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Chapter 4. Chemical Weathering of Minerals in Soils

Chapter 4. Chemical Weathering of Minerals in Soils

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I V Frequency Distribution of Minerals in Soils in Relation t o Chemical Weathering.



1 Soils in the Early Stages of Chemical Weathering

a Early Weathering Stages Occur in Soils of Both Cool and Warm



b Saline Stage of Weathering (Stage 1 )


c Calcareous Stage of Weathering (Stage 2 )


d Ferromagnesian and Feldspar Stages of Weathering (Stages

3. 4. and 5 )


2 Soils of Intermediate Stages of Chemical Weathering


a Intermediate Weathering Stages Occur in Soils of Both Cool

and Warm Regions


b Quartz Stage of Weathering (Stage 6 )


c Mica-Illite Stage of Weathering (Stage 7 ) . . . . . . . . 262

d Interstratified 2 : l Layer Silicate and Vermiculite Stage of

Weathering (Stage 8 )


e Montmorillonite Stage of Weathering (Stage 9 ) . . . . . . 270

3 Soils of Advanced Stages of Chemical Weathering


a The Laterite as the End Product of Weathering


b Kaolin Stage of Weathering (Stage 10)


c. Qibbsite-Allophane Stage of Weathering (Stage 11)


d Hematite-Goethite Stage of Weathering (Stage 12)


e Anatase-Leucoxene Stage of Weathering (Stage 1 3 ) . . . . . 289

V Weathering Release of Nutrient Elements from Soil Minerals . . . . . 291

1 General Level of Soil Fertility I s Related to Weathering Stage


a Influence of Mineral Content in the Temperate Zone . . . . 292

b Influence of Mineral Content in the Tropical Zone . . . . . 292


2 Weathering Release of Nutrient Ions from Soil Minerals

a Weathering Release of Potassium


b Weathering Release of Calcium. Magnesium. Chromium. and



c Weathering Release of Phosphorus


d Weathering Release of Nitrogen and Sulfur


e Weathering Release of Minor Elements


3 Syatema of Utilization of Soil Mineral Weathering Sources of Nutriente


a Patch Agriculture


b Paddy Culture


c Intensive Culture











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Weathering of minerals in relation to soils has two distinct phases:

( a ) Past weathering has broken down hard rocks both physically and

chemically to form the unconsolidated sands, loams, and clays from

which soils may be formed; and ( b ) present weathering of soil minerals

remains the major source of nutrients used by crops, for which the vast

chemical fertilizer industry is but a supplement. The first phase concerns the kinds of minerals found in various kinds of soil and soil parent

material. The second phase concerns the rate at which ions are released

in available form from minerals under specific conditions. As agriculture is intensified in particular areas, the proportion of the nutrients

supplied by weathering becomes relatively smaller compared to that

added as fertilizers. But even in areas of well-developed agriculture,

both in temperate and tropical regions, the weathering release of nutrient

elements is of major importance in soil fertility and crop production.

It is the purpose of this paper to examine the processes and products

of chemical weathering of minerals in soils from the standpoint of each

of these phases.

1. Definition of Weathering

The term “weathering of rocks” refers to the changes in degree of

consolidation and in composition which take place in the earth’s crust

within the sphere of influence of atmospheric and hydrospheric agencies.

Weathering has two distinct aspects, physical and chemical, each of

which can be defined separately. Physical weathering is the change of

consolidated rock to the unconsolidated state. Chemical weathering is

the change in chemical composition of consolidated o r unconsolidated

rock. For clarity, the agencies and processes of weathering (the means

by which the changes are brought about) will be carefully distinguished

from weathering itself (the changes). The term “weathered” means

changed physically or chemically, under atmospheric or hydrospheric


Changes in rock composition which are caused by deuteric and

hydrothermal agencies are termed “alteration” rather than weathering,

although some changes in mineral content of rocks arising through alteration are virtually identical to those arising through weathering. According to Merrill (1906), some scientists have not been precise in making

the distinction between weathering and alteration. He defined weathering as the superficial changes in a rock mass brought about by atmospheric agencies and resulting in a more or less complete destruction of





the rock as a geological body. Physical weathering he termed disintegration ; chemical weathering he termed decomposition.

