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Chemistry of soil aluminumE. O. McLean a ba Ohio State University , Columbusb Ohio Agricultural Research and DevelopmentCenter , WoosterPublished online: 11 Nov 2008.

To cite this article: E. O. McLean (1976) Chemistry of soil aluminum,Communications in Soil Science and Plant Analysis, 7:7, 619-636, DOI:10.1080/00103627609366672

To link to this article: http://dx.doi.org/10.1080/00103627609366672

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COMMUN. IN SOIL SCIENCE AND PLANT ANALYSIS, 7(7), 619-636 (1976)

CHEMISTRY OF SOIL ALUMINUM

KEY WORDS: Soil acidity, lime requirement, phosphorus fixation, hydroxy-aluminum, solubility products

E. O. McLeanOhio State University, Columbus

andOhio Agricultural Research and Development Center, Woostera

ABSTRACT:

Better understanding of soi l aluminum has hud dramatic effects un the

Interpretation of many aspects of so i l chemistry. Aluminum is a Group III

element, metallic in nature, and exhibits both ionic and cuvaient bonding.

It is the most plentiful of a l l metallic cations of the earth's crust. It

is released from octahedral coordination with oxygen in minerals by weather-

ing processes. Once released, the trivalent Al ion assumes octahedral

coordination with six OH2 groups each of which dissociates a H ion in

sequence as pH increases. The resulting hydroxy-Al ions are absorbed to

the cation exchange capacity of the so i l . Here they polymerize un charged

surfaces and in the interlayers of the clay minerals obstructing both the

contraction of the clay latt ice and the exchange of cations. Soluble Al

is toxic to most plants, and reacts readily with soluble phosphates con-

verting them to relatively insoluble and plant-unavailable forms. Adsorbed

and polymerized aluminum affects actual lime requirements of so i l s by i t s

acidic nature and indicated lime requirements by its effect on the buffers

aApproved for publication as Journal Article No. 76-76 of OARDC,

Wooster, Ohio 44691.

619

Copyright © 1976 by Marcel Dekker, Inc. Alt Rights Reserved. Neither this wutk nur any part iniiy be reproducedor transmitted to any form or by any weans, electronic or 111ech4nit.1l, including photocopying, microfilming,»and recording, or by any information storage and retrieval system, wiiliuul pci mtuiun in writing from the publisher.

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620 McLEAN

of the lime requirement test. The level of exchangeable Al has been

suggested as an Index of lime requirement of acid soils, but this may be an

adequate Index for liming only on highly weathered soils.

1. INTRODUCTION: ALUMINUM. THE MAJOR ACIDIC METALLIC ELEMENT OF SOILS

Recent advances In the understanding of the chemistry of soil aluminum

haw had dramatic effects on the Interpretation of many physico-chemical

properties of soils. In fact, the clarification of Its role In acid soils

during (.he past 25 years has essentially made necessary a complete rewrit-

ing of such textbouk sections as: soil acidity and lime requirement, cation

exchange capacity, acid-base buffering, phosphate reactions, plant-toxic

substances, and mineral weathering. Probably no other area of specializa-

tion In soil science -- certainly not In soil chemistry, has witnessed

such great changes In such a short period of time.

Although Iron acts as an acidic element which adsorbs Oil Ions at very

low pH levels , Al Is the major acidic metallic element In the pll range

commonly found In acid soLls.

1. Attributes of Al.

Aluminum Is one of the Group III Elements consisting of aluminum,

gallium, Indium, and thallium which have three electrons In the outer shell.

Due to their larger size and smaller lonlzation potentials, Al, as well

aa Ca, In, and Tl, are much more metallic and Ionic than B which also has

three electrons In Its outer shell. The difference In these properties

has been used to Justify the separation of B from the other four elements

In Group III. Although elemental Al Is definitely metallic, it still

exhibits both Ionic and covalent character In Its compounds. The univalent

state occurs to some extent In the Group III elements, but the order of Its

occurrence Is: Tl > In > Ga > Al. Hence, the trtvalent state which

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CHEMISTRY OF SOIL ALUMINUM 621

predominates for a l l four elements Is e s p e c i a l l y prevalent in llie case of

Al .

