Mineralogical comparison of agriculturally acidified and naturally acidic soils

Mineralogical comparison of agriculturally acidified

and naturally acidic soils

Donald G. McGahan*, Randal J. Southard, Robert J. Zasoski

Department of Land, Air and Water Resources, University of California, Davis, Davis, CA 95616, USA

Abstract

Soil acidification and acceleratedmineral weathering as a result of N fertilization may result in a soil

solution silica activity that favors the formation of short-range-order aluminosilicates, thereby

sequestering Al. Furthermore, soils formed in alluviums of differing lithology may partition Al3 + into

different pools. We hypothesized that parent material silica content controls solution silica

concentrations and short-range-order aluminosilicate formation, thereby controlling solution Al3 +

activity. As a result, we expected mineralogy to follow alteration pathways toward the minerals found

in weathered naturally acidic (acidic) soils. To test this hypothesis, we compared the mineral

assemblages in nonacidified (NA), agriculturally acidified (AA) and acidic soils formed in sialic,

mafic, and mixed alluvium. X-ray diffraction analysis revealed that the clay fraction bulk mineralogy

of AA soils is similar to their NA counterparts, but in every case, clay activity, as measured by CEC/

clay, was reduced by the acidification process, a trend consistent with mineral alteration trajectories

toward the acidic, weathered soils. A combination of selective dissolution and CEC measurements

suggested that precipitation of short-range-order hydroxy-Al and aluminosilicates were the dominant

mechanisms that control solution Al. In the AA soil derived from sialic parent material, hydroxy-Al

was the largest Al pool. In the agriculturally acidified soil derived from mafic parent materials, the

short-range-order aluminosilicate fraction was the dominant Al pool. Aluminum sinks in AA soils with

mixed lithology were dominated by hydroxy-Al and exchangeable Al. We speculate that ‘‘free’’

(noninterlayer) hydroxy-Al may be a significant Al sink in the AA soils. Our results suggest that short-

range-order aluminosilicates play some role in controlling soil solution Al activity, but the higher silica

content of the parent material did not correlate directly with increased abundance of short-range-order

aluminosilicate compounds in acidified soils. This research on soil acidification provides new

information on how ammonium fertilization affects the chemical aspects of soil quality, in particular,

the partitioning of Al to solid phases that may not be in equilibrium with the bulk clay mineralogy.

D 2003 Elsevier Science B.V. All rights reserved.

Keywords: Selective dissolution; XRD; Short-range order; Aluminosilicates; Hydroxy-aluminum

0016-7061/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved.

doi:10.1016/S0016-7061(03)00049-1

* Corresponding author.

E-mail address: [email protected] (D.G. McGahan).

www.elsevier.com/locate/geoderma

Geoderma 114 (2003) 355–368

1. Introduction

An important aspect of soil quality is how soil chemistry responds to processes of

acidification, either by long-term, intensive weathering or by the application of acidifying

fertilizers. Soils acidified as a result of natural pedogenic processes generally are marked by

low activity clays (kaolinite, halloysite, and gibbsite), hence, depressed cation exchange

capacity, lower potential for alkaline earth and alkali metal (base) cation releases upon

weathering, and increased influence of acidic cations, particularly Al (Foy, 1984; Thomas

and Hargrove, 1984). Soil acidification resulting from fertilization practices may produce

soil pH similar to the pH of naturally acidic soils. The bulk soil mineralogy of the N fertilized

soils may not reflect this acidification. Fertilization practices utilizing ammonium or urea

sources can result in soil acidification from the release of protons during nitrification, and

this can cause clay mineral degradation and reduced mineral cation exchange capacity

(Barak et al., 1997; Blake et al., 1999; Blevins et al., 1977; Bouman et al., 1995; Schwab et

al., 1989). In the western USA, the acidification effect may bemagnified by sulfur-supplying

fertilizers and direct elemental S additions (Jackson and Reisenauer, 1984; Stallings, 1991).