a. The Province of Physical Weathering. Although the scope of this

paper is limited to the province of chemical weathering, a brief statement

is given on the province of physical weathering for purposes of clarification of the interrelationships between the two. Physical weathering

is the change from solid massive rock to the unconsolidated or elastic

state, under the influence of atmospheric or hydrospheric agencies, as

already explained. Polynov (1937) defines weathering as the complex

process, cyclic in nature, which causes the breaking down of the monolith

of rock or in general, solid body, and produces a n increase in surface or

interface between it and the surrounding media. It is known that every

interface, whether it be the boundary between the solid and liquid or

solid and gaseous states, possesses specific physicodynamic properties

which become more important quantitatively and qualitatively as the

ratio of the interface to the surrounding mass, specific surface, increases.

Weathering is thus, according to Polynov (1937), any process which

breaks down the large units to smaller “more active’’ units; and he defines the “crust of weathering” as “that upper part of the lithosphere

which consists of the loose products of the disintegration of igneous and

metamorphic rocks.” Elsewhere he includes disruption of sedimentary

rocks and makes clear that he includes processes involving transporation.

Physical weathering or disintegration breaks up the rock into smaller

units by mechanical means. The smaller units retain their original identity and composition. A basalt which has been broken down by mechanical means into smaller units can still be recognized as a basalt or, if the

particles are sufficiently fine, as its component mineral grains. Physical

weathering takes place either through agencies which act on the rock in

place or which move the rock about and abrade it. The latter include:

(a) glacial ice, ( b ) moving water, (c) gravity displacement on steep

slopes, and ( d) wind, for example, sand-blasting. These agencies which

move the material about are sometimes excluded as weathering agencies

but should surely be included. Physical weathering in place is brought

about through five principal processes (Reiche, 1950) : (a) unloading,

which permits the expansion of rock masses: when confining pressures

are lessened by uplift and erosion, cracks and joints form in bedrock; (b)

thermal expansion and contraction from insolation, which was once considered a major factor (Polynov, 1937) but has now been shown to be

relatively unimportant by Blaclrwelder (1933) and Origgs (1936) ; (c)

crystal growth, which produces a prying action on rock or mineral-this

includes the action of frost and to some extent of crystals formed by

chemical weathering, as, for example, the physical weathering of quartz



crystals by iron-oxide films (Humbert and Marshall, 1943) and the crumbling of building stones in Switzerland attributed to the growth of ice

crystals and, in the cities, of sulfate crystals in cracks (Quervain, 1945) ;

( d ) colloid plucking, the physical rupture caused by shrinkage of colloidal matter with resultant flaking off of the surface of rocks; and (e )

pressure and abrasion by plants and animals, for example, disruption of

rocks by the enlargement of roots wedged in cracks (Kellogg, 1943, p.


The chief contribution of the physical weathering of rocks to chemical weathering is the increase of specific surface of the material, with

attendant increase in reaction rate. Birot (1947) found that rock Samples of granite and syenite and large crystals of biotite, muscovite, and

orthoclase lost from 1 to 6 per cent of the sample by disintegration into

sand particles after being alternately wetted a t room temperature and

dried a t 70" C. Caillere et al. (1952) obtained the breakdown of 6 per

cent of a slightly weathered gabbro into sand and some montmorin clay

by alternate wetting and drying. Simply keeping some material under

water can cause subdivision of the particles. Dekeyser et al. (1950)

boiled mica in water for a long period of time and obtained a clay-size

product. Henin and Betremieux (unpublished) obtained this same effect

at room temperature. Demolon and Bastisse (1946) found reduction of

the particle size of sieved granite after five and fifteen years of weathering in a lysimeter. Wetting and drying can also bring about the formation of new minerals and thus cannot be considered purely a physical

weathering factor. For example, Caillere et al. (1952) found that when

the gabbro suspension was evaporated and then wetted again, the gabbro

became coated with a ferruginous crust.

