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II. Aerobic Soils: Organic Acids and Phosphorus Sorption

II. Aerobic Soils: Organic Acids and Phosphorus Sorption

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ORGANIC AMENDMENTS AND PHOSPHORUS SORPTION



147



Table I

Low-Molecular-WeightAliphatic Organic Acids Identified in the Soil Solution

from a Pomona Series Soil Supporting Pine Stands in Alachua County, Florida

(Adapted from Fox and Comerford, 1990)

Stand



Horizon



Oxalic



Formic



Plantation I



A

Bh

A

Bh

A

Bh



193

362

62

683

12s

313

293

1043



19

32

63

148

89

45

64

77

tr

19



Plantation 2

Plantation 3

Plantation 4

Natural I

Natural 2



A



Bh

A

Bh

A

Bh



Natural 3



A



Natural 4



Bh

A

Bh



358

142

98

460

154

733

198

465



114



6

151



I37

9

5



Citric



Acetic



Malic



Succinic



tr



tr

tr

tr

tr

tr

tr

tr



tr

tr

tr

tr



-



tr



-



tr



tr



tr



Note. All values are in pM.

Trace amounts: peak identified at appropriate retention times but peak area not integrated.

No peak identified at appropriate retention time.



with soil type and organic amendment treatment. Stevenson (1967) reported

values of formic acid ranging from 2.5 to 4.4 mM in soil solution.



B. COMPLEXATION

REACTIONSWITH METALS

The formation of metal complexes by organic acids for such metals as A1 or Fe

which readily react with orthophosphate would increase orthophosphate availability to plants. Hue et al. (1986) demonstrated this by showing that organic

acids detoxified A1 effects in relation to plant growth, thus providing evidence of

reduced A1 activity. They further found that organic acids varied in their affinity

to form complexes and had stability constants of AI-organic acid complexes

which decreased in the order citrate > tartrate = malate > salicylate.

Carboxyl and hydroxyl functional groups are important in reactions between

metals and organic acids (Huang and Violante, 1986). An example of this is

shown by chelation of Fe with citrate with the following complexation.



148



F. NAMUREMYE AND R. P. DICK



In soils, a wide range of biochemical compounds have been identified in

forming complexes with metals. These have included aliphatic acids, amino

acids, phenolic acids, hyroxamate siderophores, 2-ketogluconic acid, and polymeric phenols (Stevenson, 1994).

The ability of FA to form stable complexes with metal ions can be attributed to

their high number of oxygen functional groups such as carboxylic, phenolic,

alcoholic, and enolic OH, (Tan, 1986). Schnitzer (1969) distinguishes two types

of reactions between humus and metal ions or hydrous oxides. A major reaction

involves the simultaneous acidic COOH and phenolic groups, and a secondary

reaction only involves the less acidic COOH groups. The complexation can be

visualized as follows.

AP+



+ RCOO-



* RCOOAP+



(7)



C. COMPETITION

FOR SORPTION

SITES

The adsorption of organic compounds such as humic acid (HA), FA, and other

organic acids on soil minerals such as A1 and Fe hydrous oxides or other clay

minerals has been established and may lead to a competitive adsorption for sites

of orthophosphate fixation but this effect varies with soil type. For example,

Appelt et al. (1975) reported that HA or FA did not decrease P sorption on a

volcanic soil at certain concentritions of HA or FA (Tables I1 and 111) because

new HA-AI(0H)y complexes form and become new source of sites of P fixation.

On other soil types organic compounds have shown a consistent reduction of

adsorption sites. Hajra and Debnath (1985) showed that HA applied to soil

decreased Fe bound P in soil treated with P fertilizer (Table IV), indicating that

HA may release P from Fe compounds and/or prevent P precipitation. Evans and

Russell (1959) showed that FA from a podzol was sorbed on lepidocrocite and

goethite. Sibanda and Young ( 1986) studied competitive adsorption on goethite,

gibbsite and tropical soils and reported that HA and FA competed strongly with

orthophosphate for the sites of adsorption at low pH, and that HA decreased

orthophosphate sorption by these soils (Figs. 3 and 4). Measurements showed

that when orthophosphate adsorption was increased there was virtually no release

of either HA, or FA into solution. Apparently the effectiveness does not rely

exclusively on the occupation of adsorption sites by carboxyl groups but also by



ORGANIC AMENDMENTS AND PHOSPHORUS SORPTION

Table I1

Effect of Humic Acid (HA) Adsorption on P Adsorption by a Puerto

Octay Subsoil (Adapted from Appelt el al., 1975)"