Weathering of the clay fraction minerals to low activity clays is one possibility, and these

clays may predominate in the latter stages of intensive soil weathering (e.g., Jackson and

Sherman, 1953). In less-weathered soils, formation of hydroxy interlayers can account for

cation exchange capacity reductions (Barnhisel and Bertsch, 1989). Additionally, Al

released during weathering of primary minerals competes effectively for exchange sites

(Rampazzo and Blum, 1992; Thomas and Hargrove, 1984). Jackson and Reisenauer (1984),

however, contended that soils in the western USA acidified by agricultural practices have

less Al contributing to exchangeable acidity than naturally acidic soils.

Soil pH influences Al partitioning into and from phyllosilicates interlayers. Rampazzo

and Blum (1992) compared X-ray diffractograms from very acidic soils (pH f 3) affected

by acid rain to nonacidic soils and reported alterations in clay fraction minerals. They

documented Al loss from Al-hydroxy interlayers in ‘‘secondary’’ Al-chlorite and found a

sharper X-ray diffraction peak corresponding to vermiculite. Conversely, in laboratory

experiments at less acidic conditions (pH 4.07–4.46), Lou and Huang (1988, 1994) and

Sakurai and Huang (1998) reported adsorption of hydroxy-aluminosilicates and hydroxy-

aluminum into montmorillonite interlayer spaces. Vermiculite also incorporates hydroxy-

aluminosilicates in the interlayer (Inoue and Satoh, 1992; Lou and Huang, 1995).

Although pH influences the partitioning of Al into interlayers, alteration of clay minerals

is often coincident with formation and persistence of X-ray amorphous hydroxy-alumi-

nosilicate products (Hem et al., 1973).

In soils with andic and spodic properties, selective dissolution is used to estimate the X-

ray amorphous (short-range order) allophane and imogolite pools and Al and Fe bound in

metal–humus complexes (Dahlgren, 1994; Parfitt and Kimble, 1989; Southard and

Southard, 1989; Wada, 1989), but the selective dissolution is equally applicable to

characterization of short-range-order compounds in other soils.

We hypothesized that lithology affects the partitioning of Al cations into different pools

in soils acidified by N fertilization. As a result, we expected the mineralogy of the acidified

soils to follow an alteration trajectory toward the mineralogy of naturally acidified soils. In

sialic alluvium, acidification and accelerated soil mineral weathering by N-fertilization

D.G. McGahan et al. / Geoderma 114 (2003) 355–368356

may produce enough silica to favor formation of short-range-order aluminosilicates that

incorporate and thereby sequester Al. Soils formed in mafic alluvium, with less Si, are not

expected to sequester Al in the short-range-order aluminosilicate pool as readily as sialic

materials. Soils formed in alluvium of mixed lithology, and intermediate silica content, are

expected to exhibit intermediate behavior bounded by sialic and mafic parent materials.

By comparing nonacidified (NA) members of soils formed in sialic, mafic, and mixed

alluvium to naturally acidic (acidic) and agriculturally acidified (NA) soils, our objective

was to assess the effect of mineralogy on Al distribution among various pools in the soils and

to assess the extent to which agricultural acidification mimicked long-term acidification and

weathering. We hypothesized that Al sequestration in a short-range-order pool would be the

greatest in AA soils formed in sialic alluvium and the least in soils from mafic alluvium.

2. Materials and methods

2.1. Field

All soils are located in California in a xeric moisture regime and have a thermic soil

temperature regime. Soils were sampled to represent NA, acidic, and AA members from

sialic, mafic, and mixed parent material (Table 1). The AA members sampled in tree crop

Table 1

Classification, location, current crop and modal pH of soils used in the study

Soil Soil Series Mapped-As Classification County Vegetation Modal pHa

Sialic

NA Dinuba: coarse-loamy, mixed, superactive,

thermic Typic Haploxeralf

Stanislaus Row crops 6.8–7.0

AA Hilmar: sandy over loamy, mixed, superactive,

calcareous, thermic Aeric Halaquept

Stanislaus Almonds 7

Acidic Montpellier: fine-loamy, mixed, superactive,


Stanislaus Annual grasses 6

Mafic

NA Vina: coarse-loamy, mixed, superactive,

thermic Pachic Haploxeroll

Tehama Walnuts 7

AA Vina: coarse-loamy, mixed, superactive,

thermic Pachic Haploxeroll

Tehama Walnuts 7

Acidic Sites: fine, parasesquic, mesic Xeric Haplohumult Plumas Mixed conifer 5.3

Mixed

NA Arbuckle: fine-loamy, mixed, superactive,


Colusa Almonds 6.2

AA Arbuckle: fine-loamy, mixed, superactive,


Colusa Almonds 6.2

Acidic Red Bluff: fine, kaolinitic, thermic Ultic Palexeralf Shasta Pasture 5

NA= nonacidified. AA= agriculturally acidified. Acidic = naturally (pedogenically) acidified.a The pH of soil material as reported in official soil series description (soil series type location) at the depth

corresponding to sampling.

