III. Nature of Soil Organic Phosphorus

87



SOIL ORGANIC PHOSPHORUS



phosphoprotein (Anderson, 1967) and sugar phosphates (Omotoso and Wild,

1970; Steward and Tate, 1971; Anderson and Malcolm, 1974) are present in soil

organic phosphorus.



A. INOSITOL PHOSPHATES



Among the phosphorus compounds that have been identified so far inositol

phosphates certainly predominate (Table I), in some cases accounting for more

than 50% of the total organic phosphate present (Anderson, 1967). The parent

cyclic polyol (inositol) can have a number of stereoisomers, of which myo-,

scyllo-, and D-chiroinositol in the form of phosphate esters have been isolated

from soil (Cosgrove, 1966, 1969; Anderson and Malcolm, 1974). Only the

myoinositol hexaphosphate isomer has been reported in plants although other

inositol isomers may be present in an unphosphorylated form. Cosgrove (1969)

has suggested that D -chiro-, scyllo-, and neoinositol hexaphosphates are synthesized by soil microorganisms by a mechanism which does not involve epimerization reactions. However, L'Annunziata (1975) indicated that the soil

D-ChirO-, scyllo-, and neoinositol hexaphosphates could be products of microbial

TABLE I

Distribution of Organic Phosphorus Compounds in Some Soils

~~



Percent of organic phosphorus

Soils from



Inositol P



Lipid P



Australia

Bangladesh

Britain

Canada

New Zealand

Nigeria

United States



0.4-38a

9-83

24-58'

11-23f

5-438

23-30:

3-5 2J



0.5-7.0 b

0.6-0.9d

0.9-2.t

0.7-3.1



'Williams and Anderson (1968).

bIslam and Ahmed (1973) and Islam and Mandal(1977).

'Anderson (1964).

dHance and Anderson (1963).

eAnderson (1961).

fKowalenko and McKercher (1971b).

gMartin and Wicken (1966).

'Baker (1976).

iOmotoso and Wild (1970).

jCaldwell and Black (1958).

kAdams et al. (1954).



-



-



Nucleic acid P

-



0.2-2.3b

0.6-2.4e

0.2-1.8k



88



R. C. DALAL



epimerization reactions of the abundant myoinositol or its hexaphosphate from

plants or microorganisms. Inositol phosphates in soil exist in complex forms,

probably bound in a complex containing carbohydrate and protein. Inositol

phosphates may also be linked chemically to larger molecules through their

phosphate groups, as in the inositides (Anderson and Hance, 1963).

Although the penta- and hexaphosphates of inositol stereoisomers have been

shown to be present in soil (Anderson, 1967; McKercher, 1968; Cosgrove,

1966), little is known about the nature of the lower isomers of inositol

phosphate. Wild and Oke (1966) found myoinositol monophosphate in CaC12

extracts of soils while Halstead and Anderson (1970) and Anderson and Malcolm

(1974) identified the neo-, chiro-, scyllo-, and myoisomers of the inositol

containing lower inositol phosphates (di-, tri-, and tetraphosphates). The quantities of lower esters are much less than the penta- and hexaphosphate fraction,

possibly because of their lower stability in soil. The ratio of hexa- to penta- is

very variable, ranging from over four to one (McKercher and Anderson, 1968).

Among the stereoisomers the myo- form is usually the predominant form in soil

organic P followed by scyllo-, chiro-, and neo- in decreasing order. However, the

cause of the different amounts of these esters present in soil is not known.



B. PHOSPHOLIPIDS



Phospholipid phosphorus content varies from 0.5 to 7.0% of total soil organic

P (Table I), with a mean value of 1%(Anderson and Malcolm, 1974). Among the

phospholipids known so far (Strickland, 1973), phosphoglycerides possibly form

the dominant fraction of the soil phospholipids although there is little information available on the other phospholipids in soil, e.g., phosphoglycolipids,

phosphodiollipids, phosphosphingolipids, and phosphonolipids (phospholipids

carrying a covalent bond between the phosphorus atom and the carbon of the

nitrogenous base).

