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Chapter 4. Organic Amendments and Phosphorus Sorption by Soils

Chapter 4. Organic Amendments and Phosphorus Sorption by Soils

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140



F. IYAMUREMYE AND R. P. DICK



highly weathered soils such as Ultisols and Oxisols. Generally, P is available to

plants in very small amounts in acid soils, due to adsorption by Fe or Al oxides or

by its precipitation with soluble A1 and Fe in acid soils, whereas in alkaline soils

phosphate readily reacts with Ca to form insoluble precipitates.

Liming soils is the traditional method used to reduce P sorption to increase its

plant availability in acid soils. However, liming is an expensive input and is less

effective on high P-fixing soils (Haynes, 1982). With the current interest in

reduced use of purchased inputs and efficient use of organic residues in

agroecosystems, there has been renewed interest in the past 10 years on use of

organic amendments to increase P efficiency and availability to plants. This idea

has a long history, but recent developments in research and analytical methods

have provided significant advances in our understanding of the role of organic

amendments in affecting P sorption in soils. Besides addressing organic amendments, the scope of this chapter includes specific components (e.g., organic

acids, transformation of organic P in soils) that are important in the relationship

between organic amendments and P sorption. Because of divergent chemical and

biological processes that occur between aerobic and waterlogged soils, this review is subdivided to separately address these two types of soil environments.

More attention is devoted to aerobic soils because considerably more research

has been done on the effect of organic residues on P sorption in aerobic than

waterlogged soils.



A. PHOSPHORUS

CYCLE

IN SOILS

Phosphorus reactions in the soil environment have been extensively reviewed

elsewhere (Wild, 1949; Larsen, 1967; Haynes, 1982; Berkheiser ef al., 1980;

Sanchez and Uehara, 1980; Sanyal and De Datta, 1991). Therefore, we will

provide a brief overview of P pools and transformations as a framework for

understanding the P dynamics when organic residues are added to soils.

The P cycle can be characterized as the flow of P between plants, animals,

microorganisms, and solid phases of the soil. Major P processes for soils shown

in Fig. 1 include P uptake by plants; biological mediated turnover of P through

mineralization/immobilizationreactions; and chemical fixation/dissolution reactions between liquid and solid phases.

When plant productivity is emphasized, P often has been partitioned into pools

based on their potential to provide inorganic orthophosphate for plant uptake.

These P pools are broadly envisioned as soil solution P, labile P, and nonlabile P.

The labile P is defined as a P reserve that can replenish soil solution orthophosphate in response to P uptake by plant roots. Conversely, nonlabile P has minimal

or nonexistent effects on soil solution orthophosphate on an annual basis. Both

labile and nonlabile P pools may contain inorganic and organic constituents.



ORGANIC AMENDMENTS AND PHOSPHORUS SORPTION



PLANT ROOTS



141



PLANT RESIDUES



Agure 1 The P cycle in soils, showing the partition of organic and inorganic forms of P into

pools based on availability to plants (adapted from Stewart, 1980).



Maintenance of adequate orthophosphate in soil solution for plant growth is

controlled by both chemical- and biological-mediated reactions. Ghoshal ( 1975)

studied the competition between these two mechanisms for controlling soil sohtion orthophosphate and concluded that, generally, chemical reactions dominate

in soils. However, the microorganisms play a critical role in mineralization of

organic inputs and partitioning P into various organic fractions and likely provide

continuous inputs of orthophosphate into soil solution during the growing season.

From this brief overview, it is obvious that organic amendments would have

impact on nearly all aspects of the P cycle in soils. The addition of such residues

will provide a source of C and stimulate biological activity. Furthermore, the

resulting residue decomposition will release organic ligands or acids that will

affect P sorption and solubility reactions. Organic amendments also can be a

source of inorganic and organic P with the latter being subject to mineralization

which releases inorganic P or forms organic P fractions.



1. Chemical Reactions of Phosphorus in Soils

Phosphorus sorption is one of the most widely studied reactions in soils.

