effects of soil K balances induced by K fertilizer applicationrates were visible in the properties of soil clay minerals, and (ii) towhat extent different particle size fractions were affected by inputsand outputs of K due to soil K fertilizing regimes. We used XRD andmeasurements of K xation capacity to investigate this.

2. Materials and methods

2.1. Soil materials

Soil was retrieved from ve long-term soil fertility experimentsestablished in 19571966 in South and Central Sweden (Carlgren and

)aCECpH8.2(cmolc kg1)b

Clay(% of b2 mm fraction)c

Phyllo-K(g kg1)d

Illiticdiocte

M-Ldiocte

Triocte

.5 15 18 7.3 82 52 62.5 7 16 9.0 102 62 82.1 14 14 4.4 42 82 31.5 8 17 6.6 42 143 52.0 25 46 12 132 102 132.2 22 59 14 143 153 213.6 9 7 3.2 42 11 n.d..3 3 2 4.4 62 n.d. n.d..5 24 52 17 223 42 183.6 19 55 22 293 31 163

77).m samples, respectively.ccording to Andrist-Rangel et al. (2006).006): Illitic dioct is illitic dioctahedral minerals; M-L dioct is mixed-layer dioctahedral

ISRIC-ISSS (1998), respectively.-ISRIC-ISSS (1998), respectively.8), respectively; Soil Taxonomy classication would be Aeric Endoaquept in the most

K fertilizer treatments were evaluated using samples subjected to aminimum of pre-treatment; i.e., those that had not been treated withTiron. These subtle differences were most apparent in the diffractionpatterns of EG-treated specimens, where we could observe smallchanges in peak area of the discrete illite peak at 10 and correlativechanges in the prole and area of peaks due to expandable minerals athigher spacings. To express these changes as a single value, wecalculated a peak area ratio as the EG-10 peak divided by the sum ofthe area of the EG-10 peak plus the peak area in the N10 range (1416 ) of collapsible clay minerals (Fig. 1). This ratio thus expresses thediffracted intensity at 10 (i.e., of illite), relative to the total diffractedintensity from 2:1 and 2:1:1 clay minerals. Since it considers onlyminerals with a basal separation distance of 10 or greater, it is notaffected by the occurrences of other minerals; e.g., kaolinite or non-phyllosilicates in the clay fraction. The ratio is hereafter referred to asthe illite layer ratio. The illite layer ratio will increase upon

111M. Simonsson et al. / Geoderma 151 (2009) 109120Mattsson, 2001) (Table 1). The experiments were designed to studyfactors inuencing long-term soil fertility including site specicfactorssoil parent material, climate, topographic and hydrologicalconditionsand management practices such as crop rotation, recy-cling of plant residues and use of organic and mineral fertilizers. Thesites were selected to cover a range of soil parent materials and soiltypes representing both rich and poor inherent soil fertility. The soilsare all developed in Quaternary deposits and represent awide texturalspan from loamy sand to clay. Their mineralogy, geochemistry and soilK reserves were investigated by Andrist-Rangel et al. (2006); see alsodata in Table 1.

The sites at Fj and Eke are located on glacial tills with a texture ofsandy loam (Table 1). The till in the Fj area is characterised byCretaceous rocks and contains various amounts of chalk (Daniel,1977); igneous rock fragments are also commonly present in the area(Westergrd, 1912). Although the till is often decalcied in the uppermetre (Daniel, 1977), the prole at Fj contains calcite at all depthsbelow 25 cm. Eke is located on a till characterised by its content ofpoorly sorted clay- and sandstones from rocks of Upper Triassicand Lower Jurassic age (Adrielsson et al., 1981); the soil at the Ekesite is non-calcareous (Kirchmann et al., 1999; Andrist-Rangel et al.,2006). Vre and Hg are located in an area underlain by Ordovicianlimestone. However, the soil parent material has a high content ofdebris derived from a vast granitic bedrock area 23 km north of thesites (Fromm, 1976), and calcite occurs only sporadically at these sitesin concentrations less than 12% (cf. Kirchmann et al., 2005; Andrist-Rangel et al., 2006). Vre is developed in a deposit of ca 50 cm ofpostglacial clay on top of a heavy glacial clay; Hg is located on aglaciouvial sand deposit (Kirchmann et al., 2005). The bedrock in theKun area is dominated by Precambrian gneissic granites and felsicmetavolcanic rocks (Mller, 1993). The Quaternary deposits overlyingthese are highly heterogeneous and include coarse to ne-grainedglaciouvial deposits, and postglacial sand and clay. The Kun site islocated on the youngest of these sediments, which is a lacustrinepostglacial gyttja clay (Mller, 1993), to some extent inuenced by ashallow groundwater table.

In 2002 and 2003 we sampled soil in the depth intervals 05, 510,1025, 2540, 4070, and 70110 cm. Samples were air-dried (~30 C)and sieved (2 mm) before analysis. During the experimental periodthe studied soils were subjected to a wide range of K fertilizerapplication rates, referred to as K fertilizer treatments A, B, C, and D.These ranged from no addition of mineral fertilizer, in treatment A, toan addition corresponding to crop off-take plus 80 kg ha1 year1 intreatment D. Treatment B wasmanaged to remain close to a zero soil Kbalance, with K inputs approximately equalling the outputs over time,and C was an intermediate of B and D. At all experimental sites, thesampled parcels are arranged in blocks (2 replicates) in a splitsplitplot design with crop rotation systems on main plots, P and Kapplication rates on subplots, and N application rates on sub-subplots(Carlgren and Mattsson, 2001). Our sampling was carried out in thesub-subplots of maximum N application.

Soil K balances, dened in Eq. (1), were estimated by Simonssonet al. (2007) for the entire experimental period (kg K ha1):

Soil K balance = Kfertilizer + Kmanure + Kdeposition Kharvest Kleaching1

2.2. X-ray diffraction of orientated samples

Air-dried and sieved soil samples (mostly the 510 cm samples)were dispersed in de-ionized water 2 min with an ultrasonic probe(600 W, Amp 100%, 25 mm probe). Stokes-law fractions ofclay (b2 m), and in samples from Fj and Kun nemedium silt(220 m), were isolated by sedimentation in ambient eld of gravity;

ne clay (b0.2 m) by centrifugation. One set of clay samples from theA and D treatments was cleansed of amorphous and poorly crystallineminerals and of organicmatter using the Tironmethod of Kodama andRoss (1991), which also results in Na saturation of the cation exchangecomplex. The remaining fractions and another set of clay sampleswere examined untreated.