HA added

(gP

~



HA adsorbed

(gP



P added



(mmotP



~



~~



P adsorbed

(mmoI)b



pH at equilibrium



6.0

12.0

17.5

23.0

28.7



5.7



~



0.0

0.0

0.0

0.0

0.0



6.0

12.0

18.0

24.0

30.0



3.8

3.8

3.8

3.8

3.8



6.0

12.0

18.0

24.0

30.0



~



-



I .4



5.8



5.9

6.0

6. I



5.8



6.0

12.0

17.2

23.0

28.5



1.4



I .4

1.4

1.4



5.9

6.0

6. I

6.1



( I Each number represents the mean value of two determinations with a

fluctuation

h Per 100 g of soil.




Table 111

Fulvic Acid (FA) Adsorption (meq1100 g soil)" and Effect on

P Adsorption (mmo1/100 g soil) by a Puerto Octay Subsoil,

When Increasing Amounts of FA and 20 mmol of P Were

Added at pH 4.0 or 5.5 (Adapted from Appelt eta/., 1975)h

__



FA adsorbcd



P adsorbed

___ -



FAadded



pH40



pH55



pH40



pH55



0.0

5.0

10.0

15.0

20.0

25 .o



-



-



20.0

20.0

20.0

20.0

20.0

20.0



20.0

20.0

20.0

20.0

20.0

20.0



S .0



10.0

15.0

20.0

25 .0



5.0

10.0

10.0

13.0

16.0



Meq represents the amount of NaOH solution necessary t o

titrate a dialyzed sample of FA from pH 35. to 8.0.

Each number represents the mean value of two determinations

with a fluctuation of <5%.



149



F. IYAMUREMYE AND R. P. DICK



150



Table IV

Effect of Humic Acid on the Transformation of Added Phosphate (Adapted from Hajra

and Debnath, 1985)

Increase in P in various fractions (mg P kg-l)



NH.,CI sol. P

Days of

incubation CcJ HA HA-C

~~



AI-P



Fe-P



.



C



HA



HA-C



C



____



HA



HA-C



76

80

80

90



67

72

72

72



-9

-8

-8

-18



75

76

78

78



70

73

75

75



C



Ca-P



HA



HA-C



-



Red soil



30

60

120



180



tr

tr

tr

tr



14



14



I1

8

9



11

8



tr

tr

tr

tr



II

10



II



11

10



II



tr



6

4



6



9



88

87

84

75



90

88

87

84



2

1



3

9



8 8

9 10

9 10

8 12



0

1



-5

-3

-3

-3



6

4

4

4



10

7

8

8



4

3

4

4



-12



7

8

8

10



10

10



3

2



9

12



2



1



4



Laterite soil



30



60

120

180

30

60

120



180



tr

tr

tr



4

8



10

10



4

4



8



68 65

66 62

64 60

64 60

84

67

56

51



92

80

71

68



-3

-4

-4

-4



Alluvial soil

7

97 85

13

118 101

21

125 101

125 I08

17



-17

-20

-17



1



C, control; tr, trace; HA, humic acid-treated soil; HA-C, net extractable P for given P fraction

(humic acid-treated soil minus untreated control).



an unfavorable electrostatic field generated around adsorbed HA molecules.

Also, part of the energy of HA must be physical (van der Waals) in nature and

would not be involved in competition with orthophosphate (Sibanda and Young,

1986).

Organic acids such as citrate, malate, and oxalate can compete with P for

sorbing sites (Struthers and Sieling, 1950; Hue, 1992; Violante and Gianfreda,

1993). Violante and Gianfreda (1993) have studied the competition between

orthophosphate ions and oxalate and showed that on a montmorillonite (chloritelike) complex, more orthophosphate than oxalate was sorbed in a system containing a constant amount of orthophosphate even when the initial concentration of

oxalate was higher than that of orthophosphate. Of the sites on the clay mineral

that were available for adsorption by both anions, 51 to 79% of the sites were

occupied by orthophosphates. Many sites were highly specific for orthophosphate, whereas others were common to both oxalate and orthophosphate, but

these sites still had greater affinity for orthophosphate than oxalate. Yet other