The diameter of the Al atom Is O.57A. This s i z e i s almost exact ly the

same as the void space between s i x oxygen atoms of diameter L.Ü2A In a.

c l o s e packed arrangement. Hence Al la general ly fuund in s o - c a l l e d s i x f o l d

coordination with oxygen In the octahedral layer of many urluury and

secondary minerals . Although l l i s larger than Si ( d l . - Ü.3VA), A) atoms

are a l s o found to some extern as a s u b s d t u t e fur Si in fuurfuld coordina-

t ion with oxygen in the tetraheJral layer of such minerals as the micas

and the clay minerals .

2 . Roles AI Plays in S o l l s .

Besides the s tructural role Al plays in various primary and secondary

mineral», I t may a l s o e x i s t and function In several other ways most of

which adversely a f f e c t the s o i l as a plant root environment. When Al i s

re leased from the s tructure of minerals by weathering processes , the Al

coordinates with 6 OH2 groups. Each Oil2 group d i s s o c i a t e s a II Ion in

sequence as the pH Increases . Some of the re su l t ing Al , (OII)Al++> and

(OIO2AI Ions remain in the s o i l s o l u t i o n , more may be adsorbed as wuuumers

to the cat ion exchange s i t e s of the s o i l , and s t i l l more may be adsorbed

and then polymerized on the surfaces of the c lay minerals , or adsorbed and

then cooplexed by s o i l organic matter, l l ie impl icat ions of the ro les these

various forms of Al play In a f f e c t i n g the physico-chemical propert ies of

s o i l s and the ir e f f e c t s on plants grown thereon are dea l t with in s e c t i o n s

to fol low.

I I . THE ALUMINUM COMPONENT OF SOILS.

1. Composition of the Earth's) Crust and of Keutesentatlvfc S o i l s .

The aluminum contents of s o i l s are more meaningful when compared:

1) to the average contents of Al and other major cons t i tuent elements of

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622 McLEAH

die ear th ' s cruse , and 11) to the contents of M and other cons t i tuent

elements of representat ive s o i l s . Aluminum Is the second most p l e n t i f u l

oxide, the third most p l e n t i f u l element Including oxygen, and the most

p l en t i fu l meta l l i c element In the ear this crust (see Table 1) . I t occurs

most commonly In the primary minerals: micas, fe ldspars , and c r y o l i t e

(N«3AlFft), In the secondary clay minerals , and In ores such as bauxite

(A100II). Aluminum makes up 8 . 1 , 8 . 2 , 2 .S , and 0.47. of Igneous, sha l e ,

sandstone, and limestone rocks, r e s p e c t i v e l y .

Total M contents of surface s o i l s are general ly of the same magnitude

as that of the ear th ' s crust (see Table 1) ' . However, they are lower In

cases where the s o i l s have a predominance of sand due to tlie Influence of

sandy parent material or where the s o i l has l u s t much of I t s Al by Inten-

s ive weathering. The Maul clay s o i l from Hawaii Is an example where s i l i c a

weathered out and Iron and titanium accumulated, but M content remained

r e l a t i v e l y unchanged.

2 . Ucatherlne of AI In S o l l s .

a) Relat ion to II Ion Concentration. When H Ion concentration In the s o i l

so lut ion Increases to a pll of U or below, the hydronlum Ions (Oil3 ) formed

cause the d i s s o l u t i o n of Al from the edges of the mineral s tructure .

Upon re l ease , Al ions become s i x f o l d coordinated with oxygen In OU2 groups,

I . e . , A l ( - a i 2 ' )(,. These Oll2 groups are e s s e n t i a l l y Al subst i tuted

hydronlum Ions, the Al having replaced one II from each of s i x hydronlum

Inns (Oil3 ) . The Al subst i tuted hydronlum Ions, c a l l e d alumlnohexahydronlum

Ions, are of ten designated as Al*6H2o' ' , or simply as Al without the

(-CHI2^6. These aliimlnohydronlum Ions sequent ia l ly d i s s o c i a t e H Ions as

base Is added (pll Increases) leaving OH Ions In place of the OH. groups:

Al(OII 2Ü - 5 + ) 6 »• A l (OH 2

0 - 5 + ) 5 . (OH 0 - 5 - ) + H+ —*• Al (OH 2 ° - 5 + ) 4 - (OH 0 - 5 ") 2 +

" + 0 - 5 + ) 3 . ( o i | ° - 5 - ) 3 + H+ ?• A l ( O l l 20 - 5 + ) 2 - ( O l l ° - 5 - ) A + H* >

- 5 - ) 5 + l |+ * Al(OH°-5*)6 + H+.

