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IV. Organic Phosphorus in Soil Solution
SOIL ORGANIC PHOSPHORUS
Effect of Drying on the Concentration of Organic Phosphorus in Soil Solution'
(under grass-grazed ley)
Inorganic P (ppm)
% of total P
Dried at 20°C
Dried at 40°C
Dried at 20°C
Dried at 40°C
'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
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from the damaged microbial cells with the phosphate ester bond intact. Thus,
since organic P in soil solution is not utilized by buckwheat, soybeans, and corn
but the plant can absorb P from phytin, lecithin, nucleic acids, nucleotides,
and calcium glycerophosphate (Pierre and Parker, 1927), this fact led Rogers
et al. (1940) to the conclusion that either organic P does not contain these
P compounds in the soil solution or that organic P in the soil solution is present
in complex form. A possible explanation is that most of the organic P in the soil
solution is actually colloidal in nature and is associated with microbial cells and
cellular debris (Hannapel et al., 1964b). The identification of the organic P
compounds in soil solution is necessary in order to improve our understanding of
their availability and significance in the P nutrition of plants.
C. AVAILABILITY OF ORGANIC PHOSPHORUS
The availability of the organic P compounds that are commonly found in soil
(Anderson, 1967) has been demonstrated by many workers. For example,
Weissflog and Mengdehl (1 933) showed that, under aseptic conditions, glycerol
phosphate, sugar phosphates, inositol hexaphosphate, and nucleic acids were as
good a source of P to maize as was inorganic phosphate. Similarly, Rogers et al.
(1940) showed that plants can absorb P from inositol hexaphosphate, lecithin,
nucleic acids, nucleotides, and calcium glycerophosphate. The availability of
inositol hexaphosphate to plants under aseptic conditions has been confirmed
subsequently (Szember, 1960; Flaig et al., 1960). Martin and Cartwright (1971)
compared the uptake of myoinositol hexaphosphate (IHP) and KH2PO4 labeled
with 32P by ryegrass (Lolium perenne). It was found that the availability of
added IHP was equal to KH2P04 from low P retention soil but that it was not
available to plants when added to high P retention soil (Table IV). One explanation is that IHP was strongly sorbed by high P retention soil. Indeed, Anderson
et al. (1974) have shown that IHP was completely sorbed by soil high in P
sorption when it was added at the rate of 4 mg P/g soil; the sorption of inorganic
P a t that rate was 65% (Table V). Therefore, the low availability of organic
phosphorus in soil may be due to the sorption as well as fixation of these
compounds by soil colloids and, possibly, by formation of insoluble Fe and Al
complexes (Anderson and Arlidge, 1962; Anderson et al., 1974).
In spite of the fact that plants can take up P from known organic P compounds, there is no unequivocal evidence that plants utilize organic P from soil
solution. Pierre and Parker (1927) observed that organic P in the soil solution
was not taken up by plants although inorganic P in the soil solution was almost
completely absorbed. However, the results of Wild and Oke (1966) suggest that
some of the organic P in the soil solution may be available to plants (Table VI).
They showed that the easily hydrolyzable fraction of organic P was taken up
SOIL ORGANIC PHOSPHORUS
Uptake of Myoinositol Hexaphosphate (IHP) and KH, PO, by Ryegrass'
Phosphorus uptakeC (mg/pot)
(low P retention)
(high P retention)
'Adapted from Martin and Cartwright (1971), by courtesy of Marcel Dekker, Inc., New
bLabeled IHP and KH,PO, were applied at 114 mg P/pot (approximately 38 mg P/kg
'Means followed by letters not in common differ significantly at P < 0.01.
readily by clover but that the fraction resistant to hydrolysis had a low
availability to plants grown under aseptic conditions. Moreover, organic phosphates forming water-soluble complexes with Fe and A1 (organometallic
phosphates) can be utilized by plants (Sinha, 1972).
Another possibility that organic phosphorus in the soil solution may be
important to P nutrition of plants is that phosphatase enzymes, excreted by the
plant roots could hydrolyze this fraction thus releasing inorganic P. That the
plant roots possess phosphatase activity has been confirmed (Ridge and Rovira,
1971; Martin, 1973). Moreover, it has been shown that P-deficiency in plants
increases phosphatase activity (Table VII). In addition, microorganisms present
in the soil may also be involved in hydrolyzing organic compounds. For example, Cosgrove (1970) isolated from a soil an organism possessing a high
Sorption of Inositol Hexaphosphate (IHP) and Inorganic Phosphorus (Pi) by Two Soils'
Phosphorus sorbed (%)
P added (mg/g soil)
Sand (low P retention)
Basic igneous (high P retention)
'Calculated from Anderson et al. (1 974). In 0.5 M acetate buffer at pH 6 .