D.G. McGahan et al. / Geoderma 114 (2003) 355–368 357

locations were sampled in the drip basins of fertigation emitters. In tree crop sampling

locations of the mafic and mixed soils, the NA and AA samplings occurred within 1–3 m

of each other. Where possible, B horizon samples were used to reduce effects from organic

matter and residual surface amendments, but for the AA and NA mafic soils, samples were

collected from surface horizons because only these were acidified by fertilization treat-

ments.

2.2. Laboratory

Soils were air dried, sieved to pass through a 2-mm sieve, and analyzed for particle size

distribution, pH, extractable cations, selective dissolution of Fe, Al, and Si, and clay

mineralogy. Particle size distribution was determined by pipette method as described by

Gee and Bauder (1986). Carbon was determined with a Fisons NA1500NC (Fisons

Instruments, Beverly, MA) by dry combustion and infrared detection of evolved CO2

(Nelson and Sommers, 1982).

Soil reaction was determined in water (1:1 soil/water), 0.01 M CaCl2 (1:2 soil/solution),

and in a saturated soil paste (McLean, 1982; National Soil Survey Center, 1996).

Extractable cations were measured by displacement with 1 M Ba(OAc)2, pH 7. Cation

exchange capacity (CEC7) was measured by the decrease in solution Ca following

displacement of the Ba (from the extractable cation procedure) by Ca from a 0.58 M

CaSO4�2H2O solution (Janitzky, 1986; Rible and Quick, 1960) and by unbuffered 1 M

NH4Cl displaced by 1 M NaCl (CECNH4Cl) and cations measured by inductively coupled

plasma (ICP) spectrometry (National Soil Survey Center, 1996).

Extractable Al (AlKCl) was extracted with 1 M KCl after 30-min equilibration with

agitation and vacuum filtration (National Soil Survey Center, 1996). The Al concentrations

were measured with ICP spectrometry.

Clay size (< 2 Am) soil materials were separated from the fine-earth fraction by repeated

centrifugation following dispersion in dilute (NaPO3)6. Clays were washed with MgCl2 or

KCl salt solutions, rinsed with deionized water to remove excess salts, and mounted as

oriented aggregates on glass slides. X-ray analyses were made with a Diano XRD 8000

diffractometer (Diano, Woburn, MA) producing Cu Ka radiation fitted with a nickel filter

and curved graphite monochromator. After the initial diffraction analysis, the MgCl2treated samples were treated with glycerol and reanalyzed. The KCl samples were

reanalyzed after 350 and 550 jC heat treatments (Whittig and Allardice, 1986).

Air-dried materials from the fine-earth fraction were extracted with sodium pyrophos-

phate, ammonium oxalate, and citrate-dithionite using the methods of the National Soil

Survey Center (1996). Extracts were analyzed for Al, Si, and Fe concentrations using ICP

spectrometry. Initial oxalate-extractable Fe (Feo) was higher than citrate-dithionite-extract-

able Fe (Fed) for the NA and AA soils derived from sialic and mafic alluvium, indicating

the presence of magnetite. We verified the presence of magnetite by removing the

magnetic fraction with a hand magnet. The presence of magnetite was confirmed in the

magnetic fraction of these soils by X-ray diffraction. We estimated short-range-order Fe by

taking the difference between Feo of the whole soil and Feo (from magnetite) of a

subsample after citrate-dithionite extraction. All analyses were preformed without repli-

cation and are reported as single sample values.


3. Results and discussion

Across the range of soils and parent materials, agricultural practices have produced

significant acidification (Table 2). In all soils except the mafic acidic sample, pH increased

with dilution and decreased with salt addition, suggesting that negatively charged clays

dominate the soil exchange properties. Only in the mafic acidic sample are the soil

properties dominated by oxyhydroxides (Thomas and Hargrove, 1984).