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CHEMISTRY OF SOIL ALUMINUM 623

o

o

> O

5 S IH g

u ou >o. H

'Oe

Siuu

•A M O O O ^^O> I

o o o o

O »Hn O -

«4 «M .

rt O *<3 Z

•-< O in O

eo

•4 U*•• C

UOOOO WO

O <*i r *& m *4 «o o o o«Dvâ«*4OO*-*OOOOO

o co coO O CO vo -H O O

0-.00-JOJOOOOO

u au u

oo.

§

eo

< O O O rsmO f<4 t>JO CMO O O O O <*1-4>-<»)««J N U « N C OM < h U Z » ( l H B J W

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624 McLEAN

The aliimtnoliexuliydronliim Is a weak acid comparable In strength to acetic

lcld, the dissociation constant of the former being 1.08 x 10 compared to

1.8 x 10* for the latter.

If the above sequence of Al Ions Is written with the progressively

smaller number of Oil2 groups omitted as Is often done, we have the following

progression of Ions including the Insoluble uncharged A K ü H ) ^ with Increased

pH (see Fig. 2 ) :

Al(OH)2t" *

Al(Hin2+ > Al(OII) + I(H

Al(OH)3 *• Al(0H)4" + ll+

Al(l«l)4* * Al(OH) 52" + ll+

A1(OII)52* *> A1(OH) 6

3" + ll+

Some of. the aliimlnohexahydronlum Ions may remain In solution, but most

of them are adsorbed on soil cation exchange sites from which they are easily

displaced with ordinary unbuffered salt solution such as IN KC1, If the pll

Is below 5. If the pH Is higher, Oll-Al** ur (OII)2*Al+ is formed either

befure or after the Ions are adsorbed to the soil cation exchange sites.

These Ions polymerize as continuous layers or discontinuous Islands on the

Interlayer surfaces of clay minerals, or they complex with reactive groups

of su 11 organic matter, neither of which are exchangeable with unbuffered

salt solution (See Fig. 1). Since these Ions both as monomers and as

polymers are only partially neutralized, they are acid and hence require a

base such as lime for neutralization. Also, when polymerized on the surface

of clay minerals or complexed with organic matter, they are less accessible

for being quickly neutralized when lime Is added, and they obstruct the

exchange sites of the soil for exchange of other cations,

b. Sequential Buildup and Subsidence of At. Release of Al from mineral

structures beulns only after a sequence In which basic cations are weathered

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Hydroxy -AluminumInter- Layers

Acid Soil Limed Soil

\i i i i i i i i i i i i i i i i i i1/

Clay/ i i i i i i i i i i i i i i i i i i n

\i i i i 11 i ii i i i ii i i i i1 /

/ i i i i 11 i i 11 i i i i i i i i i\

Continuous/ I l l i r i l l l l l | l l 1 l l l l \ \ r\U - / M M I I M I N I M I I I l | \

I • -—'-r \ wn ,, -|j

\ l I I I M I I I I I I I I I I I I l/ l H \ l I I I I I I I I I I I I I I I I I l/

Clay j K/ I I I I I I I I I I I I I I I I I I l \ \ 1 / I I I I I I I I I I I I I I I I I I l \

Vermiculite

\ i i i i i 11 i i i i i i i 11 11 i / /

/ i i i i i11 i i i i i i i i i 11 i \

Islands"••• • • • • •

++++•+

Organic MatterComplexing

\ i i i i i i i i 11 i i i i i i 1 1 1 / /

/ i i i i i i i i i i i i i i i i i i i \

HO-C-N: —AI-OH

OH

\i 111111111111111111/

/ 111 1111111111 11111\

\l I I I I I I I I I I I I I I I I I 1/

/i i i i i i i i i i i i i i i i i i ry

OHHO-C-N

<?+ AI (OHL

inHSO

se

* Exchangeable CationFigure 1. Mechanisms by Which Hydroxy-Alumlnutn Alters CaCion Exchange Capacities of Acid Soils with Change

in pH. (pH-Dependent Obstruction of Permanent Charges of Clay and pH-Dependent Complex»tionwith Organic Matter).