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Phosphorus Availability to Clover of the Three Fractions of the Soluble
OIganic P in Broad Series‘
‘Taken from Wild and Oke (1966). I
bThree fractions accounted for 3 0 4 0 % of organic P in soil solution. Myoinositol monophosphate was the dominant constituent in P, and P, and probably in P, fractions.
‘Compared with availability of inorganic phosphate taken as 100.
activity of the specific enzyme inositol hexaphosphate phosphohydrolase.
Greaves and Webley (1965) found that 3 0 3 0 % of bacterial isolates from the
rhizosphere of pasture grasses possessed phytase activity; however it is doubtful
whether the occurrence of bacteria possessing the phytase activity in the rhizosphere will increase the dephosphorylation of myoinositol hexaphosphate at the
root surface above the activity due to plant enzymes (Martin, 1973). Further, it
is uncertain whether phosphatase activity in the presence of low concentration
of phosphate esters in the solution has significance (Bieleski, 1973).
Recently there has been a considerable interest in the possible increase in
availability of organic phosphorus to plants resulting from the infection of plant
roots by mycorrhizae. Paterson and Bowen (1968, cited in Bowen, 1973)
showed that ectomycorrhizal fungi in culture could use sugar phosphates,
nucleotides, and inositol hexaphosphate as sources of energy and phosphate and
that mycorrhizae of P. radiata exhibited surface phosphatase activity. The
phosphatase activity of mycorrhizal and nonmycorrhizal roots are compared in
Table VIII. Since mycorrhizal association occurs commonly even in cultivated
Phosphatase Activity of the Roots of Spirodeta oligorrhiza under Phosphorus Sufficiency
and Deficiency Conditions‘
Enzyme activity (ex.) b
P deficiency (1 1 days)
P deficiency (14 days)
‘Calculated from Bieleski and Johnson (1972).
bl e.u. hydrolyzes 1 ,mole p-nitrophenyl phosphate per minute per gram fresh weight at
SOIL ORGANIC PHOSPHORUS
Phosphatase Activity of Mycorrhizal and Nonmycorrhizal Rootsu
Date of sampling
‘From Williamson and Alexander (1 975).
of p-nitrophenyl phosphate hydrolyzed/mm2 root surface h-’ .
plants (Strezemska, 1974), it may be of considerable importance in P nutrition
The advantage of mycorrhizal association in the use of organic phosphorus is
the ability of mycelia to penetrate soil pores and soil organic matter at distances
away from the root, thus exploiting a greater soil volume than uninfected plants
as well as competing positively with other soil microorganisms. In that way
mycorrhizal-infected plants can absorb greater amounts of phosphorus. Further,
since certain mycorrhizal fungi can grow at low water potentials, when other
organisms are senescing and releasing organic phosphates, it would be of considerable advantage to the mycorrhizal-infected plants (Bowen, 1973) in dephosphorylating, absorbing, and translocating the absorbed P. However, Hayman
and Mosse (1972) observed that the plant roots could not utilize organic
phosphate even in the presence of vesicular-arbuscular mycorrhiza. They concluded that the main role of mycorrhiza was the provision of extra nutrientabsorbing surface. Because of these conflicting reports, it may be useful to
investigate the significance of organic P t o mycorrhizal-infected plants.
In summary, it can be concluded that: (a) the concentration of organic
phosphorus in soil solution exceeds that of organic phosphate, (b) the hydrolyzable soluble organic phosphate can be utilized by plants, and therefore it
is necessary to characterize the organic phosphorus compounds in soil solution,
and (c) mycorrhizae may increase the availability of organic phosphate by
producing dephosphorylating enzymes. It is necessary to determine whether the
soluble organic phosphorus can be replenished when its concentration is reduced
by plant uptake. Moreover, the environmental factors that govern not only the
concentration of soluble organic P but also its turnover (in whole or in part)
should be studied. Since the organic phosphorus in the soil solution is more
mobile than the inorganic phosphorus [and indeed in calcareous soils, Hannapel
et al. (1964a,b) showed that 95% phosphorus movement in the soil is in organic
form], it would be of interest to investigate this phenomenon especially in soils
where organic phosphorus is of the predominant form in the soil solution.