Among the phosphoglycerides, choline phosphoglyceride has been found to be

the predominant soil phospholipid (- 40%) followed by ethanolamine phosphoglyceride (- 30%); remaining phospholipids, as determined by current techniques, are present in small amounts (Kowalenko and McKercher, 1971b).

The phospholipids in soil may be contributed by plant debris, animal wastes,

and microbial biomass. Kowalenko and McKercher (1971a) suggested that a

characterization of the fatty acid association with phosphate may be useful in

establishing the origin of the phospholipid since bacterial fatty acids tend to be

saturated and are either branched or cyclic in form, whereas plant fatty acids

tend to be unsaturated and are substituted normally at the 2-position on the

choline moiety. They suggested that phospholipids are accumulated in soil from

bacterial and fungal biomass. The fact that phospholipids comprise the major



SOIL ORGANIC PHOSPHORUS



89



part of total organic phosphorus in plant tissue (Bieleski, 1973) but only a small

amount in soil organic phosphorus (Anderson, 1967) shows that their synthesis

and degradation may be fairly rapid in soil. The soil phospholipids may be

important in supplying phosphorus to plants; however, this possibility needs to

be investigated.



C. NUCLEIC ACIDS A N D THEIR DERIVATIVES



Only a small proportion (up to 3%) of soil organic phosphorus exists as nucleic

acids or their derivatives (Table I) in spite of the fact that these are probably

added to the soil through decomposing microbial, plant, and animal remains in

greater amounts than most other phosphate esters (Anderson, 1967). For example, Bieleski (1973) quoted a typical ratio of DNA:RNA:lipid-P:ester-P of the

plant tissue (nonseed portion) as 0.2:2: 1.5: 1 (yrnoles per gram fresh weight). It

appears that nucleic acids added to soil are either rapidly degraded or resynthesized and combined with other soil constituents in a form not extracted by

existing techniques. The evidence available so far shows that nucleic acids can be

rapidly mineralized in soil and incorporated into microbial biomass.

Anderson (1961) demonstrated the presence of four nucleic acid bases

(adenine, guanine, cytosine, and thymine) in a bound form in humic acid

fractions, and from the proportions in which they occurred it was inferred that

DNA-derived polynucleotides of microbial origin were present in soil. This was

confirmed when two pyrimidine nucleoside diphosphates, thymidine 3‘,5‘-diphosphate and deoxyuridine 3’,5’-diphosphate, were isolated from soil (Anderson, 1970; Anderson and Malcolm, 1974). However, not enough information is

available about the RNA-derived polynucleotides in soil (Anderson, 1967). The

results of Adams ef al. (1954) and Islam and Ahmed (1973) show that RNA and

its derivatives are present in smaller amounts. It is expected that more refined

analytical techniques regarding extraction, separation, and identification of the

nucleic acid fraction of soil organic phosphorus would assist in understanding

the contribution of this fraction to the phosphorus turnover in soil.



D. OTHER PHOSPHATE ESTERS



The three groups of organic phosphates mentioned in the preceding sections

account for about half the amount of organic phosphorus in soil and, therefore,

much of the soil organic phosphorus is present in as yet unidentified forms.

Steward and Tate (1971) isolated phosphorylated compound by gelchromatographic technique from soil organic phosphorus. Electrophoresis revealed that two major components were a nonreducing and a reducing sugar