Orientated samples were mounted on glass slides according tothe Millipore lter transfer method (Moore and Reynolds, 1997) forne clay and clay fractions, and by evaporation of a few ml ofpreconcentrated (by centrifugation) suspension for the nemediumsilt fraction. Each slide was subjected to air-drying (AD), ethyleneglycol solvation (EG), and heating to 300 C for 1 h (H300); after eachof these steps, the XRD pattern was recorded from 2=2 to 45with a Siemens D5000 diffractometer using Co K radiation and adiffracted beam monochromator. The scanning was performed in0.02 steps, counting for 1 s per step.

Based on data from Tiron treated samples, semi-quantitative site-average (K fertilizer treatments A and D, each from two blocks)composition of phyllosilicates in the clay (b2 m) fraction wasassessed using peak areas of basal reexions (00l) and the referenceintensity ratios (RIR) of Table 2 calculated as described in Moore andReynolds (1997) and Hillier (2003) with instrument parametersappropriate for the Siemens D5000. Phyllosilicates were operationallydivided into kaolinite, chlorite (assuming an equal numbers of atomsof Fe and Mg in octahedral sites of the mineral), illite, and collapsibleclayminerals.Discrete illite wasmeasured as the peak area of the 10 peak that remained in this position following the shift of expandableclay minerals to higher spacings upon glycolation. Collapsibleclay minerals were calculated from the peak area difference at 10 between the EG and H300 patterns, and therefore included smectites,vermiculites and all other mixed-layer clays that collapse to thisposition following heating.

The subtle differences in 2:1 clay mineral composition between

Table 2Reference intensity ratios (RIR) used in semi-quantitative evaluation of claymineralogy; the sum of the clay minerals in the table was set to 100%.

Mineral Sample treatmenta Peak order d () RIR

Kaolinite EG 001 7.10 2.45EG 002 3.58 1.70EG 003 2.38 0.12

Chlorite (FeMg) EG 001 14.20 0.59EG 002 7.10 3.32EG 003 4.72 0.81EG 004 3.52 1.77

Illite EG 001 10.00 1.00Illite + collapsibleb H300 001 10.00 1.00

a EG = ethylene glycolated, H300 = heated to 300 C for 1 h.b Sum of minerals with d(001)=10 in H300 pattern, including original illites, as

well as N10 minerals that collapsed to 10 upon heating (smectites, vermiculites andtheir intergrades with illite).incorporation of K in clay minerals, and decrease upon K depletion.

2.4. Evaluating potassium xation capacity

The assessment of K xation capacity was a rather complexprocedure, and various uncertainties in the measurements had to beconsidered when interpreting the results. For this reason the

Fig. 2. Example graphs showing the evaluation of K xation capacity in Fjrdingslv A,510 cm. (a) Mass balance for the subsample with 5 cmolc kg1 addition of KCl.(b) Range of subsamples showing the relationship between the single point method(exemplied with the same numbers as in a) and the plateau method. Error barsrepresent combined standard uncertainties multiplied with a coverage factor of 2, toyield an expanded uncertainty with a level of condence of approximately 95%.

112 M. Simonsson et al. / Geoderma 151 (2009) 109120Formation of 10 domains in clay minerals, which diffract as discreteillite and contribute to the peak at 10 , will increase the ratio byincreasing its numerator. Incorporation of K as illite layers in mixed-layer clays, on the other hand, will also increase the ratio, although bydecreasing the denominator. The latter is due to the fact that relativelymore illitic mixed-layer clay minerals, of any character, will haverelatively less diffracted intensity in the N10 region compared totheir more expandable, less illitic, counterparts (Moore and Reynolds,1997). The illite layer ratio therefore reects changes in both theamount of discrete illite in the sample and in the amount of illiticlayers in mixed-layer minerals.

Care was taken to ensure that the intensity ratio was measuredconsistently with respect to selection of the background. Mostsamples contained chlorite, which will also have an effect on theabsolute value of the ratio. However, the amounts of chlorite weresmall and constant for a given experimental site, such that it will haveno inuence on the comparison of different fertilizer treatments fromthe same site. Before calculating the ratio, the intensity at N10 wasdivided by a factor of 5. This was not an essential step, but was done inan attempt to put the measured intensities on a common scale.Otherwise, expandable clay minerals usually give peak intensities upto ve times the intensity of an equal weight of illite, although mixed-layeringwith illite layers tends to reduce the difference (Hillier, 2003).

2.3. Potassium xation capacity

Fig. 1. Example graph showing how the illite layer ratio was calculated from anethylene glycol treated XRD pattern. The peak area ratio was evaluated as B/(A+B).Incorporation of K in clay minerals will increase the ratio, depletion will decrease it.Dry K xation capacity was determined in samples from the depths05, 510,1025, and 2540 cm at Fj; 510 and 1025 cm at Hg; andonly 510 cm at Eke, Vre, and Kun. For each soil sample, at least vesubsamples were subjected to progressive additions of K. Therationale is that the amount of K xed is a positive function of themagnitude of K addition, until such a point where the soil is saturatedand the xation stabilizes at a level that represents the soil's K xationcapacity.

Subsamples (5.0 g) were put into plastic 50-ml centrifuge tubesand received de-ionized water +1 M KCl (total volume, 6 ml); Kadditions ranged from zero to an amount corresponding to 12 timesthe CECpH8.2. Tubes with soil and solution were shaken overnight(16 h) and then dried at 105 C for 24 h. All subsamples underwent twoadditional wettingdrying cycles, which each involved shakingwith 5 ml de-ionized water during 6 h, and drying at 105 C for 16 h.Finally, non-xed K (Knon-xed; i.e., exchangeable and free salt K) wasdetermined by repeated extractions in 1 M NH4Ac pH 7.0 (440 ml)and analysis of the extractswith ame atomic absorption spectroscopy(AAS) following centrifugation (2000 rpm10 min) and ltrationthrough 00H lter paper.procedure used to evaluate uncertainty is described in some detail.For individual subsamples with KCl added to it, xation of added

Table 3Site average semi-quantitative composition of phyllosilicates in Tiron treated orientatedclay (b2 m) samples.