The CEC measured by unbuffered salt (CECNH4Cl) decreases with acidification. This is

particularly relevant in the mafic and mixed lithology soils since they are pairs collected

only a few meters apart. If the CEC to clay ratio is considered, the CEC decrease

normalized to clay content is extended through the acidic soil with each lithology (Table

2).

3.1. Sialic parent material

Clay fraction diffractograms of soil derived from sialic alluvium exhibited a dominance

of mica (persistent 1.0-nm peak), kaolinite (0.72-nm peak disappears with 550 jC heat

treatment), and a minor amount of vermiculite (1.4-nm peak collapses to 1.0 nm with

heat). The AA soil diffractogram had a broad low shoulder between 1.4 and 1.0 nm that

could indicate incipient interlayering of the vermiculite and result in the observed

incomplete closure to 1.0 nm with heat. Quantification of kaolinite content was not

attempted, but the 0.72-nm peak intensities are greater relative to other peaks in the acidic

soil than the NA and AA soils (Fig. 1). The CEC/clay ratios also decrease from the NA to

AA. This decrease in CEC/clay, together with greater relative 0.72-nm peak intensities, is

Table 2

Selected soil properties for the soils used in the study

Soil pHa Extractable cations (cmol kg� 1) CEC7 CECNH4ClCEC/ Sand Silt Clay

K Ca Mg Na(cmol

kg� 1

)

(cmol

kg� 1

)

clayb

(%<

2 mm)

(%<

2 mm)

(%<

2 mm)

Sialic

NA 7.78 1.1 4.7 0.6 0.7 5.5 2.9 1.19 90.1 7.5 2.4

AA 5.35 0.1 0.4 0.1 < 0.1 3.5 1.3 0.92 91.4 7.2 1.4

Acidic 5.23 0.3 2.1 0.4 < 0.1 6.0 3.8 0.75 70.7 24.1 5.1

Mafic

NA 6.26 1.5 9.7 4.8 < 0.1 22.5 14.7 1.14 50.1 37.0 12.9

AA 5.55 0.6 5.4 4.0 0.2 17.5 12.1 1.08 54.5 34.3 11.2

Acidic 4.55 0.1 0.1 0.1 < 0.1 9.5 7.0 0.20 20.3 44.5 35.3

Mixed

NA 6.54 0.1 5.2 2.1 0.1 12.0 6.3 0.43 56.4 28.8 14.8

AA 3.94 0.1 5.1 2.1 0.1 13.0 4.5 0.34 53.2 33.5 13.3

Acidic 4.67 0.4 0.6 0.2 < 0.1 8.5 7.7 0.25 34.3 35.2 30.5

NA= nonacidified. AA= agriculturally acidified. Acidic = naturally (pedogenically) acidified.a pH in 1:1 soil/water.b Calculated ratio of unbuffered CEC (CECNH4Cl

) to percent clay.


Fig. 1. Clay fraction (< 2 Am) X-ray diffractograms from soils formed in sialic alluvium. Peak index numbers are

d-spacings in nanometers.


interpreted to indicate an increase in the kaolinite content from NA through AA to the

acidic soil, wherein kaolinite and mica appear to be co-dominant.

Citrate-dithionite-extractable Al (Ald) and organically bound Al (pyrophosphate

extractable; Alp) both increase in acidic soils relative to NA soils (Table 3). The Alp is

as great as Ald in the NA and AA soils suggesting that citrate-dithionite is extracting

primarily organically bound Al (Table 3). Extractable Al (AlKCl) increased in the AA soil

relative to NA, but is less abundant in the acidic soil despite the increase in Ald and short-

range-order fraction Al (Alo) (Table 3). Since the short-range-order Si pool (Sio) did not

increase as AlKCl or Alo increased in the AA soils (Table 3), the source of the Al is most

likely a hydroxy-Al pool rather than an aluminosilicate pool. The XRD and CEC (Table 2)

results suggest that some interlayering of 2:1 phyllosilicates may be occurring.