Sorption may include adsorption and precipitation reactions. Sposito (1986)



F. IYAMUREMYE AND R. P. DICK



142



defined adsorption as bidimensional and precipitation as tridimensional. However, he recognized that the two mechanisms are difficult to distinguish, and both

mechanisms are described by the same mathematical model. Therefore, for the

purposes of this paper, P sorption is defined as the loss of orthophosphate to the

solid phases of soils which can occur by either adsorption or precipitation.

Bohn et al. (1979) described two adsorption reactions. The first is specific

adsorption or ligand exchange where a phosphate anion replaces the hydroxyl on

the crystal of hydrous A1 or Fe structures. This mechanism is kinetically described as very rapid and completed within a few days. The second mechanism,

nonspecific adsorption, is mediated through the protonation of the hydroxyl

surface that creates positive charges and attracts negative charged anions such as

orthophosphate.

Goldberg and Sposito (1984) have visualized the surface adsorption models as

follows:



+ H,PO, S-H2P0, + H 2 0

S-OH + HZPO, * S-HPO, + HZO

S-OH + HPO, i3 S-PO, + H,O ,



S-OH



(1)

(2)

(3)



where S refers to the surface of the solid phase.

Other anions such as fluorine, sulfate, and organic acids may be sorbed at the

surfaces of A1 or Fe crystals (Stumm et al., 1980; Violante and Gianfreda, 1993;

Sibanda and Young, 1986; Parfitt, 1978) similarly to orthophosphate adsorption. These anions then may be in competition with orthophosphate for sorption

sites.

The precipitation reaction process can be slow and may take years for completion to occur (Bohn et al. 1979; Syers, 1971). As shown in Fig. 2, pH has a

major role in P precipitation. Under alkaline conditions, CaZ+ controls P solubility where orthophosphate readily forms less soluble di- and tricalcium phosphates. Under acid conditions, AP+ and Fe3- control solubility of P with orthophosphate readily precipitating as highly insoluble phosphate compounds by the

following reactions:



+ PO,3- + 2H,O AlPO, 2H20,,, (amorphous variscite)

Fe3+ + Po,’- + 2H,O a FePO, 2H20(,, (strengite) .



A13+



*



*



(4)



(5)



2. Biological Transformation of P and Organic P Fractions

Phosphorus is an important element in all biological systems, participating in

most metabolic pathways and as a structural component of nucleic acids, coenzymes, phosphoproteins, and phospholipids (Tate, 1984). Biological cycling of P



ORGANIC AMENDMENTS AND PHOSPHORUS SORPTION



143



PH

Figure 2 Stability diagram for selected discrete phosphate compounds in aqueous environments

(adapted from Berkheiser rt d..1979).



in ecosystems is driven by the presence of plants, soil fauna, and microorganisms.

Soil biological activity controls the turnover of P by decomposition of organic

P inputs to soils which releases inorganic phosphate or forms organic P fractions,

immobilization of P in viable cellular biomass, and solubilization of insoluble

mineral P forms through release of chelating agents and/or organic acids. The

mineralization/immobilization processes occur simultaneously and net rates are

difficult to measure because the end product of mineralization, orthophosphate,

is readily subject to fixation reactions in soils.

Soil organic P generally makes up 20-80% of total soil P (Stevenson, 1986),

and of the P in the soil solution, 20-70% may exist in organically bound forms

(Speir and Ross, 1978).

Soil organic P compounds can be classified into three groups (Anderson,

1980): ( 1 ) inositol phosphate which composes up to 60% of soil organic P (Tate,

1984), ( 2 ) nucleic acids, and (3) phospholipids.