Site Depth (cm) Kaolinite Chlorite Illite Collapsibleclay minerals

Fj 510 41a 11 2010 75970110 31 00 249 7210

Eke 510 168 83 185 581170110 144 00 212 655

Vre 510 31 21 449 51970110 42 21 4814 4616

Hg 510 44 105 176 691070110 52 64 3617 5421

Kun 510 21 42 6013 331270110 20 30 6817 2717

a Uncertainty limits (t95%standard error of 4 individual samples from differentblocks or treatments in the eld) reect only variability in the data, not the accuracy ofthe method.

K (Kxed, cmolc kg1 of ne earth) was calculated according to thefollowing mass balance; Eq. (2):

Knonfixed; before + Kadded = Knonfixed; after + Kfixed 2

The mass balance is illustrated in Fig. 2a. Before and after refer tothe addition of K, and the drying and wetting procedure leading toxation of K. Kadded is concentrationvolume of the KCl solutionadded to the subsample in question. In practice, conditions before

and after were measured in different subsamples, without and withK added, respectively. Rearrangement of Eq. (2) yields:

Kfixed = Kadded Knonfixed; after

+ Knonfixed; before 3

In Fig. 2b, the difference (KaddedKnon-xed) is shown for a range ofsubsamples with different K additions, and for one with zero additionof K. For the latter, the y variable is simply Knon-xed, before, so thexation according to Eq. (3) in a subsample with K added equals the

K fe

113M. Simonsson et al. / Geoderma 151 (2009) 109120Fig. 3. X-ray diffraction patterns of clay (b2 m) fractions from topsoil (510 cm) within

ethylene glycol solvated (EG), and heated to 300 C for 1 h.rtilizer treatment A. Orientated samples of suspensions treated with Tiron; air-dry (AD)

K (e.g.; in Fj, the one at 5 cmolc kg1); this methodwill be referred toas the single point method.

In practice, the plateau method is more likely to yield an unbiasedestimate of K xation capacity. Therefore, we used it wheninvestigating differences between the sites. The single point methodwas, however, more precise (in terms of MSE; see next paragraph),and was used when calculating differences in K xation capacityresulting from the fertilizer treatments within a site. Although a singlepoint may underestimate the maximum K xation capacity in a singlesoil (Fig. 2b), the comparison of soil samples from different fertilizertreatments is valid, if, as was, the K addition is standardized and largecompared to the changes in non-exchangeable K that has taken placein the eld. In each case, the K addition selected for the single pointmethod was 25 times greater than the span of K xation capacitiesseen among treatments AD. A similar single point approach has beenused previously, e.g., by gaard and Krogstad (2005).

114 M. Simonsson et al. / Geoderma 151 (2009) 109120Fig. 4.Diffraction patterns of clay (b2 m) fractions from topsoil (510 cm). Comparisonvertical distance between its point in the graph and the point ofthe subsample with no K added. Note that in the subsample with noK added, K xation is zero by denition. Therefore, any xation, orrelease, of K caused in this particular subsample by the wetting anddrying cycle, or by the extraction following it, will go unaccounted for.The xation measured is only the one caused by the addition of K.

For the judicious selection of points representing the plateau of Ksaturation in Fig. 2b, measurement uncertainties (vertical bars) wereestimated according to the guidelines of ISO (1995), also available at theEurachem/CITAC Internet site (http://www.measurementuncertainty.org/mu/guide/index.html; accessed 2009-03-05). Considered sourcesof uncertainty in Kxed (cmolc kg1) includedvolumeand concentrationof the added 1 M KCl solution; volume of NH4Ac extracts and measuredconcentrations of K in them; uncertainty in subsample mass; andpossible heterogeneity between subsamples from the same container(critical when assuming that the zero-addition subsample representsthe same soil as the subsamples with K added). The latter was (over)estimated by comparing Knon-xed, before of the two blocks within each Kfertilizer treatment.

Two approaches were used to evaluate K xation capacity. The rstapproach, whereby the plateau of K saturation (upper dashedhorizontal line in Fig. 2b) was considered, will be referred to as theplateau method. For instance, to calculate K xation capacity in theFj samples, we used the average (KaddedKnon-xed) for K additionsof 1020 cmolc kg1. Uncertainties in (KaddedKnon-xed) generallyincreased with increasing K additions (as illustrated by error bars inFig. 2b). The reason is that uncertainties in Kadded and in Kmeasured inthe extracts (Knon-xed) were proportional. As a result of this, the largerthe additions of K, the more (KaddedKnon-xed) suffered from being asmall difference between two large numbers. Therefore, we alsoevaluated K xation capacity using only one of the smaller additions of

of orientated, ethylene glycol (EG) solvated samples from fertilizer treatments A (nofertilizer K) and D (maximum fertilizer K). Fertilizer applications increased the reectedintensity at 10 and decreased it at N10 . Samples were not treated with Tiron.Once K xation capacity (cmolc kg1) had been assessed forindividual eld plots, a pooled mean standard error (MSE), with 4degrees of freedom, was calculated for each site in the following way:rst, variances were calculated between the two blocks within eachtreatment (AD). Then, the square-root of the average of the fourvariances was divided by 2 (=4). Finally, K xation capacity wasconverted to kg ha1 values using previously published dry bulkdensities (Kirchmann, 1991; Kirchmann et al., 1999, 2005) togetherwith ne-earth (b2mm) percentages from our samplings in 2002 and2003 (ranging from 90 to 100%). In cases where only one layer (510 cm) within the topsoil had been analysed, the result wasgeneralized to the entire topsoil (ca 25 cm), which was presumablyhomogenized by soil tillage. The effect of this generalization wasfound negligible in Fj and Hg, where additional topsoil layers wereanalysed. In all cases, the uncertainty of topsoil mass was estimated bya comparison of repeated soil prole descriptions from the sites. Thesesuggested a relative standard error of Ap horizon depth that wasbetween 4 and 5%. Therefore, the MSE of K xation capacity wascombined with a relative standard uncertainty of 5%. The resultingcombined standard uncertainty in K xation capacity (kg ha1) wasmultiplied by a coverage factor of 2, which yields an expandeduncertainty with a level of condence of approximately 95%.