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626 McLEAN

AI (ppm)

300

200

100

Al(0H)3_A

6 7 8pH of Solution

10

Figure 2. Solubility of Aluminum In Water Solution as Affected by pll ofSolution (Probable forms of aluminum lona In solution at thevarious plis are Indicated).

free of the mineral structure and are Init ial ly adsorbed to soil exchange

si tes and later are leached away with a resulting buildup of exchangeable H

Ions. At tills point, as mentioned previously, when II Ion concentration

reaches pll 4 or below, they are dissipated Into the mineral crystal wLth

equivalent amounts of Al released. The result of II disappearing Into

the mineral Is Increased pit of the soLl solution and OH-A1 ions formed,

which then polymerize or become complexed. In time, as the more weather-

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CHEMISTRY OF SOIL ALUMINUM 627

able minerals disappear or cheir Al become* less accessible, the rate of

release of Al declines. Sell! later, the initially accumulated At may

gradually be leached out or recomblned into stable mineral species such as

glbbaite. Eventually most of the Al remaining In the soil may be in the

form of relatively stable kaolin!te and very stable crystalline gibbslte.

In these fonts, Al Is no longer a source of II luns to the soil solution

and is less likely to fix as much phosphorus In unavailable forms as was

the case when much more was present in alumliiohydronluia form. Also, it is

not as likely to be toxic to plants as it formerly was.

3. Forms of AI In Solls: Relative Amounts.

a) Native Mineral. By far Che largest portion of the Al in most soils

is In clay mineral crystals in octahedral and to a lesser extent in tutra-

hedral coordination with oxygen. Even a relatively highly weathered soil

usually has the bulk of Its Al remaining as part of the aliuninosillcate

minerals (Table 1), with considerably less than 1% in Na-dlthionite

extractable Al.

b) llydroxy Al. This form of Al is relatively small, but may range from

none to the equivalent of 45 or more tons of lime per hectare (20 or more

T/A). The lacter would be equivalent to approximately Û.5Z Al in this form.

As already mentioned, under Intensive weathering conditions such as exist in

the Humid Tropics, much of the hydroxy-Al ends up as crystalline glbbsite;

and in such cases, a relatively high proportion ot Üie toLat Al may be in

this form.

c) Aluminum Phosphate. Much of the phusphorus applied to soils is tied up

as relatively insoluble Al-P. Also, during the weathering process, us the

two component elements, Al and P, are solubilized and brought together from

separate sources, Al-P Is formed. Most soils contain only trow a few kilo-

grams to a few hundred kilograms of Al-P per hectare of suit.

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628 HcLEAN

4. Solubility Criteria of Aluminum.

a) Relationship to pll. The solubilities of aluminum In water solution at

various solution plis are shown In Fig. 2 . These data were obtained by

measuring the concentration of Al remaining In solution when a definite

quantity of A^tSO^)} was dissolved In water In a series of flasks and

increasing Increments of N.iOH were added to bring the solutions to the

Indicated pll values. It Is evident that from about pH 4.7 to 7.5 the sol-

ubility of Al Is quite low. Tills Is the pll range where the Al Is precipitated

and remains su as lite relatively Insoluble Al(0H>3. Since, in this study,

the precipitate was not given opportunity tu age, It likely was amorphous

which accounts for detectable solubility which would have been even much

less had the precipitate progressed to crystalline glbbslte. Both below

pll 4.7 and above 7.5 the concentrations of Al remaining In solution Increase

rapidly and accelerate drastically to very high concentrations below pH 4.0

and above pll 9.2. The forms of Al Ions probably most prevalent at the

various pll s are Indicated (Fig. 2 ) . Due to the effect of the charges on the

clay and organic matter In a soil, the pH value is higher In the soil Itself

when Oil-A I ions are Inactivated as Al(OH)j than is Indicated here for solu-

tlons .