I n soils, particles of sand size are almost entirely products of physical

weathering (concretions excepted). Practically all particles of silt size

are products of physical weathering in cool and temperate climates but

are largely products of chemical weathering in tropical climates. Particles of clay size have such high specific surface that every stage of

chemical weathering is represented in this fraction, from early stages in

glacial rock flour (hornblende, biotite, albite) and intermediate stages

(montmorin, kaolin) to highly resistant products and residues (allophane,

anatase, ilmenite) .

b . The Province of Chemical Weathering. Chemical weathering is

the change in chemical composition of rocks under the influence of atmospheric or hydrospheric agencies. Sometimes termed the " decomposition" of rocks and minerals, chemical weathering is better viewed

in more conservative terms : The products of chemical weathering must

either ( a ) be formed as a new mineral (synthesis mechanism) or (b)





remain as a residue when other constituents are removed (residue mechanism). Polynov (1937) termed these “products of accumulation” and

“ ortho-eluvium, ” respectively, although both are accumulations by different mechanisms. Solutes which pass into the ground water are also

“products” of weathering but are of less concern to soil mineral content

except as they may re-enter the cycle through synthesis (case 1). Likewise, ions which are released from minerals and temporarily held in

adsorbed forms which are available to plants are products of chemical

weathering and are of first importance to crop production; however,

they are not emphasized i n the usual consideration of chemical weathering because their quantities are so small compared to that of the weathered mass.

Decomposition is indeed involved in chemical weathering in the release of reactants for the synthesis mechanism ; likewise, decomposition

of a more readily weatherable substance and its complete removal in

solution are necessary for the operation of the residue mechanism. However, the products of chemical weathering (aside from solutes in ground

water) are in either case something other than a “decomposed” substance. The use of such terms as “decomposition” (Merrill, 1906) and

“katamorphism” (Van Hise, 1904) grew out of the fact that the

products of chemical weathering were formed or left behind from decomposition of something else and were frequently colloidal. Mineralochemical analysis had not developed to the point that the products formed

could be determined as new minerals, and this aspect was not emphasized in early literature as it came to be later.

The province of chemical weathering is, therefore, the formation or

residual accumulation of minerals as a function of their relative stability

to the chemical weathering processes (Sections I1 and 111) ; it includes

the frequency distribution of minerals in soils (Section IV) and the rate

of weathering release of nutrient elements from soil minerals (Section


c. Weathering Akin. t o Deuteric m d Hydrothermal Alt,eration of

Minerals. The layer silicates and other minerals frequently encountered

in weathering studies are also formed by deuteric and hydrothermal

alteration. Consequently, these layer silicates occur in the geologic column at much greater depths than the depths of initiation of weathering.

Although these layer silicates enter soils when the rock formation is exposed at the surface by erosion, these mineral species in such soils do not

re0ect chemical weathering, except by their relative resistance to weathering subsequent to exposure.

Deuteric alterations occur late in the crystallization of rocks (Bowen,

1922). In gabbro, for example, the alkali- and silica-rich interstitial



solutions react with the olivine, pyroxene, and amphibole, which typically crystallize out earlier, to form Mg-rich layer silicates such as chlorites and serpentines. They also react with feldspars to form sericite.

The complex mixture can be seen in the rock by means of a microscope.

Similarly, in the metamorphosis of rocks, chlorite may form from garnetbiotite shist and sericite from feldspars (Harker, 1932).

The term “hydrothermal alterations, ” following the usage of Shand

(1944), refers to the changes in rocks resulting from the movement of

hot solutions along well-defined channels or fractures within the earth.

A few examples of hydrothermal alteration are given to show the close

relation of this type of change to weathering. A quartz monzonite was

hydrothermally altered (Sales and Meyer, 1950) to sericite for 20 feet

from the fracture through which the hydrothermal solution circulated.

The potassium was furnished from orthoclase. An isothermal front in

the 300400” C. range is suggested as defining the limit of sericitization.

The next 20 feet were altered to kaolinite, with the orthoclase being unaffected. The next 30-60 feet were altered to montmorillonite. Gradation followed through chloritized rock biotite to the unaltered rock. The

degree of alteration thus varied with the distance from the hot solutions

in the fracture. The rock alteration described above was clearly caused

by solutions moving deep within the earth; yet, there are examples of

similar alterations caused by hot solutions near the surface (Schmitt,

1950). At Yellowstone Park, hot spring waters have converted volcanic

rocks to rocks containing beidellite and kaolinite, quartz and orthoclase,

and zeolites and calcite. The alterations there differ from the deeper

types in that most of the water involved is of meteoric origin warmed

by superheated steam from depth. It is thus a type of surface alteration

activated by a deep-seated heat source.