2.5. HCl extractable potassium

The ne-earth fraction (b2 mm) was analysed for exchangeable(Kex) and 2 M HCl extractable K (KHCl) following the procedures ofEgnr et al. (1960). The method for Kex involves extraction in 0.1 Mammonium lactate +0.4 M acetic acid (pH 3.75) at room temperature(2025 C). For KHCl, the soil is extracted by 2MHCl in a boiling water-bath (100 C) for 2 h. Extracts were analysed for K by ame AAS.Subtracting Kex from KHCl yielded a non-exchangeable fraction,KHCl corr, which is strongly correlated to K in phyllosilicate minerals

Table 4Illite layer ratio in XRD patterns from topsoil and subsoil total clay (b2 m), not treatedwith Tiron, from K fertilizer treatment A and D (n=2).

Site Treat Depth (cm) SEMa

510 70110

Fj A 0.26 0.41 0.06D 0.52 0.50 0.06

Eke A 0.36 0.66 0.02D 0.33 0.65 0.02

Vre A 0.69 0.78 0.07D 0.83 0.81 0.07

Hg A 0.28 0.74 0.08D 0.39 0.70 0.08

Kun A 0.87 0.94 0.01D 0.91 0.94 0.01

a SEM = standard error of the mean over two blocks.

(Andrist-Rangel et al., 2006), and typically constitutes approximately1030% of the total amount of K held in phyllosilicates in the soil(Sthlberg, 1958; Andrist-Rangel et al., 2006).

3. Results

3.1. Clay mineralogy

Semi-quantitative clay mineralogy (b2 m) is presented in Table 3and the pertaining XRD patterns are shown in Fig. 3. In the topsoil, thehighest contents of illitewere found at Vre and Kun. These soils also hadthe highest clay content (Table 1). Eke had a larger concentration ofkaolinite compared to the other sites, probably reecting a contributionof kaolinite from its Triassic and Jurassic parent materials. Smallamounts of chlorite occur in most samples. As for collapsible clays(Table 3), Kun was at the lower end, with a concentration clearly lessthan that of Fj, Eke and Hg. These features are no doubt largely

inherited from the parentmaterial, as indicated by the small differences(Table 3) between topsoil (510 cm) and deep subsoil (70110 cm).

As mentioned in the Methods section, the collapsible clay categoryrepresents an operationally dened group of clay minerals. Althoughsome features were common to all sites, there were some obviousbetween-site differences in the nature of the collapsible (andexpandable) clay minerals present. Commonly, peaks near 14 developed or sharpened following EG solvation and disappeared uponheating. These peaks represent vermiculite components and are mostreadily observed as discrete maxima in the Tiron treated samples(Fig. 3). In most samples there was also signicant diffracted intensityin the EG trace to both the low and the high angle side of the peak near14 , indicating the presence of more or less expandable mineralsincluding mixed-layer clays. Heating produced a sharp peak at 10 ,but often with a tail or hump at higher spacings. Peak and humprepresent, respectively, the effects of complete and partial collapse ofvermiculite andmixed-layer clays. As for between-site differences, the

ilt, 2lite l

115M. Simonsson et al. / Geoderma 151 (2009) 109120Fig. 5. Illite layer ratio in XRD patterns of different particle size fractions (nemedium sextractable K of total ne earth (b2 mm). All samples are from 5 to 10 cm depth. The il

lines), and the different particle size fractions appeared similarly associated with changes i20 m, FMS; total clay, b2 m, Clay; ne clay, b0.2 m, FC), as a function of 2 M HClayer ratio was signicantly correlated with HCl extractable K in most cases (regression

n this K pool. Note different scale on the x-axis for the different sites.

mixed-layer clays appeared somewhat more expandable in Vre, Kunand particularly in Fj than in the other soils. In both Vre and Kunthere is also evidence of 12 spacings in the EG patterns, especiallyevident in the Tiron treated samples (Fig. 3). This spacing usuallyindicates ordered mica/vermiculite mixed-layers. In Eke and Hg, onthe other hand, the vermiculitic character is dominant and there ismore evidence of resistance to heating, as indicated by an incompletecollapse upon heating.

As for other minerals, small amounts of quartz and feldspars areevident in the clay fractions of most samples, but particularly so in Vreand Kun, as indicated by the peaks, e.g., at 3133 2 (Fig. 3). In thelatter two, also amphibole was visible as a small peak between thelarger 10 and 7 peaks in Fig. 3.

3.2. Changes in XRD patterns

Fig. 4 showsXRDpatterns of topsoil total clay fraction (b2 m) fromK fertilizer application rates A (no K fertilizer) and D (maximumapplication of K fertilizer). Thematerials of Fig. 4were not treatedwith

Tiron. As a result, it is much more difcult to recognize the individualcomponents, or populations, of clays that contribute to the diffractionpatterns, presumably mainly as a result of poor basal orientationcompared to the fractions cleansed with Tiron. Note that diffractedintensity is very sensitive to orientation (Moore and Reynolds, 1997);the Tiron treated clay samples produced diffraction patterns withapproximately four times the number of counts per second, comparedto the untreated clays. The untreated clay fractions, however, werechosen for the study of the effects of K fertilizer application ratesbecause of the potential for the Tiron to affect the interlayer K studied.

Table 4 lists the illite layer ratio for the total clay fractions of thetopsoil (510 cm). According to an analysis of the variance, thereweresignicant overall effects of K fertilizer application rate at all sitesexcept Eke; the more K fertilizer applied, the larger the proportion ofdiscrete illite and/or illite layers in mixed-layer clays in the soilfractions. Although we elected to use a ratio as a measure of thechanges in the diffraction patterns between different treatments, insome samples these changes are clearly evident simply by visualinspection of the patterns (Fig. 4). Thus, both the discrete illite peaks

thilize

116 M. Simonsson et al. / Geoderma 151 (2009) 109120Fig. 6. Potassium xation capacity (cmolc kg1) as a function of depth determined withrepresent 95% condence intervals based on a site-wise analysis of the variance over fert

have been slightly off-set vertically, to make them distinguishable from each other.e plateau method. Sites differed markedly, depending on clay content of the soil. Barsr treatments (AD), two blocks (n=2), and all available depths. Markers at each depth

are seen to change in intensity, and the proles of the moreexpandable clays also show changes in intensity and shape. In deepsubsoil (70100 cm), on the other hand, there was no signicantoverall effect of K fertilizer application rate; i.e., there was no changein the illite layer ratio. Compared to deep subsoil, the topsoil total clayfraction was either similar (Fj, Vre, Kun), or appeared depleted in10 materials (Eke, Hg) (Table 4).