Dalal (1977) and Tate (1984) reported that soils contained phosphoproteins,

sugar phosphates, and glycerophosphates. A large portion of unidentified organic

P occurs as insoluble comFlexes bound with clay minerals and organic matter

(Tate, 1984). Forms of organic P resistant to both chemical and enzymatic

hydrolysis are thought to result from the incorporation of organic compounds

into humic materials during oxidative polymerization of polyphenols.

lnositol phosphate, which can have mono-, di-, or triesters of phosphate

groups, constitutes the most important fraction of organic P (Hawkes et a/.,



144



F. IYAMUKEMYE AND K.P. DICK



1984; Gil-Sotres et al., 1990). Some esters are quickly broken down in soil

(Anderson, 1980); however, they can react and form stable complexes with Fe

and A1 as organic P (Saxena, 1979) or be stabilized by colloids (clay, sesquioxides) through sorption reactions (Stewart and Tiessen, 1987). These may be the

reasons why they tend to accumulate in soil compared to phospholipids (Cole et

al., 1977).

Another very important dynamic P pool is the biomass P which is 1 to 2% of

the total soil P (Stevenson, 1986) and is directly correlated with biomass C

(Brookes et a / ., 1984). Isotopic double-labeling techniques have shown that

recently added organic residue P apparently is an important component of the

microbial biomass P. A field experiment showed that 22-28% of the 33Papplied

in medic plant residues was recovered in the microbial biomass (McLaughlin et

al., 1988a). Furthermore, there appears to be rapid transformation of plant P to

organic P fractions in soils. For example, McLaughlin er al. (1988b) reported

that after 7 days 40% of the plant residue 33P was incorporated into organic P

fractions of soil.

The microbial biomass C represent 2-3% of the total organic C in soil (Sanyal

and De Datta, 1991) but it is a key site for soil organic P mineralization (Brookes

et a l . , 1984). An early incubation experiment of soil provided evidence that

organic P is mineralized because organic P decreased similarly to increases in

extractable inorganic P (Van Diest and Black, 1959). Other evidences of organic

P mineralization are provided by observations that organic P decreases in cultivated soils (Sanyal and De Datta, 1991; Haas, 1961).



B. EARLYHISTORY

Phosphorus fixation was first demonstrated in 1850 by Way (1850) with simple

percolation experiments. Calcium phosphate dissolved in dilute sulfuric acid was

passed through soils which resulted in leachate with no detectable levels of

orthophosphate. This indicated that there was a rapid reaction of orthophosphate

with soil constituents.

Liebig (1858) and Sachs (1 865) demonstrated that polished plates of marble

and ostheolite were etched by the roots of different plants. Later views held that

plants excreted acids from their roots, which were proposed to be effective agents

for solubilizing mineral forms of nutrients like P (Dyer, 1894; Quartaroli, 1905;

Pfeiffer and Thurmann, 1896; Palladin, 1911; Maze, 1911). In particular, Shulov

(1912) proposed that excretion of malic acid from plant roots solubilized orthophosphate in soils. Yet as late as 1931, Miller (1931) argued that there was no

positive proof that carbonic acid was excreted by plant roots. Later Gerretsen

( 1948) provided evidence that microorganisms in the rhizosphere can solubilize

insoluble mineral P forms.



ORGANIC AMENDMENTS AND PHOSPHORUS SORPTION



145



At the turn of the century, there was recognition that rock phosphate was best

utilized when in intimate contact with decaying organic matter (Hopkins, 1910).

It was thought that organic acids, carbonic acids, and nitrous acids resulting from

decomposition were the active agents that were solubilizing P (Bauer, 1921).

During the second decade of the 20th century there already were reports that

organic amendments could affect P reactions in soils and plant availability.

Jensen (1917) performed an experiment which included the use of alfalfa hay,

sweet clover, barley hay, and stable manure added to soil. In addition, soils were

extracted with: (a) soluble organic matter obtained from decomposing green

manure and from stable manures; (b) soluble organic matter obtained from thoroughly decomposed green manure; (c) artificially prepared humus; and (d) osmosed organic solutions derived from the decomposition of organic matter.