The layer silicates formed by alteration enter soils when the rocks

are exposed a t the surface, but these layer silicate species in no way reflect the climate in which the soil was formed. These examples are cited

as illustrative of the numerous occurrences in relatively young soils of

minerals of hydrothermal origin, which cannot be related to chemical


No11 (1936a, 1936b, 1 9 3 6 ~ )demonstrated hydrothermal synthesis of

several layer silicates in the laboratory in the system A1208.Si02.Hz0.

Kaolinite formed below 400” C., and pyrophyllite was formed above this

temperature. With the addition of alkali and alkaline earth metallic ions

to the system, kaolinite formed when the metallic ion concentration relative to alumina and silica was low, but montmorillonite was favored by

increasing ratios of these metallic cations, particularly that of MgO. As

the relative potassium content was raised, sericite was formed and then





feldspar at still higher potassium ratios and higher temperatures. These

experiments in the laboratory are of principal interest in the elucidation

of alteration mechanisms but may be indicative of chemical weathering


2. Pedochemical versus Geochemical Weathering

Viewpoints differ greatly as to the extent to which chemical weathering is identified with soil formation as distinct from parent material formation. The term “pedochemical weathering” is here defined to apply

to the chemical weathering which has taken place in the soil during its

formation and to that which is taking place in the soil currently. The

term “geochemical weathering” is defined to apply to the chemical

weathering which is considered to have taken place in the parent material

before the start of soil formation (granted that geochemical weathering

in a broader sense includes pedochemical weathering also). The general

term “chemical weathering” is employed to include both subdivisions

without distinction. Although i t is not always possible to distinguish

which of these two subdivisions of chemical weathering is primarily responsible for the origin of minerals in a given soil, and although the

distinction is sometimes a matter of definition of “soil formation,’’ different authors often wish to state a preference of viewpoint, and the use

of the two terms as defined is therefore extremely useful for clarity.

That physical weathering is largely completed during soil parent

material formation seems to be generally agreed upon. However, some

writers regard all chemical weathering as soil formation (pedochemical

weathering identical to geochemical weathering), whereas others restrict

the definition of soil to the upper portion of the geologic column which

is greatly influenced by the biological forces. Holding the latter view,

Marbut (1951) stated that “the soil is that layer of the earth’s crust

lying within the reach of those forces which influence, control, and develop organic life.” The geologic material beneath was termed the “soil

material’’ or “soil parent material. ’’ It is ordinarily unconsolidatedthat is, it is physically weathered. Beneath the soil material is consolidated geologic material. These views presented many years ago are adhered to by most of the soil scientists who followed. Kellogg (1943)

discusses weathering under the heading “building materials for soils” ;

this concurs with Marbut’s view of weathering as parent material formation. Physical, chemical, and biological “processes meet and mingle

in the surface film of the earth’s crust to form the soil which is neither

strictly physical nor strictly biological but a combination of both. ” Likewise, according to Robinson (1949, p. 60), “. . it is desirable to bear

strictly in mind the distinction between weathering, which produces the




material from which the soil is developed, and pedogenesis, which results

in the development of a soil profile.” As will be brought out subsequently, chemical weathering has had the major part in producing the

colloidal minerals present in soils and thus the properties of soils, regardless of whether it is considered to be largely pedochemical or geochemical.

a. Situations of 21zindrnal Pedochemical Weatherilzg. When the definition of soil is restricted to that portion of the geologic column upon

which biological forces are acting, many soils have developed with little

pedochemical weathering of minerals. The test of little pedochemical

weathering under this restriction is that the mineral content of the solum

be similar to that of the C horizon.

In general, chemical weathering of minerals is slow relative to the time

required for a n unconsolidated material to be called soil. As soon as

biological forces have taken hold on a fresh alluvium, a n azonal soil is

recognized; but the mineral weathering stage (Jackson et al., 1948) is

that of the soil or rock column from which the alluvium was derived.

The same applies to young soils (Regosols) on till, loess, or other unconsolidated deposits. In these cases, pedochemical weathering is minimal.

The accumulation of organic matter and other effects of biological activity, quite apart from weathering, are recognized as initiating soil formation.