In Fig. 5, illite layer ratios of the different particle size fractionsare plotted against KHCl corr of the entire ne earth. An analysis ofthe slopes showed that all tested fractions varied signicantly withKHCl corr at Fj, Vre, and Hg, and, with the exception of FMS, also atKun (pb0.05). However, the responsiveness (the slope) did not showany signicant differences between fractions at any of the sites. Thevertical position of the lines in Fig. 5 probably reects the proportionof various clay minerals in the different size fractions. At Fj and Vre,the nest fractions had lowest illite layer ratio. At Eke, however, itwas the other way round, whereas at Hg and Kun the fractionsinvestigated were similar in this respect.

3.3. Potassium xation capacity and the effect of soil K balance

Overall, the K xation capacity measured by the plateau method(cmolc kg1) differed signicantly between the sites. In the topsoillayer 510 cm it followed the sequence KunNVreNFjNEkeHg,which is roughly concordant with the content of clay-sized particles(Table 1). An exception to this was Eke, which ranked lower thanexpected, regarding K xation. Overall, there were also signicant

effects of K fertilizer application rates, with a generally largerK xation capacity in the topsoil of unfertilized treatment A than intreatments C and D. Fig. 6 shows K xation capacity by site and depth.At Fj, where the prole was investigated to 40 cm depth, K xationcapacity was smaller in the topsoil (025 cm) than in the uppermostsubsoil (2540 cm).

At all sites,we also evaluatedKxation capacity (kg ha1) in theAphorizon against soil K balance in the past. We estimated changes in thetopsoil, and assumed that differences in K xation capacity due to Kfertilizer treatments were negligible in the subsoil. In Fig. 7, topsoilvalues of K xation capacity (kg ha1) calculated by the single pointmethod are plotted against soil K balance (as calculated in Simonssonet al., 2007). The slope (henceforth called x/bal) is of specialinterest, because it expresses the change in K xation capacity as apercentage of soil K balance. Fig. 7 presentsx/balwith uncertaintylimits at a level of condence of approximately 95%. The semi-quantitative interpretation of Fig. 7 must be that K xation capacitywas sensitive to soil K balance at Fj, whereas it behaved in a moreinert way at Eke and Hg. At Vre and Kun, soil K balances had onlymarginal effects on the very large K xation capacity; therefore, x/bal was highly uncertain there.

4. Discussion

In the present study, we investigated which particle size fractionsparticipated in the release and xation of K in a long-term agriculturalexperiment with varying K fertilizer application rates. We also

Dd lonta

117M. Simonsson et al. / Geoderma 151 (2009) 109120Fig. 7. Potassium xation capacity (kg ha1) of the different K fertilizer treatments (Aexperiment. Fixation capacity was highest where no fertilizer K had been applied (A), anindicates a large inuence of reversible xation and release on soil K dynamics. Horizo

coverage factor of 2, to yield an expanded uncertainty with a level of condence of approxi) plotted against their respective soil K balance (kg ha1) during ca 40 years of eldwest in the treatment with maximum K application rate (D). A strongly negative slopel and vertical error bars represent combined standard uncertainties multiplied with a

mately 95%.

118 M. Simonsson et al. / Geoderma 151 (2009) 109120measured the extent to which K release in the eld was reversible byK xation under laboratory conditions.

According to a recent investigation at the same experimental sites(Simonsson et al., 2007), the topsoil had developed a range of HClextractable K as a result of varying K fertilizer application rates duringthe several decades that the eld experiments had been running. Theslope of illite layer ratio against KHCl corr (Fig. 5) expresses therelationship between non-exchangeable K in the soil, and the relativeabundance of 10 materials in the different size fractions; either asdiscrete illite, or illite layers in mixed-layer clays. By plotting differentparticle size fractions separately, we intended to highlight anydifferences in reactivity regarding K release and xation between thedifferent particle size fractions. However, the slopes of differentfractions were similar, which indicates that the degree to whichphyllosilicateswere affected by K fertilizer application rates during theexperimental period (ca 40 years) did not differ signicantly betweensilt and clay size fractions from different sites. Hence, according toFig. 5, the fractions appear to have participated in K release andxationto a similar degree. Murashkina et al. (2007) investigated K xationcapacity of different particle size fractions in Californian soils derivedfrom granitic alluvium. They reported silt and sand fractions that wereactually more active than the clay fraction regarding K xation. Theircoarser fractions were generally richer in vermiculites and thereforeprone to x K, whereas the clay fractions were dominated by smectiteor mica, resulting in a lower capacity to x K. Among the soils of thepresent study, a high clay content seems to be associated with both ahigh K release potential (Andrist-Rangel et al., 2006; Simonsson et al.,2007) and a large K xation capacity (compare Fig. 6with Table 1). It isnevertheless noteworthy that the investigated fractions were similarlyreactive to differences in K application rates.

As to reversibility of K release, the soil proles at Vre and Kuncontain a large amount of clay and phyllosilicate K inherited from theparent material (Table 1), and were unquestionably able to deliver(Simonsson et al., 2007) and to x (Fig. 6) large amounts of K.Although illite layer ratios were signicantly affected by fertilizertreatments, any changes in K xation capacity were difcult toestablish, because xation capacities were several times the values ofthe negative and positive soil K balances (Fig. 7), and so changes in Kxation capacity were swamped in proportional measurementuncertainties. Consequently, x/bal was highly uncertain, with alower limit of the order 0.7. This means that changes in K xationcapacity could account for anything from zero up to 70% of the net Kinput or output to or from the soil. However, the high K xationcapacity seen in this study, and the moderate K leaching from thehighest K fertilizer application rates (Simonsson et al., 2007), suggesta substantial reversibility of K release. We note also that in the eldstudy of gaard and Krogstad (2005), the most clayey soils showedthe highest degree to which soil K outputs were reected by anincrease in K xation capacity (43%).