90



R. C. DALAL



phosphate whose properties suggested that it was a monophosphorylated uronic

acid, uronolactone, or a related compound. This compound accounted for 40%

of the organic P in 0.1 M NaOH extract. Anderson and Malcolm (1974), from

3 M NaOH soil extracts, detected several monophosphorylated carboxylic acids

with C to P ratios of approximately 7 or 8 to 1 and two esters each containing

glycerol, myoinositol, chiroinositol, and an unidentified component. It is known

that a number of phosphorylated polymers are found in microorganisms (Cosgrove, 1967). It has been suggested that teichoic acids (polymers of ribitol

phosphate containing ester-linked alanine), which sometimes comprise up to

50% of cell walls of gram-positivebacteria, may possibly account for some of the

unidentified organic phosphorus in soil. Advances in the techniques for extraction, separation, and identification of soil organic phosphorus compounds may

reveal hitherto unknown phosphorus compounds which may be contributing

significantly to the phosphorus turnover in soil and hence to the phosphorus

supply to plants.

IV. Organic Phosphorus in Soil Solution



Since the observation of Pierre and Parker (1927) that the amount of organic

phosphorus in soil solution may exceed that of inorganic phosphorus, considerable interest has been generated in the amount and nature of organic P in soil

solution and its availability to plants.



A. ORGANIC PHOSPHORUS CONTENT



Pierre and Parker (1927) found that the average contents of inorganic and

organic P in displaced soil solution of twenty soils were 0.9 and 0.47 ppm P,

TABLE I1

Phosphorus in Soil Solution'

Organic phosphorus

Soil type



Inorganic phosphorus (ppm)



PPm



% of total P



Sand (l)b

Sandy loam (5)b

Sit loam (8)b



0.02

0.12

0.12



0.28

0.57

0.44



93

83

79



'In extracts obtained by displacement method. Compiled from Pierre and Parker (1927).

0 1927 The Williams & Wilkins Co., Baltimore.

bFigures in parentheses are the number of soils examined.



91



SOIL ORGANIC PHOSPHORUS

TABLE 111

Effect of Drying on the Concentration of Organic Phosphorus in Soil Solution'



Organic phosphorus



Soil type

Broad seriesb

(under grass-grazed ley)



Sonning seriesb

(under cultivation)



Treatment



Inorganic P (ppm)



ppm



% of total P



Fresh soil

Dried at 20°C

Dried at 40°C



0.09



0.09



0.14

0.33

1.49



61

69

94



Fresh soil

Dried at 20°C

Dried at 40°C



0.34

0.44

0.14



0.10

0.11

0.95



23

20

56



0.15



'In 1:2 soil: CaCl, extracts. From Wild and Oke (1966).

bTotal soil organic P in Broad series and Sonning series are 620 and 240 ppm, respectively.



respectively; in 1 5 soi1:water extracts the respective values were 0.35 and 0.22

ppm P. Coarse textured soils contained a greater proportion of their solution P

in organic form than fine textured soils (Table 11). Fuller and McGeorge (1951)

observed that a substantial portion of the total water- and C 0 2 - extractable

phosphorus in twenty calcareous soils was present in the organic form. Similarly,

Wild (1959) found that the concentration of organic phosphate in CaC12

extracts of soils considerably exceeded that of the inorganic phosphate.

The concentration of organic P in soil solution increases considerably upon air

drying soil. Thus Wild and Oke (1966) observed that air drying the soil at 40°C

increased the proportion of organic P in CaC12 extracts from 61 to 94% in soil

under grazed ley, and from 23 to 56% in soil under cultivation (Table 111). The

significance of the effect of changes in soil environment due to different cultural

practices on the organic P in soil solution should be investigated because of the

possibility that it plays a considerable role not only in P movement in soil

(Hannapel et aZ., 1964a,b) but also in plant nutrition (Wild and Oke, 1966).



B. NATURE OF ORGANIC PHOSPHORUS



Relatively little information is available on the nature of organic phosphorus in

the soil solution. Wild and Oke (1966) identified the myoinositol monophosphate as the major constituent of organic P in the CaCI, extract of soil. Martin

(1970) obtained some evidence of phosphate esters in cold water extract of soil,

but the other components could not be identified. It appears that a significant

proportion of the intracellular organic phosphorus is released into soil solution