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V. Organic Phosphorus Turnover in Soil
Since organic phosphorus is a part of soil organic matter, it tends to follow the
pattern of accumulation and loss of organic matter as a whole. The process of
buildup of organic phosphorus may be termed immobilization, i.e., available
inorganic phosphorus is converted biologically into organic phosphorus compounds which are unavailable to plants. The microbial conversion of soil organic
phosphorus into inorganic phosphorus is termed mineralization. The immobilization and mineralization of phosphorus can occur concurrently in soil.
Halm et al. (1971) studied the phosphorus cycle in a native grassland system
and presented it diagrammatically in Fig. 1. Examination of Fig. 1 shows that
the large concentration of phosphorus is in the soil fauna and microorganisms,
and the organic and inorganic fractions. The rate at which these fractions are
made available to the soil solution controls the phosphorus supply. The amount
of phosphorus in the birds, grasshoppers, small mammals, and other inverteorates (consumers), and in the above-ground plant material, at any given period
is very small when compared to the extremely large amount tied up in the
organic and inorganic phosphorus fraction of the soil. It is also interesting to
note that soil fauna and microorganisms together contain more phosphorus than
t!ie total amount in the plant material.
The phosphorus cycle of a native grassland ecosystem is summarized as follows
(Halm et al., 1971). The phosphorus in litter is attacked by fungi and is
physically moved into the soil in fungal hyphae which are then attacked by
FIG. 1. Phosphorus cycle in a native grassland system (in parentheses is P expressed in kg
per hectare per 30 cm soil depth). Adapted from Halm ef al. (1971).
SOIL ORGANIC PHOSPHORUS
bacteria providing a continuing source of organic phosphorus (Clark and Paul,
1970). The more soluble fraction of this phosphorus is immobilized by new
microbial tissue or converted into more resistant compounds forming soil
humus. On mineralization, it goes into soil solution where it may be taken up by
plants, adsorbed by soil colloids, and fixed into unavailable inorganic form or
again appropriated by microorganisms. Thus both processes, immobilization of
inorganic P and mineralization of organic P, occur simultaneously in the soil and
only the difference in the rates of immobilization and mineralization of organic
P can be observed at any given time.
A. IMMOBILIZATION OF INORGANIC PHOSPHORUS INTO
The available literature on phosphorus turnover in soil reflects that more
studies have been carried out on the factors which govern phosphorus mineralization than on organic phosphorus buildup in soil.
Considerable amount of native inorganic phosphorus has been transformed
into soil organic phosphorus over the years (Walker and Adams, 1958).
Since carbon, nitrogen, sulfur, and phosphorus are associated in fairly definite
proprotions in soil organic matter, a deficiency of either sulfur or phosphorus
may limit nitrogen fixation by legumes or microorganisms. In areas where
sufficient sulfur is supplied from the atmosphere, the organic matter buildup and
hence organic phosphorus accumulation of soil would be determined by phosphorus content of the parent material. Indeed, Walker and Adams (1958)
observed that the phosphorus content of the parent material was a major factor
governing the accumulation of organic phosphorus in soil. Subsequently a
number of workers have observed a close relationship between organic P and
total phosphorus content of soil (Kaila, 1963; Syers and Walker, 1969; Walker
and Syers, 1976).
In soils where native inorganic phosphorus is low, as in many Australian soils
(Jackson, 1966), the application of inorganic phosphorus, especially to legumegrass pastures, should result in an organic phosphorus buildup. Donald and
Williams (1954) found that the application of superphosphate to subterranean
clover (Trifolium subterraneum L.) grown in podzolic soils for 26 years resulted
in an increase in organic P from 53 to 9 6 pprn (increase of 4 ppm organic P per
9.5 kg of P applied). The results of Jackman (1955) and Rixon (1966) (Table
IX) show that organic phosphorus buildup can be fairly rapid under favorable
conditions although the rate of accumulation would be different depending
upon a number of environmental, soil, and plant factors.
Factors other than inorganic phosphorus supply may limit organic phosphorus
accumulation in soil. Williams and Donald (1957) suggested that the rate of
organic P accumulation under legume pastures may be limited by the insufficient