Compared with well-off Vre and Kun, the soil at Fj may becharacterized as poor but industrious; it appeared highly dynamicregarding K release and xation. Like Eke, it had an intermediate claycontent (Kirchmann et al., 1999), and an intermediate-sized pool ofphyllosilicate K (Andrist-Rangel et al., 2006). Regarding the illite layerratio, the nemedium silt fractionwas similar to that of Kun, whereasclay fractions were considerably poorer, at least when not fertilizedwith K (Fig. 5). However, the soil at Fj was able to deliver and x largeamounts of K according to Simonsson et al. (2007). This fact isconrmed by the results of the present investigation. Thus Fj reactedreadily in terms of both changes in illite layer ratios and K xationcapacity. The condence interval of x/bal suggests that reversiblerelease/xation of K accounted for 3060% of the soil K balances inthis soil. A circum-neutral pH, buffered by the presence of smallquantities of calcite at least in the subsoil (Kirchmann et al., 1999;Andrist-Rangel et al., 2006), is likely to prevent the formation of

hydroxyaluminium interlayers (Rich, 1968; Barnhisel and Bertsch,1989). This may allow K depleted phyllosilicates to remain availablefor xation of K.

The 1314 minerals at Eke and Hg were not readily expandableto higher spacings upon glycolation (Figs. 3 and 4). By contrast, theycollapsed, although somewhat heterogeneously, to lower spacingsupon heating (Fig. 3). Together these characteristics are sufcient toidentify an important vermiculite component in these soils, theheterogeneous collapse indicating an element of hydroxyinterlayer-ing. Both soils also had only a modest K xation capacity (Fig. 6),resulting in a substantial leaching of K from treatments that wereamply fertilized with K (Simonsson et al., 2007). The K xationcapacity at Eke was lower than expected from its clay content. Thex/bal indicated that only about 020% of the soil K balance wasrecovered as a change in K xation capacity. Thus, a large proportion(perhaps 80%) of any release of non-exchangeable K is probablyirreversible, and application of K fertilizer is hardly able to form 10 illitic layers to any great extent at these sites. Table 4 suggests thatwhilst fertilizer applications, before or during the 40 years of eldexperiment, may have kept or restored topsoil 10 materials close tosubsoil levels at the other sites, Eke and Hg may have had a longhistory of irremediable loss of K from 2:1-minerals in the topsoil.

Most marks of poverty encountered at Hg may simply beexplained by its coarse parent material, poor in clay minerals able todeliver K. Although illite layer ratios of clay fractions were similar tothose at Fj (Fig. 5), clay content was only about half as great. Itscapacity to release non-exchangeable K was small (Simonsson et al.,2007), and Hgwas poorest in phyllosilicate K among the investigatedsites (3.2 g kg1; Andrist-Rangel et al., 2006). The soil at Eke, on theother hand, was able to release adequate quantities of K to growingcrops (Simonsson et al., 2007). The recalcitrance to expansion andcollapse seen in Fig. 3 was even more evident comparing AD, H300(not shown) and EG diffraction traces (Fig. 4) from samples notcleansed from poorly crystalline components using Tiron. We mayspeculate that formation of hydroxyaluminium interlayers in 2:1-minerals, favoured by the lower pH in Eke (Rich, 1968; Barnhisel andBertsch, 1989), might explain the recalcitrant character of clayminerals and the poor reversibility of K release. General soil fertilityis lower at Eke than Fj (Kirchmann et al., 1999). It is also notable thatthis soil has a much higher kaolinite content compared to the othersoils, indicating a more advanced stage of alteration in the clayminerals forming the sedimentary rock fromwhich this soil is derived.

Finally, focussing on the parentmaterials and their inuence on thebehaviour of the studied soils with respect to K release and xation, itmay be argued that some general patterns have emerged. Thus Vre andKun are ne textured and rich in rst weathering cycle phyllosilicatesalong with other readily weatherable minerals, such as amphibole(Andrist-Rangel, 2008). For reasons discussed above, they are dynamicwith respect to both xation and release. Amphibole and otherweatherableminerals are commonly found in clay fractions of Swedishsoils developed in young clayey parent materials (e.g., born, 1991).The calcareous nature of the parent material of Fj also preserves adynamic character. A contrast to this are parent materials that havealready been through one ormore cycles of weathering, such as Eke, orsimply has a much lower content of less easily weatherable minerals,most especially if they are also coarser textured like Hg. In the presentstudy, they showed to be less dynamic. It may be expected, for soilsdeveloped on such parentmaterials, that release of K is less likely to berestored by xation following K fertilizer applications.

5. Conclusions

In spite of quite different clay content and clay mineralogy, not leastderived from the great variability among soil parent material, thesoils all responded in a similar way under different K management.Thus K fertilizer practices had markedly inuenced the soil mineral

composition of most soils. The content of illitic (10 ) materials in

119M. Simonsson et al. / Geoderma 151 (2009) 109120clay fractions, both as discrete illite and as illite layers inmixed-layerclays, was higher at high K application rates than at low rates.

A comparison of soils from different fertilizer treatments suggestedthat various particle size fractions (ne clay, total clay, nemediumsilt) were affected to a similar degree by K inputs and outputs (the Kbalance) during the experimental period of ca 40 years.

K release was most likely reversible by xation in the two silty clays(Vre and Kun). Among the remaining (sandyloamy) soils, a soil(Fj) with a neutralslightly alkaline pH appeared highly dynamic,whereas the other two soils (Eke, Hg) showed evidence ofirreversible loss of K from phyllosilicate minerals. The latter twosoils had a slightly lower pH and we suggest that occurrence ofhydroxyaluminium interlayers in 2:1-minerals hampered K xation.

In general terms, it may be suggested that clay minerals in soilscontaining calcite, or easily weatherable primary minerals in theclay fraction, are preserved in a dynamic condition, which allows Kto be released and xed reversibly. In contrast, K release may to agreat extent be irreversible in soils that are poor in this respect.

Acknowledgements

We are indebted to Kristina Grill, at the time working at SLU,for carrying out the K xation capacity experiments; Pierre Barr atthe BIOEMCO laboratories in Paris is acknowledged for valuablediscussions regarding the data analysis. In addition, two anonymousreviewers contributed constructive criticism. The study was per-formed within the project Potassium dynamics in agricultural soilsquantifying sources and sinks and identifying soils in need of Ksupplementation, nanced by Formas (The Swedish Research councilfor Environment, Agricultural Sciences and Spatial Planning).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.geoderma.2009.03.018.