ORGANIC AMENDMENTS



0.2 0.4



AND PHOSPHORUS SORPTION



0.6 0.8 1.0



1.2



1.51



1.4



PHOSPHATE IN SOLUTION (pg P cm-J)



PHOSPHATE IN SOLUTION (pg P mJ)



Fcgure 3 Phosphate adsorption isotherms on Salisbury (Harare) 5E2 series topsoils (0-20 cm),

conducted at the measured pH of the soil in 0.1 M NaCl and at 25°C. (a) Salisbury (Harare) 5E2

series. and (b) Marandellas (Marondera) 7G2 series. Four levels of hurnic acid are compared: A, 0;

B , 0.4%; C. 1.6%; D. 3.0% (adapted from Sibdnda and Young, 1986).



sites were specific for oxalate. Maximum reduction of adsorption of orthophosphate occurred when oxalate was added before orthophosphate addition and,

conversely, the minimum occurred when orthophosphate was added first. Addition of phytic acid to soil strongly reduced P sorption whereas cinnamic and

benzoic acid had no effect on P sorption (Evans, 1985).

Cations bound to P by organic acids resulted in the release of P in the soil

solution. In the case of oxalate, ligand exchange sites are complexed by the

following reaction.



1



1

,I



,AI:i



1



u-L



+



2OH-



F. IYAMUREMYE AND R. P. DICK



I52

f



6.0 r



P



(a)



0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

PHOSPHATE IN SOLUTION (pg cm-7



U



Y



5.0



D



a:



0



$

a



4.0



3.0



W



48

0



2



2.0

1.0

d



0



0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2



PHOSPHATE IN SOLUTION (pg ~ r n - ~ )



Figure 4 Phosphate adsorption isotherms o n synthetic geothite, conducted in 0.1 M NaCl and at

25°C. (a) at pH 4 and (b) at pH 7 . Four levels of humic acid are compared: A , 0; B , 11.8%;

C, 47.1%;D, 88.2% (adapted from Sibanda and Young, 1986).



The presence of certain organic anions can greatly reduce the amount of

orthophosphate precipitated. Swenson et al. (1949) showed that humus and

lignin had the ability to prevent orthophosphate fixation (Fig. 5). They concluded

that humus and lignin were effective in replacing phosphate from the basic Fe

phosphates, probably because of the formation of stable compounds or complexes between the active Fe and humus or lignin. Several organic anions including aromatic hydroxy acids and aliphatic hydroxy-organic acid were effective in

preventing orthophosphate from combining chemically with A1 or Fe or in replacing the chemically combined orthophosphate.

The pH has a major effect on competition between organic acid and P for

adsorption sites (Violante et af., 1991; Hue, 1992; Lopez-Hernandez et a / . ,

1986; Nagarajah et al., 1970). Struthers and Sieling (1950) showed that citrate

decreased P sorption at any pH (Fig. 6 ) and concluded that the most effective

acids were citric, oxalic, kartaric, malonic, malic, and lactic acids. Generally



ORGANIC AMENDMENTS W D PHOSPHORUS SORPTION



153



L



0.4



I



I



I



0.2



0.4



L



0.6



0.8



I



1.0



1.2



ORGANIC MATTER ADDED (9)

Figure 5 Ability of humus and lignin to prevent phosphate fixation. Three milliequivalents of

Fe,O,-Fe was reacted with I meq phosphate with or without organic amendments. Saloid-bound

phosphate is amount of phosphate in solution when only Fe,O, and phosphate were in solution

(adapted from Swenson er a / . . 1949).



3



4



5



PH



6



7



8



PH



Figure 6 Influence of pH on effectiveness of lactate. malate. citrate, and a-aminopropionate

ions in preventing phosphate precipitation by iron ( a ) and by aluminum (b) (adapted from Struthers

and Sieling. 1950).



154



F. NAMUREMYEC AND K. P. DICK



these acids were the most effective complexing agents at a more acid pH which is

fortuitous because this coincides with soil pH values when Fe and A1 are most

active in phosphate sorption, thus helping to prevent fixation.



D. DISSOLUTION

OF PRECIPITATED

PHOSPHATE

AND

FORMATION

OF SOLIDPHOSPHATE

PHASES

Dissolution of precipitated phosphate by organic acids is another probable

mechanism that makes P available when organic residues are added to soils. The

solubilization of phosphate by organic acids involves the reactions of carboxyl

and hydroxyl functional groups of organic acids with Al, Fe, and Ca cations to

form complexes.