b) Solubility Products of Aluminum Compounds. The solubility products of

Al(0ll)3 an(i AIPO4 expressed as negative logarithms, (p), ate given In Table

2 along with analogous compounds of Fe and several Ca phosphates for com-

parison . In general, Ca phosphates arc more soluble than Al phosphates,

and the latter are more soluble than Fe phosphates. (When the negative

logarithms are smaller, the solubility is greater).

The relatively Insoluble phosphates are most Insoluble in acid soils

where the "common Ions" Al and Fe are most plentiful, or In alkaline soils

where the "common Ion" Ca predominates. An example of the consequence of

this Is given In a later section. Since the hydroxides of Al and Fe are

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CHEMISTRY OF SOIL ALUMINUM 629

TABLE 2

Solubility Products of Several Compounds Exprussud us pKsp Values.

Compound

A1(OII)3

Alr-O4 '2ll2Û

Fe(OH)3

FePO4

C« 3 (PO 4 ) 2

Ca4(PO4)3-3H2O

CaHP0.-2H,0

Eauation

pKsp * pAl

|>Ksp » pAl"*"1

pKsp - pFe3"*

pKsp • pFe

pKsp " 3pCa

pKsp - ApCa

pKsp - pCa^

for Calc.

• + 3 „ O U -

• + p i u w .

' + 3poir

' + pH2mA

"*" + 2pllR)

+ 3pllW)

' + pHPOi2

»Ksii

" + 2pUl'

" + 2\Ml'

l}' - 2pll+

-

uKsn

32.7

2a-32"

37

33-35*

6-11,"

9-l2Vi

6.6

Specific values depend on such factors as wheLliei' determination was made

by dissolution or precipitation, and what sulld-io-solutlun ratio or

concentration of precipitation reagents were used .

less soluble than the phosphates (Table 2), there is a slight tentlency for

Increased OH ion concentration - such as by lining, to Increase the avail-

ability of P by favoring AIPO4 dissolution and A 1(011)3 precipitation.

Approximately twice as high a concentration of P dissolves from AIPO4 in

Na-acetate buffer solution as dissolves from FePU^ at Che same solution pll

with the amount of each dissolved almost doubling between pll 3.8 and 7.1 .

Acid soils usually contain considerable exchangeable Al, AKOll)}, or poly-

merized hydroxy-Al in the lnterlayers of clays as well as varying amounts

of Fe oxides, but the activity of Al is generally greater than that of Fe

due to Its greater solubility. Hence, even though FePUj; is the least solu-'

ble and Fe may ultimately attract and hold most of the PO4 ions, formation

of A IPO4 may predominate initially due to greater activity of Al ions (see

Fig. 3, to be discussed later). Also, due to the greater solubility of

AIPO4 than FePO^, approximately the same concentrations of l^WJ^" can be

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Fe-P

McLEAH

Figure 3. Changes with Time of Al- and Fe-Pliosphatea Formed .fromPreviously-Added Soluble P In Three Soils Kept Moist.

maintained In water solutions at various p»s with AKOID3 present as with

Fe2l)> present . Since the solubilities of both Al and Fe phosphates are so

low relative to Ca phosphates., and since at least small amounts of free Al

and Fe oxides or hydroxides occur even In neutral soils, Al and Fe phosphates

are the main phosphates formed when soluble phosphates are added to such

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CHEMISTRY OF SOIL ALUMINUM 631

s o i l s . Only when Ca a c t i v i t y g r e a t l y exceeds those of AI und Ke docs amount

of Ca-P formation exceed Chose of AI -P mid Ft:-I' In s u i U .

5 . Alum Ilium S u r f a c e s .

a) Chemical Adsorption of Phosphates. IL lias been suggested thai phos-

phorus may be fixed In acid soils by chemical adsorption . This Is a special

case of precipitation which essentially follows the solubility product

principle. However, the cations concerned teiuaiu as constituents of the

soil mineral or of Al and Fe hydroxide or oxide components, yet they react

with the phosphate because of residual positive charges on the surfaces.