References

Adrielsson, L., Mohrn, E., Daniel, E., 1981. Beskrivning till jordartskartan HelsingborgSV. SGU Ae 16. (Description of the quaternary map Helsingborg SV). SGU, Uppsala(in Swedish with English summary).

Aide, M.T., Cwick, G.J., Cummings, M.F., 1999. Clay mineralogy and potassium status ofselected soils in the glacial Lake Agassiz region of central Manitoba. Can. J. Soil Sci.79, 141148.

Andersson, S., Simonsson, M., Mattsson, L., Edwards, A.C., born, I., 2007. Response ofsoil exchangeable and crop potassium concentrations to variable fertilizer andcropping regimes in long-term eld experiments on different soil types. Soil UseManage. 23, 1019.

Andrist-Rangel, Y., 2008. QuantifyingMineral Sources of Potassium in Agricultural Soils.Doctoral Diss., Dept. of Soil and Environment, SLU. Acta Universitatis AgriculturaeSueciae, vol. 2008:53, Uppsala, Sweden. Available at http://epsilon.slu.se/eng/index.html.

Andrist-Rangel, Y., Simonsson, M., Andersson, S., born, I., Hillier, S., 2006. Miner-alogical budgeting of potassium in soil: a basis for understanding standardmeasures of reserve potassium. J. Plant Nutr. Soil Sci. 169, 605615.

Barnhisel, R.I., Bertsch, P.M., 1989. Chlorites and hydroxy-interlayered vermiculite andsmectite. In: Dixon, J.B., Weed, S.B. (Eds.), Minerals in Soil Environments. SoilScience Society of America, Madison, Wisconsin, USA, pp. 729788.

Barr, P., Velde, B., Catel, N., Abbadie, L., 2007. Soilplant potassium transfer: impactof plant activity on clay minerals as seen from X-ray diffraction. Plant Soil 292,137146.

Bertsch, P.M., Thomas, G.W.,1985. Potassium status of temperate region soils. In: Munson,R.D. (Ed.), Potassium in Agriculture. Soil Science Society of America, Madinson, USA,pp. 131162.

Blake, L., Mercik, S., Koerschens, M., Goulding, K.W.T., Stempen, S., Weigel, A., Poulton,P.R., Powlson, D.S., 1999. Potassium content in soil, uptake in plants and thepotassium balance in three European long-term eld experiments. Plant Soil 216,114.

Bouabid, R., Badraoui, M., Bloom, P.R., 1991. Potassium xation and charge character-istics of soil clays. Soil Sci. Soc. Am. J. 55, 14931498.

Carlgren, K., Mattsson, L., 2001. Swedish soil fertility experiments. Acta Agric. Scand.,Sect. B, Soil and Plant Sci. 51, 4978.

Chen, S.Z., Low, P.F., Roth, C.B., 1987. Relation between potassium xation and theoxidation state of octahedral iron. Soil Sci. Soc. Am. J. 51, 8286.Daniel, E., 1977. Beskrivning till jordartskartan Trelleborg NO. SGUAe 33. (Description ofthe quaternary map Trelleborg NO). SGU, Stockholm (in Swedish with Englishsummary).

Egnr, H., Riehm, H., Domingo, W.R., 1960. Untersuchungen ber die chemischeBodenanalyse als Grundlage fr die Beuteilung des Nhstoffzustandes der Bden. II.Chemische Extraktionsmethoden zur Phosphor- und Kaliumbestimmung. Kungl.Lantbr.hgsk. Ann. 26, 199215.

Fanning, D.S., Keramidas, V.Z., El-Desoky, M.A., 1989. Micas. In: Dixon, J.B., Weed, S.B.(Eds.), Minerals in Soil Environments. Soil Science Society of America, Madison,Wisconsin, USA, pp. 551634.

FAO,1988. Soil Map of theWorld. Revised Legend. World Soil Resources Report 60. Foodand Agriculture Organization of the United Nations, Rome.

FAO-ISRIC-ISSS, 1998. World Reference Base for Soil Resources. World Soil ResourcesReport 84. Food and Agriculture Organization of the United Nations, Rome.

Feigenbaum, S., Edelstein, R., Shainberg, I.,1981. Release rate of potassium and structuralcations from micas to ion exchangers in dilute solutions. Soil Sci. Soc. Am. J. 45,501506.

Feng, K., Wang, X.L., Wnag, X.Z., Tang, Y., 2003. Relationship between 2:1 mineralstructure and the xation and release of cations. Pedosphere 13, 8186.

Fromm, E., 1976. Beskrivning till jordartskartan Linkping NO. SGU Ae 19. (Descriptionof the quaternary map Linkping NO). SGU, Stockholm (in Swedish with Englishsummary).

Hillier, S., 2003. Quantitative analysis of clay and other minerals in sandstones by X-raypowder diffraction (XRPD). Int. Assoc. Sedimentol. Spec. Publ. 34, 207245.

Hinsinger, P., Elsass, F., Jaillard, B., Robert, M., 1993. Root-induced irreversibletransformation of a trioctahedral mica in the rhizosphere of rape. J. Soil Sci. 44,535545.

Huang, P.M., 2005. Chemistry of potassium in soils. In: Tabatabai, M.A., Sparks, D.L.(Eds.), Chemical Processes in Soils. Soil Science Society of America, Madison, WI,USA, pp. 227292.

ISO, 1995. Guide to the Expression of Uncertainty in Measurement (GUM). ISO/IEC Guide98:1995. International Organization for Standardization, Geneva, Switzerland.

Kirchmann, H., 1991. Properties and classication of soils of the Swedish long-termfertility experiments: I. Sites at Fors andKungsngen. Acta Agric. Scand. 41, 227242.

Kirchmann, H., Eriksson, J., Snll, S., 1999. Properties and classication of soils of theSwedish long-term fertility experiments: IV. Sites at Ekebo and Fjrdingslv. ActaAgric. Scand., Sect. B, Soil Plant Sci. 49, 25638.