McKeague and Day (1966) found XRD pattern enhancement (sharper peaks) and

expansion with glycerol after both citrate-dithionite and acid–ammonium-oxalate treat-

ments of artificially prepared hydroxy-interlayered materials (HIM). Iyengar et al. (1981)

reached similar conclusions, but also concluded that acid–ammonium-oxalate removed

less Al than citrate-dithionite treatments. Based on these studies, if hydroxy-Al interlayer-

ing were the dominant Al sink, we would have expected significantly more extracted Ald.

The fact that Alo increased without an increase in (Ald–Alp) in the AA soil suggests that

the Al is not in an interlayer pool (Table 3). Similarly, this Al is not subject to KCl

extraction but is subject to acid–ammonium-oxalate extraction. Southard and Southard

(1989) speculated that in California Andisols a large pool of Al extractable by acid–

ammonium-oxalate, but not by citrate-dithionite, may exist as free short-range-order Al-

hydroxy compounds. The alteration of naturally weathered soils often leads to low activity

clays (kaolinite, halloysite, and gibbsite), but intermediate metastable mineralogy fre-

quently shows some hydroxy-interlayering, either as biotite is altered to vermiculite, or as

vermiculite becomes a sink for the Al. The Al pool of the AA soil is Si poor despite the Si

rich parent material, and the bulk of this Al pool is apparently not associated with the

interlayers. The fate of Si produced by mineral weathering is also not clear, but we

speculate that because silica is more mobile than the Al it is leached to the deeper subsoil

by irrigation water.

3.2. Mafic parent material

The clay fractions of the NA and AA soils derived from mafic alluvium are dominated

by smectite (1.56 nm Mg-saturated peak that expanded with glycerol solvation) with a

minor amount of kaolinite (0.72-nm peak that disappeared with heating to 550 jC) (Fig.2). In the mafic NA and AA samples, smectite had large interlayer distances in the Mg

saturated samples and exhibited a variable expansion with glycerol that resulted in

indistinguishable peaks. This behavior could indicate a low charge smectite. The acidic

soils are dominated by halloysite (0.73- and 0.445-nm peak that disappeared with heating

to 550 jC), gibbsite (0.489-nm peak that disappeared with heating to 550 jC), andgoethite (0.415-nm peak that disappeared with heating to 550 jC) (Fig. 2).

Increased Fed contents from NA through AA to the acidic soil were similar to the sialic

lithology and suggest acidification attributed to fertilization enhances weathering of the

Fe-bearing minerals. Among all samples, the acidic mafic soil contains the highest level of


Table 3

Selective dissolution chemistrya of the soils

Soil Fed(g kg� 1)

Feob

(g kg� 1)

Fep(g kg� 1)

Ald(g kg� 1)

Alo(g kg� 1)

Alp(g kg� 1)

Sio(g kg� 1)

AlKCl(g kg� 1)

Alo–Alp(g kg� 1)

Ald–Alp(g kg� 1)

Alo–Alp/

Sioc,d

Alo–Ald/

Sioc,e

(Ald–Alp)/

[(Ald–Alp)+

(Fed–Fep)]c,f

(%)