Highly weathered geologic columns present a similar situation of minimal pedochemical weathering, if the term “soil” is restricted to that

portion of the geologic column upon which biologic forces have acted.

The main chemical weathering has, under this restriction, occurred in

the soil parent material formation, as forcefully advocated by Nikiforoff

(1949). The test of pedochemical weathering under this restriction is

that mineral content in the solum differ from that in the immediately

underlying parent material. In the Low Humic Latosols of Hawaii

(unpublished data of the authors and associates) there appears to be

little mineral weathering change within the range of biological forces or

immediately beneath. Pedochemical weathering is minimal, even though

the weathering stage is advanced. Deep in the column, the geochemical

weathering profile shows a transition from kaolin to basaltic rock, a t

about 25 feet.

Mineral analyses of the A and B horizons of several soils of intermediate weathering (Coleman and Jackson, 1946) showed similarity to

those of C horizons, and thus illustrate minimal pedochemical weathering

in soils of intermediate weathering stages. The occurrence of sodiumrich feldspars, quartz, muscovite, and even some biotite in weathered rock

of Switzerland led Niggli (1926) to the conclusion that the chemistry of

the soil was similar to that of the parent rock.



b. Siturctiolzs in Which Pedochemical Weathering I s Clearly Evident.

Situations in which pedochemical weathering is clearly evident include

the following :

1. Release of cations and anions from minerals in the soil, including

those taken up by plants and those lost by leaching.

2. Pedochemical deposition of sesquioxides and layer silicates (“clay

formation”) in the B horizons.

3. Pedochemical accumulation of sesquioxide and titaniferous surface

horizons and crusts.

4. Coincidence of the depth of chemical weathering with the depth of

biological activity and soil development.

The importance of pedochemical weathering in situation (1) both in soil

formation and in practical agriculture is clearly evident and is discussed

further in Section V. Ionic release is the first step in weathering reactions which lead to new weathering products.

To the extent that new minerals colloids are formed and accumulate

in the B horizon, situation (2) arises. The occurrences of the orterde

and ortstein of the Podzol and the clay accumulation in the B horizon

in the Cray-Brown Podzolic, Brown, and Planosol groups seem clearly

to represent mineral colloid formation by pedochemical weathering, in

addition to varying amounts of illuviation and residual accumulations.

Smith (1942) concludes that the development of soils from loess i n

Illinois “is toward the condition of a claypan soil, or Planosol.” Whiteside and Marshall (1944) concluded that clay formation and its distribution within the profile have been the chief result of soil formation in

the Putnam and Cowden soils (Planosols) of Missouri and Illinois. Allen

(1930) and Bray (1935) have described firm evidence of illuviation of

a colloidal montmorin series mineral (beidellite-nontronite) which “fills

cracks and cavities and has the cleavage plates oriented parallel to sides

of the deposit” (Allen, 1930). “The downward movement through

cracks and channels, as a result of water action, of discrete particles’’

was observed by Bray (1935). Bray in unpublished work (personal

communication) states that the same deposition of clay was observed in

worm holes. These observations support the concept of claypan formation by illuviation, as distinguished from clay formation by weathering

in place. Yet the formation of some montmorin series minerals by

weathering is suggested by the quantitative relationships, since there

is an increase with time in total.quantity of montmorin in the profile

(Bray, 1934). Bray (1937a, 1937b) concludes that weathering of

micas produces the expanding layer silicates and finally minerals of

more advanced (‘ ‘lateritic’ ’) stages, as indicated by the silica-sesquioxide ratios.



Accumulations of surface horizons and crusts in the form of ferruginous and titaniferous materials, situation (3), seem clearly to be the

result of pedochemical weathering in the Humic Ferruginous Latosols

(Sherman et al., 1949a). Numerous similar examples undoubtedly occur.

I n situation (4), geochemical weathering has not kept ahead of soil

formation and/or erosion. To the normal processes of geochemical weathing have been added the biological and other processes peculiar to soils.

When the weathering is shallower than biological forces normally extend,

the soil is termed a Lithosol. When soils form on bare rocks, the inception of soil formation is recognized when lichens or other low forms

of life begin to grow. As weathering loosens up some debris on the surfaces of rocks and weathering of some mineral constituents sets in, the

weathering is coincident with soil formation.