The conclusions were:

1. The amount of phosphoric acid dissolved by organic extracts from the soil

exceeded the amount dissolved by water from 1.7 to 5.4 times regardless of the

amendment;

2 . The solvent action of these organic extracts on the soil minerals appeared to

be due both to the inorganic salts present in the organic solvents and to the

organic compounds.

3. Mixing green plant manures or stable animal manure with soil (3% w/w)

followed by incubation increased the solubility of phosphoric acid in the soils

from 30 to 100%.

4. Artificial humus solution, free from calcium, magnesium, Fe, and phosphoric acid, increased the solubility of soil phosphoric acid.



Another approach to increasing the plant availability of inorganic P, recognized early in the 20th century, was to mix inorganic P with animal manure prior

to incorporating in soil. Tottingham and Hoffmann (1913) found that mono- and

tricalcium phosphate mixed with animal manure increased P uptake in barley

grown in pots. Later, Midgley and Dunklee (1945) found that pellets formed

from a mixture of phosphate and manure markedly increased P availability to

plants. Increasing the pellet size increased P availability to the crop on highphosphate-fixing soils.

Bear and Toth ( 1942) suggested that phosphate fixation can be greatly reduced

by incorporating large amounts of organic matter into the soil. They explained

that humic acid produced during organic residue decomposition reacts with soluble Al and Fe to form humates which are less soluble than orthophosphate. Like

the hydroxyl ion, humic acids also function as a replacing agent for adsorbed

phosphate and may be used as partial substitutes for liming material and organic

matter (Bear and Toth, 1942).

Copeland and Merkle (1941) found that soils receiving manure had lower

phosphate adsorption quotients. They hypothesized that the biologically active



146



F. IYAMUREMYE AND R. P. DICK



manure either exerts a protective effect upon the soil mineral colloids or helps to

release fixed phosphate or both. They observed that addition of animal manure

had a large effect on P availability, but soils with high organic matter did not have

a significant advantage over soils with lower organic matter in making P available to plants. From this, they suggested that it is the “biologically active”

organic matter that is important in P availability in soils. Swenson el a!. (1949)

demonstrated that several organic anions form stable complexes with Fe and Al.

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. Citric, oxalic, malic, tartaric, malonic, malic, and lactic acids were most effective in preventing P precipitation by

Fe or A1 and citrate was the most active anion in preventing orthophosphate

precipitation between pH 4 and 6 (Struthers and Sieling, 1950).

Dalton et al. (1952) concluded that organic matter added to the soil as an

amendment is effective in increasing the availability of soil orthophosphate. This

was attributed to microbiological decomposition of organic amendments which

releases metabolic products to form stable complexes with Fe and A1 in acid

soils.



11. AEROBIC SOILS: ORGANIC ACIDS AND

PHOSPHORUS SORPTION

A. ORGANIC

ACIDSIN SOILS

When organic residues are added to soils, organic acids may be added directly

to soils with the residue or be produced as by-products during decomposition of

the residues by microbial activity.

Organic acids commonly encountered in soil solution tend to be significantly

lower in cultivated soils than in the same soil under native vegetation (Fox and

Comerford, 1990; Hue et al., 1986). This is shown in Table I, in which a

plantation site managed with monoculture pine is compared to a natural site

which had mixed treelshrub vegetation. Hue et al. (1986) compared subsoils of

cultivated and noncultivated soil on the same soil types and found that oxalic acid

ranged from 4 to 22

in forested soil whereas in cultivated soil it ranged from

< 1 to 3.4 p,M with similar trends among other organic acids measured (citric,

malic, malonic, succinic, lactic, formic, and phthalic). Stevenson and Ardakani

(1972) reported values as high as 3.7-5.0 and 0-1.0-4.0 mM of acetic and

malic acid, respectively. Iyamuremye et al. ( I 99%) detected organic acids (oxalic, malic, maleic, malonic, succinic, formic and acetic acid) in the soil solution

of five different types of soils. The concentration of these organic acids varied



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.



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