Sialic

NA 2.37 1.64 0.07 0.03 0.11 0.04 0.13 0.006 0.07 – 0.5 0.6 –

AA 2.97 1.60 0.13 0.09 0.31 0.08 0.08 0.015 0.22 0.01 3.1 2.9 0.6

Acidic 7.67 2.04 0.27 0.16 0.40 0.10 0.19 0.006 0.29 0.05 1.6 1.3 1.5

Mafic

NA 13.77 7.13 0.69 0.99 1.76 0.50 2.47 0.010 1.26 0.50 0.4 0.3 7.3

AA 14.32 5.21 0.82 1.20 2.43 0.65 2.95 0.010 1.78 0.55 0.6 0.4 7.8

Acidic 99.66 0.50 0.11 14.24 4.01 0.68 1.29 0.001 3.33 13.56 2.7 – 22.0

Mixed

NA 10.54 1.50 0.14 0.08 0.46 0.12 0.15 0.008 0.35 – 2.4 2.6 –

AA 13.37 1.83 0.73 0.24 1.59 0.82 0.19 0.255 0.77 – 6.3 7.1 –

Acidic 21.77 4.21 0.84 2.27 2.01 0.89 0.25 0.138 1.11 1.38 4.7 – 12.0

NA= nonacidified. AA= agriculturally acidified. Acidic = naturally (pedogenically) acidified.a d subscript: citrate-dithionite extractable; o subscript: acid–ammonium-oxalate extractable; p subscript: pyrophosphate extractable.b Estimate of short-range-order Fe of the whole soil adjusted for Feo from magnetite.c Mole basis.d Estimate of Al/Si ratio in short-range-order aluminosilicates after adjustment for Al in organic complexes (Alp).e Estimate of Al/Si ratio in short-range-order aluminosilicates after adjustment for Al in organic complexes (Alp) and in Al substituted Fe-oxyhydroxides (Ald).f Ratio representing the percent Al substitution in Fe-oxyhydroxides.

D.G.McG

ahanet

al./Geoderm

a114(2003)355–368

362

Fig. 2. Clay fraction (< 2 Am) X-ray diffractograms from soils formed in mafic alluvium. Peak index numbers are



Ald. This pattern follows that of Fed, and most likely reflects the isomorphous substitution

of Al for Fe in goethite (Norrish and Taylor, 1961). The representation of the Al

substitution in Fe-oxyhydroxides, Ald–Alp/(Ald–Alp + Fed–Fep), assumes that all of the

Ald–Alp is extracted from the Fe-oxyhydroxides and not from interlayers, short-range-

order aluminosilicates, or organic pools. Goethite was abundant enough, or sufficiently

crystalline, to be identified by XRD, and Ald>Alo in only the acidic mafic soil. Because

goethite was not detected by XRD, it is difficult to argue for Al-substituted goethite in the

NA and AA mafic soils.

Extractable Al (AlKCl) in samples of AAmafic soil did not increase relative to the NA soil

(unlike the sialic soil) despite a decreased pH and increased Ald and Alo pools (Table 3).

Increased Alo and Sio in AA samples relative to samples of NA in mafic soil indicate that

more Al was present in a short-range-order aluminosilicate pool. The Ald increase was less

than the Alo increase. Because citrate-dithionite appears to be more aggressive than oxalate

at removing interlayers (Iyengar et al., 1981), the Alo–Ald value seems to represent Al in

short-range compounds but corrects for Al extracted from Al substituted in Fe-oxyhydr-

oxides or poorly ordered Fe-silicates (Dahlgren, 1994). The Alo–Alp/Sio values for the

mafic NA and AA soils are greater than the Alo–Ald/Sio values for these soils (Table 3).

These ratios do not take into account acid–ammonium-oxalate extracted silica in Fe-rich 2:1

layer silicates, Si-bearing ferrihydrites or other Fe rich allophane analogs (Kohyama and

Sudo, 1976; Carlson and Schwertmann, 1981; Eggleton et al., 1983; Shayan, 1984), which

could contribute to lower-than-expected Alo–Alp/Sio and Alo–Ald/Sio values. Assuming

that citrate-dithionite is more aggressive than oxalate at removing interlayer Al, these results

suggest the occurrence of a noninterlayer, short-range-order aluminosilicate.

We hypothesized that the mafic soils would contain less short-range-order Al associated

with Si than soils derived from parent materials with greater Si content. Our results suggest

instead that Si is retained in the AA sample and could favor the formation of short-range-

order aluminosilicates and perhaps short-range-order ferrosilicates. We speculate that the

mafic parent material, relative to the other parent materials, is dominated by more easily

weathered primary minerals that release silica and other weathering products rapidly. It

may be that higher Fe activity in the mafic parent material plays an important role in

control of Si and Al chemistry in these soils (see Feo, Table 3). Formation of a low charge,

and possibly low crystallinity, smectite as a metastable mineral may be the result of

incorporation of the weathering products via the short-range-order aluminosilicate inter-

mediate. We further speculate that the low charge of the smectite may reduce the

electrostatic attraction of ionic Al forms, thereby favoring reaction of Al with solution

silica to form the short-range-order aluminosilicates.