The effects of organic acids on P availability starts at the early stage of soil

formation. Organic acids are capable of solubilizing the phosphate minerals

during the weathering process and therefore can release phosphate bound in these

minerals. Bolton (1882) stressed the role of organic acids in the weathering

process of rocks and subsequent release of nutrients. This ability of organic acids

to solubilize P from minerals has been used to determine plant available P in

different soils (Bolton, 1882; Morgan, 1941).

The reaction of organic acids with soluble A1 results in the reduction of the

activity of A1 and Fe and a corresponding increase of P activity in the soil

solution as shown in the case of variscite which controls A1 solubility as follows

(Lindsay ef al., 1959).

Al(OH),H2P04 & A13+ + 20HpAl



+ H,PO,'-



+ 2,OH + ,H2P04 = pK,,



=



30.5



(9)

(10)



Because the K s p remains constant, a decrease in A1 with no change in pH

results in a release of phosphate. This means that additional variscite may solubilize. The reaction above shows that organic acids may prevent P precipitation

or increase P concentration by solubilization of minerals. Organic acids also may

react by ligand exchange with orthophosphate sorbed on the sites of Al, Fe, or Ca

oxides and hydrous acids (Lopez-Hernandez et al., 1979). The kinetics of release

seems to be related to the type of organic acid. Traina et al. (1986) reported a

more rapid initial release of orthophosphate from an acid montmorillonitic soil in

the presence of citrate compared with formic acid. The ligand that formed more

stable complexes with A1 increased the rate of orthophosphate release.

Recently, Fox et al. (1990) proposed two mechanisms for the role of organic

acids in solubilization of P: (i) replacing P sorbed on metal hydroxides (Stumm,

1986), and (ii) dissolution of P sorbed at the metal-oxide surface (Stumm and

Morgan, 1981; Martell et al., 1988).



ORGANIC AMENDMENTS AND PHOSPHORUS SORPTION



15s



The organic compounds may change physical properties of crystalline A1 or

Fe. Schwertmann (1966) demonstrated that crystallization of A1 and Fe compounds was inhibited in the presence of HA and FA. Inoue and Huang (1983,

1984) demonstrated that complexing Al prevented the growth of A1 silicate

crystals because of the disruption of the hydroxyl bridging, which is indispensable for the polymerization of A1 ions.



E. EFFECTSON SURFACE

CHARGE

Organic acids can have a significant effect on surface charge and precipitation

reactions. Our review focuses on the literature for this topic as it relates to P

sorption. Other reviews, such as Huang and Violante (1986), provide an in-depth

discussion of the interaction of organics with soil minerals.

Soil organic matter content affects the degree of surface charge. Indirect

evidence of this relationship is provided by Moshi et al. (1974) who studied the

distribution of charges and orthophosphate adsorption characteristics through

entire soil profiles. They found that the negative charges were highest in the

surface horizon of forest soils, and within a profile the negative charge decreased

with depth, becoming almost constant below 30 cm in the cultivated profile and

below 60 cm in the forest soil. Positive charge decreased with increasing organic

matter which corresponded to a decrease in orthophosphate sorption. However,

they suggested that the two phenomena are probably independent of each other,

though this was not shown experimentally.

Organic matter increases negative charges which may contribute to the decrease of P sorption by a repulsion of orthophosphate ions by a negatively

charged surface (Moshi et a / . , 1974). Perrott (1978) found that treatment with

extracts of humified clover caused the net surface charge to become more negative. Also, the positive charge on A1 silicates is reduced by organic treatments.

The effect of organic treatment on charge characteristics of allophanic soil clay

was similar to that of aluminosilicate clays. Perrott (1978) explained that the

increase in negative charges in some soils was due to the selective adsorption of

organic molecules containing a higher concentration of anionic groups. In the

samples where this explanation holds, these authors suggested that the increase

in negative charges was due to the removal of charges balancing Al hydroxy

species, corresponding to the removal of A1 by organic matter from the structural

models described by Cloos et al. ( 1968) and de Villiers (1971). Also, adsorption

of organic matter at positively charged sites could expose negative charges by the

conversion of the charge balancing A1 hydroxy polymers to insoluble organomineral complexes, similar to insoluble hydroxy-Al complexes (Hsu, 1968,

1979; and Huang and Violante, 1986). The organic matter may change surface



156



F. WAMUREMYE AND R. P. DICK



charges through the organic acids produced during mineralization or by their

action on pH and exchangeable Al.