Kirchmann, H., Snll, S., Eriksson, J., Mattsson, L., 2005. Properties and classication ofsoils of the Swedish long-term fertility experiments: V. Sites at Vreta Kloster andHgsa. Acta Agric. Scand., Sect. B, Soil Plant Sci. 55, 98110.

Kodama, H., Ross, G.J., 1991. Tiron dissolution method used to remove and characterizeinorganic components in soils. Soil Sci. Soc. Am. J. 55, 11801187.

Komadel, P., Madejova, J., Stucki, J.W., 2006. Structural Fe(III) reduction in smectites.Appl. Clay Sci. 34, 8894.

Liu, G., Nielsen, N.E., Hansen, H.C.B., Borggaard, O.K., 1997. Mineral changes in a DanishAlsol caused by 30 years of potassium depletion in the eld. Acta Agric. Scand.,Sect. B, Soil and Plant Sci. 47, 16.

Malavolta, E.,1985. Potassium status of tropical and subtropical region soils. In: Munson,R.D. (Ed.), Potassium in Agriculture. American Society of Agronomy, Madison,Wisconsin, USA, pp. 595617.

McBride, M.B., 1994. Environmental Chemistry of Soils. Oxford University Press, NewYork.

McLean, E.O., Watson, M.E., 1985. Soil measurements of plant available potassium.In: Munson, R.D. (Ed.), Potassium in Agriculture. Soil Science Society of America,Madinson, USA, pp. 277308.

Mberg, J.P., Nilsen, J.D., 1983. Mineralogical changes in soils used for potassiumdepletion experiments for some years in pots and in the eld. Acta Agric. Scand. 33,2127.

Mller, H., 1993. Beskrivning till jordartskartan Uppsala NV. SGU Ae 113. (Descriptionof the quaternary map Uppsala NV). SGU, Uppsala (in Swedish with Englishsummary).

Moore, D.M., Reynolds, R.C., 1997. X-Ray Diffraction and the Identication and Analysisof Clay Minerals. Oxford University Press, New York.

Murashkina, M.A., Southard, R.J., Pettygrove, G.S., 2007. Silt and ne sand fractionsdominate K xation in soils derived from granitic alluvium of the San JoaquinValley, California. Geoderma 141, 283293.

born, I., 1991. Some effects of chemical weathering in three cultivated acid sulfate soilsin Sweden. In: Wright, R.J., Baligar, V.C., Murrmann, R.P. (Eds.), Developmentsin Plant and Soil Sciences. Kluwer Academic Publishers, Dodrecht, Netherlands,pp. 5563.

born, I., Andrist-Rangel, Y., Askegaard, M., Grant, C.A., Watson, C.A., Edwards, A.C.,2005. Critical aspects of potassium management in agricultural systems. Soil UseManage. 21, 102112.

gaard, A.F., Krogstad, T., 2005. Release of interlayer potassium in Norwegian grasslandsoils. J. Plant Nutr. Soil Sci. 168, 8088.

Polemio, M., Rhoades, J.D., 1977. Determining cation exchange capacity: a newprocedure for calcareous and gypsiferous soils. Soil Sci. Soc. Am. J. 41, 524528.

Rich, C.I., 1968. Hydroxy interlayers in expansible layer silicates. Clays Clay Miner. 16,1530.

Saha, U.K., Inoue, K., 1998. Hydroxy-interlayers in expansible layer silicates and theirrelation to potassium xation. Clays Clay Miner. 46, 556566.

Scott, A.D., Smith, S.J., 1966. Susceptibility of interlayer potassium in micas to exchangewith sodium. Clays Clay Miner. 14, 6981.

Simonsson, M., Andersson, S., Andrist-Rangel, Y., Hiller, S., Mattsson, L., born, I., 2007.Potassium release and xation as a function of fertilizer application rate and soilparent material. Geoderma 140, 188198.

Singh, B., Goulding, K.W.T., 1997. Changes with time in the potassium content andphyllosilicates in the soil of the Broadbalk continuous wheat experiment atRothamsted. Eur. J. Soil Sci. 48, 651659.

Soil Survey Staff, 1987. Keys to Soil Taxonomy, 3rd edition. United States Department ofAgriculture, Washington, DC.

Soil Survey Staff, 1998. Keys to Soil Taxonomy, 8th edition. United States Department ofAgriculture, Washington, DC.

Soil Survey Staff, 2006. Keys to Soil Taxonomy,10th edition. United States Department ofAgriculture, Washington, DC.

Sparks, D.L., Huang, P.M., 1985. Physical chemistry of soil potassium. In: Munson, R.D.(Ed.), Potassium in Agriculture. Soil Science Society of America, Madinson, USA,pp. 201276.

Sthlberg, S., 1958. Om markens kaliumtillgngar och metoder fr deras vrdering.Kungl. Skogs- och lantbruksakademiens tidskrift 389402.

Stucki, J.W., Lee, K., Zhang, L.Z., Larson, R.A., 2002. Effects of iron oxidation state on thesurface and structural properties of smectites. Pure Appl. Chem. 74, 21452158.

van derMarel, H.W.,1954. Potassium xation in Dutch soils: mineralogical analyses. SoilSci. 78, 163179.

Velde, B., Peck, T., 2002. Clay mineral changes in the Morrow experimental plots,University of Illinois. Clays Clay Miner. 50, 364370.

Westergrd, A.H., 1912. Beskrifning till kartbladet Trelleborg. SGU Aa 146. Norstedts,Stockholm (in Swedish).

Whitehead, D.C., 2000. Nutrient elements in grassland. SoilPlantAnimal Relationships.CAB International, Wallingford, UK.

Wiklander, L., Koutler-Andersson, E., 1959. Kalkens markeffekt III. Kemiska under-skningar av ett lngvarigt kalkningsfrsk p skifte IV vid Lanna. Grundfrbttring1, 140.

120 M. Simonsson et al. / Geoderma 151 (2009) 109120

Changes in clay minerals and potassium fixation capacity as a result of release and fixation of.....IntroductionMaterials and methodsSoil materialsX-ray diffraction of orientated samplesPotassium fixation capacityEvaluating potassium fixation capacityHCl extractable potassium

ResultsClay mineralogyChanges in XRD patternsPotassium fixation capacity and the effect of soil K balance

DiscussionConclusionsAcknowledgementsSupplementary dataReferences