This Is different from the conventional concept of precipitation where the

Ions In question are Initially active but react to form relatively Insoluble

compounds. The limitation of solubility and hence ionic activity imposed by

solution pit (see Fig. 2) Is not Imposed on chemical adsorption to these

mineral or hydroxy oxide surfaces .

b) Mechanical Obstructions to Cation Exchange. The polymerized hydroxy-Al

and neutralized AHOIO3 mentioned In previous sections which forms Interlayer

surfaces In clays, called chlorltlzed vennlcullte or montiuoillloulte, Is a

mechanical obstruction which prevents the free exchange of one cation for

another (Fig. 1). Instead of individual cations of one or more specific

elements being adsorbed to the negative charges of the clay surfaces from

which they may be readily displaced by cations of another element, the poly-

merized hydroxy-Al layer Is a multlvalent-catlonlc layer which Is too large

and strongly adsorbed to be displaced In normal catlou exchange. It neutral-

izes the negative charges of clay minerals preventing other cations from being

adsorbed to and desorbed from these si tes.

The polymerized hydroxy-Al layers also obstruct the collapse of clay

lattices upon drying preventing entrapment of cations such as K+ and Nll^+.

In addition, they may Interfere with the release of K Ions from clay

lattices by weathering processes.

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632 HcLEAN

c) Reaction Sites (or H or Oil. Since the polymerized 0I1-A1 layers are

capable of releasing additional H Ions when pH Is Increased or of adsorbing

H Ions when pll Is decreased, they act as a buffer system (Fig. 1). Further-

more, since llie number of H Ions released reduces the number of posLtlve

charges on the (MI-AI layer or island, the number of negative charges on the

clny now free for adsorbing additional cations Increases. The effect of this

Is to Increase the cation exchange capacity uf such soils by liming '

lit. ALUMINUM RELATIONSHIPS TO PIANTS AND L1HE REQUIREMENT OF SOILS.

1. Toxlcltv to Plants.

Aluminum appears to be toxic to plants In several ways. It probably

has adverse effects on the protoplasm of the cells. It has been observed

to cause precipitation of phosphate just Inside cell walls. Roots and tops

alike are stunted severely In the presence of toxic levels of Al. The effect

on the roots Is further characterized by a disorganization of the root cap,

Q

the root apex, and the vascular elements . Different species or varieties

of crops, as well as different cultlvars of a given species or variety, vary

markedly In response to the effects of toxic levels of Al 1 . Furthermore,

q

Al-sensltlve varieties lower the so i l pll near the roots . Hence plant-

variety differences In Al tolerance may be both because of differences In

Induced levels of soluble Al around theLr roots and because of differences

In specific tolerance to a given concentration of Al.

2. Effects of Aluminum on Lime Requirement.

a) Effect of Al on SolL Acidity per se. Soll acidity and lime requirement

of acid so i l s Increase with amount of OH-A1 they contain and with the degree

to which 0H-A1 deviates from neutrality, I . e . , Al(0ll)3. Some so i l s may

have as much Oil-AI as the equivalent of a lime requirement of 45 tons CaCO]

per hectare (20 T/A) for a seven Inch plow depth.

b) Effect of Al on Buffer-Indicated Lime Requirement of Soi l s . The

relative degree of depression of pH of the buffer used to Indicate lime

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CHEMISTRY OF SOIL ALUMINUM 633

requirement of acid s o i l s per unit of t o ta l a c i d i t y decreases In Lite fullow-+ i i i

Ing order H > Al > Oll-AI inonouier > Oïl-Al polymer. Hie consequence of

this sequence Is chat a buffer-indicated lime requi remette CesC ul" a highly

Weathered acid soi l saturated with Olt-Al (essentially devoid uf bases) will

be considerably less than le would be If the same soil were leached with HC1

acid and hence made H Ion-saturated . When the buffer-Indicated 1 lute

requirement test has been adjusted to reJd correutly with UH-Al pulytuer

present, It will not read correctly, If the predominant form of the acid

Is changed to something e lse . This Is one reason why It Is not easy to

adapt a given method to widely different soil conditions.