3.3. Mixed parent material

The NA and AA soils derived from mixed alluvium contain mostly vermiculite (1.4 nm

collapses to 1.0 nm) and hydroxy-interlayered material (HIM, 1.4 nm collapses to 1.17

nm), plus minor amounts of mica (persistent 1.0 nm) and kaolinite. The naturally acidic

soil derived from mixed parent material was dominated by kaolinite and contains only a

minor amount of mica and goethite (Fig. 3). Clay mineralogy of the AA soil most closely

resembled that of the NA soil.


Fig. 3. Clay fraction (< 2 Am) X-ray diffractograms from soils formed in mixed alluvium. Peak index numbers are



The decrease of the CEC from NA to AA soil as measured by unbuffered salt

(CECNH4Cl, Table 2) may be the result of nonexchangeable aluminum on the exchange

complex, further pillaring in the interlayer, presumably by hydroxy-Al polymers that block

exchange sites and/or a decrease in pH dependent exchange sites. The increase of CEC7 in

the AA soil is interpreted as the increase in pH dependent charge that more than offsets

any decrease in permanent charge with hydroxy-Al interlayering or nonexchangeable

surface associated short-range-order compounds (Inoue and Satoh, 1992).

Extractable Al (AlKCl) in samples of AA soil from mixed lithology was greater than in

any other soil, indicating that some of the Al released is not polymerized, but probably

occurs as Al3 + on exchange sites. However, if calculated as Al3 + the AlKCl exceeds a

reasonable value for Al3 + on the exchange sites. Potassium chloride is, therefore,

extracting more than just exchangeable Al3 +. This increased concentration of exchange-

able Al occurs in the AA soil that has a pH of 3.94, which was about 1.5 pH units lower

than other AA soils. The increased extractable pools of both Alo and Alp accompanied by a

smaller increase in the Sio pool suggests that hydroxy-Al is also an Al sink in the AA soil

with mixed parent materials. Similar to the sialic soils, the Ald pool in mixed soils was not

significantly greater than the Alo pool, indicating that the hydroxy-Al pool may not be in

the interlayer.

We hypothesized that soils with mixed parent materials having an intermediate Si

content would have intermediate levels of short-range-order Al associated with Si. Clearly,

the mixed lithology NA and AA soils are not dominated by low activity clays, but have

relatively low activity nonetheless, as measured by CEC/clay, possibly due to hydroxy-

interlayering. Along the mineralogical alteration trajectory toward a bulk clay mineralogy

dominated by low activity clays, interlayers acting as a sink for Al become less pronounced

as interlayers become filled or blocked by hydroxy-Al. This soil, however, demonstrates

that the presence of HIM should not necessarily lead to the conclusion that the interlayers

are the only sink for Al released by weathering. In fact, exchangeable Al and noninterlayer

hydroxy-Al appear to be more important sinks for Al here than in either the sialic or mafic

soil. The data from agriculturally acidified soils with mixed parent material demonstrates

the complexity of determining the fate Al in soils dominated by HIM.

In summary, we suggest that acidification associated with ammonium-based N-

fertilization tends to increase short-range-order pools of hydroxy-Al or aluminosilicates

in all soils, may cause interlayering of 2:1 minerals, and significantly increased exchange-

able Al with the most intensive acidification. In none of the cases did the agricultural

acidification significantly affect the bulk soil mineralogy, but in every case agricultural

acidification caused a decrease in clay activity as measured by CEC/clay, suggesting

mineral weathering trajectories toward the mineralogy of the acidic, weathered soils. A

similar trend in all three parent materials, suggests that these short-range-order alumi-

nosilicates may not be in equilibrium with the solid crystalline phases. The short-range-

order Al pool was most pronounced in the mafic parent material soil. Interlayering of Al

appeared to be most pronounced in the sialic and mixed parent material soils, but a

significant portion of the hydroxy-Al pool appears to occur as ‘‘free’’ forms. These results

do not support our hypothesis that silica content of the parent material affects the

partitioning of Al. The presence and type of 2:1 minerals, degree of acidification, and

weatherability of the primary minerals appear to be major determinants of Al sinks upon


acidification. This research shows that soil acidification by ammonium fertilization can

have a significant impact on chemical aspects of soil quality, in particular, on the

partitioning of Al into solid phases.

Acknowledgements

We are grateful to the Kearney Foundation for support of this research.

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Mineralogical comparison of agriculturally acidified and naturally acidic soils

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