The coincidence of the depth of rock weathering with soil formation

arouses the interest of soil scientists in chemical weathering, because the

soil can form only because of the vast changes in rocks wrought by chemical weathering. Thus Jenny (1941) includes weathering as part of soil

formation, while recognizing that many soil scientists sharply distinguish

weathering from soil formation. Haseman and Marshall (1945) state

that a deeper soil layer than that just below the B horizon must be used

to obtain the basal Zr content for calculation of the soil volume changes

resulting from weathering. This further illustrates that the geochemical

weathering profile may be deeper than the solum and the soil profile as

usually defined.

It should be pointed out that chemical weathering in situation (4 )

is no more fundamental than, and often is little different from, that occurring in deeper columns beyond the solum. It is therefore not surprising to find a number of soil scientists who prefer to extend their

definition of soil to include a11 weathered zones, a viewpoint set forth

by Bnshnell (1944, 1950, 1951) and employed in deep profiles studied

by Weaver et al. (1949).

c. Chemical Weathering I m p o r t a n t in Determining Soil Colloids.

From the examples given, both those in which pedochemical weathering

is minimal and those in which it is clearly evident, it is uniformly apparent that chemical weathering is largely the avenue by which the colloidal minerals of soils are supplied. As already mentioned, deuteric

and hydrothermal alteration supplies some of the colloidal minerals

to soils. It is also evident that the minerals of coarser sizes persist in

soils after more easily weathered ones are removed by chemical weathering. It is not critical for our present review whether the definition of

soil includes the entire weathering profile (geochemical plus pedochemical) or not. The degree of chemical weathering and the processes of



it remain the same, whether classified ( a ) as being geochemical, i.e., occurring in the parent material (Kellogg, 1943 ; Nikiforoff, 1949 ;Marbut,

1951), or (b) as being pedochemical, i.e., occurring in the soil profile,

however deep (Jenny, 1941; Bushnell, 1944, 1951; Haseman and Marshall, 1945 ; Weaver e t al., 1949). I n either case, chemical weathering of

minerals somewhere, not necessarily in their present site, for the most

part determines the nature of the minerals present in soil colloids. If it

is held that most of the chemical weathering occurs in the parent material

formation, case (a), rather than in soil formation, then the attendant

conclusion must be accepted that the mineralogy of soil colloids (and

many important soil properties, both chemical and physical) is determined to like extent by the parent material.

3. Terminology for Weathering Products

The term “clay fraction” will include the particles less than 2 p in

equivalent diameter, whereas the term “mineral colloid” will refer to

the finer clay fractions, including particles less than 0.2, 0.1, 0.08, etc.,

p in diameter, according to the individual report. The term “clay

mineral” refers to minerals in the clay fraction, including both layer

and other silicates and feralitic clay minerals. I n the interest of preciseness and clarity, the term “layer silicate” is employed for that structural

group of silicates.

Because colloidal layer silicates are universally involved in a consideration of the results of chemical weathering, passing reference will be

made to the extensive reviews available on their structure and nomenclature. Modern concepts of the layer silicates minerals (those silicates

with closely knit layer units of crystal structure) have been reviewed in

detail by Grim (1942), Ross and Kerr (1931), Ross and Hendricks (1945),

and Gieseking (1949). The nomenclatural and broad structural relationships of layer silicates to other silicates are reviewed by Jackson et al.

(1949), and details of layer silicate structures and properties are reviewed in detail by Marshall (1949) and Brindley (1951). These topics

will therefore be excluded from present consideration.

In agreement with Ross and Kerr (1931), the term “kaolin” will be

employed as a general name for the 1:1 family of layer silicates, including kaolinite, halloysite, and their hydrates and polymorphic relatives.

Following the reasoning of Correns (1950,1951), the term “montmorin”

is employed to include the isomorphous series of 2 :1layer silicates, including montmorillonite, beidellite, sa.ponite, nontronite, and other

isomorphous relatives, all having the property of expansion to approximately 18 A. in the presence of glycerol. The terms “montmorin,”

‘‘montmorin series, ” and ‘‘montmorin isomorphous series” have iden-

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Chapter 4. Chemical Weathering of Minerals in Soils

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