5. Hole of Aluminum in Iliosuluirus Fixation.

a) Aluminum Phosphate Fraction. It was pointed out earlier that when

soluble P Is added to acid so i l s , much - perhaps most, of the added P ul l l be

converted toAl-P. Initially this form of P Is relatively unstable and thus

available to plants. In time, It becomes much leas available when It either

crystall izes to A1PO >2H2O with a solubility product (Ksp) of 10"-10 (see

Table 3), or reverts Co, FeP (see Fig. 3). An example of the "coiiauon Ion"

effect of Al from Che soi l on the concentration of H lti " from A1 ft)/,-

fo11ows:Al(On)2-ll2PO4 v •* Al(l)ll)2+ + "2™**'

Ksp - IO°° - (A1(OH)2')

(U2K)<,") - VlO"30 - 10'1 5 from

But If the soil has a concentration of (Al(üll)2+> - 10"5 then

(U2PO4") - IP'3 0 - 1O"25 which Is 1010 fold less concentrated than It was10-5

from A IPO4 without the "common Ion" present. It has been shown that AIPU4

Is a reasonably good source of P for plants in the absence of the Al as a

"common Ion" such as In sand, while It Is a very poor source of plant-avail-

able P In a soil high In exchangeable Al1 2 .

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634 McLEAN

b) It was mentioned e a r l i e r : I) that the combination of r e l a t i v e a c t i v i t y

of AI Ions or surfaces In acid s o i l s and of the r e l a t i v e I n s o l u b i l i t y of

A1-1' formeil caused comparatively large amounts of Al-P to be formed I n i t i a l l y ,

and II ) tli.it with time Al-P nuiy then revert to Fe-P due to s t i l l lower s o l u -

b i l i t y uf the l a t t e r . Tilts Is very we l l l l u s t r a t e d In F ig . 3 where the gain

In Fe-t* in 120 days Is general ly of the same order of magnitude as the l o s s

in Al-P [or three d i f f erent kinds or condit ions of s o i l s . The same amounts

of so luble P lind been iidduil to each s o i l , l ite concentrations of Ca-P and

of other ("onus of I* changed l i t t l e with time. The I n i t i a l data (I day)

are for increases In concentrations of Al-P and Fe-P from the added P, and

the curves indicate the changes in Fe-P and Al-P upon standing under moist

condi t ions .

It i s evident that the "Red" s o i l I n i t i a l l y fixed s imi lar amounts of

P both as Al- and as Fe-P and changed r e l a t i v e l y l i t t l e with time (Fig . 3 ) .

Probably the a c t i v i t y of Fe Ions or lurfaccs was s u f f i c i e n t to f ix a re la -

t i v e l y large amount uf P, and Increases with time were not great . "Common

Ion" e f f e c t s from Al on the Al-P solubLll ty might have contributed to the

s luggish conversion to Fe-P. Also , If r e l a t i v e l y few addit ional Fe Ions

or surfaces e x i s t e d , not much conversion tn Fc-P would be expected with time.

4 . Tests for Reactive Aluminum In S o i l s ,

a) Exchangeable Al . Hie r e a c t i v i t y of Al In s o i l s varies with the form

In which It occurs , decreasing In order from w.iter-soluble Al or 0II-A1i i i

monomers > adsorbed (exchangeable) Al or Oil-A I monomers > CM-A1 polymers

> A1 (Uli)3 > AI in coordination with oxygen In mineral crystals. Since solu-

ble Al may be so lnw In concentration that It is hardly measurable except

In highly weathered soi ls of low pll, and since that In the mineral crystals

can be considered as a relatively non-reactive form, tests for reactive Al

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CHEMISTRY OF SOIL ALUMINUM 635

are general ly designed to remove a fract ion from intermediate forms l i s t e d

above such as the exchangeable and various Oil-A I cx trac tab le forms.