Easterwood and Sartain (1990) reported a change in the nature of surface

charges of soil samples previously coated with Fe oxides or with clover. Samples

treated with clover had increased negative charges and resulted in more solution

orthophosphate and less retention of P than in soil treated with Fe oxides.



F. PHYTOAVMLABILI-I-Y

OF PHOSPHATE

To confirm the increased P availability in the presence of organic acids, some

bioassays have been conducted to determine the effects of organic acids on P

phytoavailability. Hue (1991) showed that dry matter production of lettuce was

increased when inorganic P was added with organic acids such as malic or

protocatechuic acid (Fig. 7). Hue (1992) further concluded that the efficiency of

P fertilizer would increase significantly when applied along with organic acids or

particularly when acid-producing materials such as green manure or animal

wastes were incorporated into soil. Moshi et al. (1974) reported that the amount

of fertilizer P required to attain 0.2 mg liter' in the soil solution decreased from

90 to 22 rng kg-1 in an Oxisol when soil C increased from 3.8 to 6.5%.



111. AEROBIC SOILS: PLANT RESIDUES

ANDANIMALMANURES



A. SOILPH

Addition of organic residues can cause a significant increase in soil pH (Sharpley et al., 1984; Hoyt and Turner, 1975). Hue (1992) and Mnkeni and MacKenzie (1985) obtained higher pH values in soil solutions treated with organic

residues than in those amended with CaCO,. Table V provides an example of pH

changes due to organic amendments on some acidic soils from Oregon and

Rwanda (lyamurernye er al., 1995a).

The pH changes have been attributed to the high concentrations of basic

cations in the organic matter used (Hue, 1992; Hoyt and Turner, 1975) and to the

reduction of higher valence Mn oxides (Hoyt and 'Ibmer, 1975) or Fe oxides and

hydrous oxides (Hue, 1992) in soils. The latter reaction leads to the self-liming

effects observed in submerged sails caused by oxido-reduction reactions:



* Mn2+ + 20HFeO(0H) + e- + H,O * Fez+ + 30H- .

MnO,



+ 2H+ + 2e-



( 1 1)



(12)



ORGANIC AMENDMENTS AND PHOSPHORUS SORPTION

h



5 1.50



c.



h



g 1.50



3



'zpl



v



a



g 1.00

I=

a



Ea



2 0.50



'.OO



2 0.50



z

a:

n



>-



a



f2 0.00



157



None Plot.



Mal.



None Prot.



Mal.



+ 100 mg P/kg



NO P



None Prot.



Mal.



None P r O l



None Prot.



Mel.



None Prot



Mal.



Mml.



+ 400 mg Plkg



NO P



None Prot.



+ 40 mg Pkg



NO P



0.00



MaI.



None Prot.



Mml.



+ 400 mg Plkg



NO P



Figure 7 Yield increases of lettuce in the presence of malic (Mal.)'or protocatechuic (Prot.)

acid. Vertical bars are standard errors. Label " H " represents 2.0 mrnol/kg in the Oxisol or 5.0

mmolikg in the other soils; label "L" represents 0.5 mnlolikg in the Oxisol and 2.0 mmollkg in the

other soils (adapted from Hue. 1991).

Table V



The p H Values of Five Soils Treated with Organic or Inorganic Amendments

after 28-Day Incubation (Adapted from lyamuremye et al., 1995a)

Soil amendment

-



~



~



~



Soil



Control



ManureU



Alfalfao



Wheat straw"



CaCO,*



CaSO,h



Jory (Ultisol)

Mata (Ultisol)

Tolo (Andisol)

Kinigi(Andiso1)

Kibeho(Ultiso1)



5.4

4.7

5.8

4.7

4. I



6.4

6.0

6. I

5.3

5.8



6.7

6.2

6.7

6.1

5.9



5.5

5.3

6.2

5.5



5.8

5.9

6.2



5.3

4.5



5.0



4.8

5.3



5.4

4.6

4. I



-



Amended to m1 at 5% ( w i w ) .

Amended to soil at rate of three times the equivalents needed to neutralize exchangeable Al.



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