Exchangeable Al Is normally that which i s displaced by leaching wich an

unbuffered sa l e s o l u t i o n such as IN KC1. Since appreciable amounts of

exchangeable Al are tox ic Co many plane s p e c i e s , iC has been suggested that

th i s Al be neutral ized by liming the s u l l a t tlie rale of a mult ip le uf the

equiva lents of exchangeable A i d ! , to 2) which depends on (.lie s e n s i t i v i t y of

14the crop to Al . Such rates uf lluie a p p l i c a t i o n are low r e l a t i v e tu the

amount required to bring the s u i l to pil 6 . 5 . In f a d , exchangeable Al may

be p r a c t i c a l l y e l iminated when s o i l |JI i s Increased to a value as low US 5 . 5 .

This may be an adequate pll l e v e l fur uplliuum plant growth under highly

weathered cond i t ions , but considerably l e s s than adequate under l e s s weath-

ered condi t ions .

b) Extractable Al . One normal ammonium a c e t a t e of pll 4 . Ö has been used

rattier widely as an extractant for Al . It removes both exchangeable and a

fract ion of the 0H-A1 polymerized on s u i l surfaces and uf chat cuiuplexed

8 16by organic matter . This fraction of Al evidently is sufficiently

active to react with soluble phosphates

Sodlum-dlthlonite-clcrale buffer solution Is often used lo extract

free (non-mineral) iron from suits . This extractant, when buffered at

pll 4.6-4.7, probably removes a l l of those forms uf Al extracted by IN NII OAc

of pll 4.8 plus much of the Al oxides or hydroxides coining the surfaces of

soil particles. This form of extractable Al can be used <is an Index o.f

degree of weathering and of accumulation uf non-structural Al in sui ts .

All of these extractants are tools for examining an aspect of suil

chemistry which, when properly used and the results carefully Interpreted,

reveal Co us yec another physico-chemical attribute or the soil which we seek

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636 McLEAN

to understand. The picture Is c l e a r e r , but such t o o l s may help sharpen up

some of the remaining blurs on the canvas.

REFERENCES

1. N. T. Coleman and C. W. Thomas, Soi l S c i . Soc. Amer. Proc. 28:187(1964) .

2 . M. L. Jackson, "Chemistry of the S o i l " Amer. Chem. Soc. Monog. S e r i e s ,Reinhold Pub. Co. , N. Y. pp. 71-141 (1964) .

3 . N. C. Brady, "The Nature and Propert ies of S o i l s " Eighth Ed. TheMacmillen Co. pp. 19-39 (1974) .

4 . H. L. Jackson, Proc. 7th Int . Cong. So i l S c i . Madison, Wisc. 2:445(1960) .

5. O. C: Maglstad, Soi l S c i . 20:181 (1925) .

6. S. C. Chang and M. L. Jackson, So i l S c i . Soc. Amer. Proc. 21:265 (1957) .

7. P. B. Hsu, So i l S c i . Soc. Amer. Proc. 28:474 (1964) .

8 . E. O. McLean, D. C. Relcosky, and C. Lakshmanan, So i l S c i . Soc. Amer.Proc. 29:374 (1965) .

9 . A. L. Fleming and C. D. Foy, Agron. J. 60:172 (1968) .

10. C. D. Foy and J. C. Brown, So i l S c i . Soc. Amer. Proc. 28:27 (1964) .

11. W. K. Hourlgan, Ph D Diaser ta t ion , Ohio Sta te Univers i ty , Columbus,Ohio. pp. 1-101 (1960) .

12. E. O. McLean and R. W. Wheeler, S o i l S c i . Soc. Amer. Proc. 28:545(1964) .

13 . U. N. Hazra, Ph D D i s s e r t a t i o n , Bldhan Chandra Krishl Viswa Vidyalaya,Kalyani, Nadi, West Bengal, India.

14. E. J . Kamprath, S o i l S c i . Soc. Amer. Proc. 34:252 (1970) .

15. E. O. McLean, Crops and S o i l s Science Soc ie ty of Fla. Proc. 31:189(1971) .

16. D. R. Keeney and R. B. Corey, S o i l S c i . Soc. Amer. Proc. 27:277 (1973) .

17. D. E. Coff in, Can. J. Soi l S c i . 43:7 (1963) .

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