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V. Organic Phosphorus Turnover in Soil
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
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Increase in Soil Organic Phosphorusunder Irrigated Pastures"
by year 5
Organic P by year 1 (ppm)
Wimmera ryegrass (Lolium rigidum Gaud.)
Subterranean clover (Trifolium subterraneum L.)
Perennial ryegrass (L. perenne)
White clover (T.repens L.)
'Calculated from Rixon (1966). Superphosphate added, at the rate of approximately 50
kg P per hectare per year, in the years 2, 3, and 4.
amounts of sulfur supplied in the superphosphate. The results of the experiments conducted by McLachlan and Norman (1962) did not support their
suggestion, however; they indicated that phosphorus rather than sulfur could be
the limiting factor in organic phosphorus buildup. Jackson (1966) has indicated
the situations in pastures where phosphorus or sulfur could be limiting for
organic matter and hence organic phosphorus accumulation.
The addition of soil carbohydrate or organic material having a large carbon to
phosphorus ratio leads to increased microbial activity and the formation of
organic phosphorus from a pool of inorganic phosphorus in soil (Cosgrove,
1967). Van Diest (1968) noted accumulation of soil organic phosphorus in
Coastal Plain Soils of New Jersey owing to the addition of fertilizer P and energy
materials through crop residues. Ghoshal and Jansson (1975) have elegantly
demonstrated the immobilization of inorganic phosphorus when glucose-C as the
energy source was added (Table X).
The organic carbon:organic phosphorus ratio in soil and plant residues added
to soil has been used to predict net immobilization and mineralization of
phosphorus in soil (Black and Goring, 1953; Thompson et a l , 1954; Alexander,
1961; Tisdale and Nelson, 1975). It has been suggested that if the organic
carbon:organic P ratio is 200: 1 or less, mineralization of phosphorus occurs and
if the ratios are 300:l or more than immobilization occurs. Thus the critical
level of phosphorus in organic material which serves as a balance between
immobilization and mineralization is about 0.2%(Alexander, 1961). However,
Enwezor (1967) observed that organic C:organic P ratio in the soil was an
unreliable index for predicting immobilization or mineralization. Birch (1961)
suggested that the cause of the inconsistency of the C:P ratio as an index of
SOIL ORGANIC PHOSPHORUS
Immobilization of Added Phosphorus as Affected by Phosphorus and Glucose-Carbon'
Phosphorus added (ppm)
Glucose-C added (%)
P immobilizedb (ppm)
'From Ghoshal and Jansson (1975).
bIncubated for 15 days.
immobilization or mineralization of P may be due to the variable but significant
amounts of inorganic phosphate present in the organic materials. Further,
Barrow (1960) suggested that this inconsistency could be a consequence of the
suboptimal supply of nitrogen and/or sulfur in the soil; in either case the
formation of soil organic matter would be inhibited. The addition of N through
either fertilizers or atmospheric fixation by legumes could result in the immobilization of inorganic phosphorus. However, it needs to be confirmed by
labeling the phosphate pool with 33Pin the added organic matter and then
following its decomposition in relation to mineralization and immobilization
Soil organic phosphorus accumulation is also governed by environmental
factors other than the nutrient supply. Organic phosphorus accumulation decreases with increase in leaching as a result of rainfall (Walker and Adams, 1959).
Higher temperatures cause greater mineralization and hence less net immobilization although suboptimal temperature also decreases immobilization rate because of reduced biological activity. Suboptimal moisture has a similar effect.
Type of vegetation also influences organic phosphorus accumulation in soil.
Ipinmidun (1972) found that soils from southern Guinea savanna had a higher
organic phosphorus content than those from the Sudan vegetation zone. Soils
under forest immobilize higher amounts of phosphorus than those under grass
(Enwezor and Moore, 1966). Rixon (1966) observed that organic P accumulation was significantly lower under Wimmera ryegrass than under clover or
perennial ryegrass (Table IX). Increase in soil acidity could restrict organic P
accumulation. Thus Simpson et aZ. (1974) observed that organic P remained
unaffected by the 9-fold increase in superphosphate application from 64 kg P/ha
to 579 kg P/ha; the possible increase in acidity due to N fertilizers appeared to
be the cause.
The mechanism of organic phosphorus accumulation in soil has not been
completely worked out. Van Diest (1968) suggested that increases in soil organic
P might be induced either by increase in the soil microflora following application
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of fertilizer P, or by accumulation of crop residues containing a fraction of
organic P resistant to rapid hydrolysis, or by a combination of these two
processes. Evidence for both the processes exists in the literature. Birch (1961)
found no pronounced mineralization of plant organic P (incubated with sand)
during decomposition periods up to 3 months. In soil, protection from decomposition might be even stronger due to possible sorption of organic phosphorus compounds on clay minerals (Pinck and Allison, 1951; Goring and
Bartholomew, 1950, 1952) and hydrated oxides of iron and aluminum (Anderson and Arlidge, 1962). Such sorption decreases the rate of mineralization by
enzymes (Greaves and Webley, 1969). In fact, Anderson ef al. (1974) showed
that the stability of phosphate esters was due to their sorption by soil, and
possibly the formation of insoluble Fe and Al complexes with esters.
Since phosphorus accumulation in the soil organic matter follows the organic
C, N, and organic S accumulation in soil (Jackman, 1964), organic phosphorus is
almost certainly accumulated in soil as the result of microbial activity. For
example, before 1963, soil phytin (metal salts of inositol hexaphosphate) was
assumed to be entirely of plant origin (Birch, 1961). However, Cosgrove (1963,
1966) and others (L'Annunziata, 1975) have shown that phosphate esters of
D -chiro-, scyllo-, and neoinositol are synthesized by microorganisms in soil
(Section 111, A). Although the origin of soil phospholipids is not known (Cosgrove, 1967), it has been suggested that these are accumulated in soil from
bacterial and fungal biomass (Section 111, B) whereas nucleic acid phosphorus in
soil has been found to be of bacterial origin (Anderson, 1967). As shown earlier
(Table X), organic phosphorus can be synthesized by microflora from inorganic
phosphorus and available organic C. However, long-term studies by Halstead and
Sowden (1968), where organic amendments to a sand and a clay soil were added,
gave no evidence to either support or deny the assumption that accumulation of
organic phosphorus was caused by microbial rather than by plant residue effects.
It is hoped that the better understanding of soil organic phosphorus through
improved techniques such as ion exchange and gel-chromatography , nuclear
magnetic resonance and mass spectrometry, as suggested by Halstead and Mc
Kercher (1975), would assist in elucidating the mechanism of organic phosphorus buildup and its possible relationship to other constituents in soil organic
matter (organic C, N, and S), and their subsequent (and possibly concurrent)
mineralization in soil.
B. MINERALIZATION O F ORGANIC PHOSPHORUS AND
FACTORS AFFECTING THE PROCESS
Soil organic phosphorus contributes to the phosphorus nutrition of plants
primarily after being mineralized into inorganic phosphorus. It has been shown
in the laboratory that in incubated soil, organic phosphorus decreases with
SOIL ORGANIC PHOSPHORUS
Changes in Organic and Inorganic Phosphorus upon Cropping'
Percent change from virgin soil
'Estimated from Haas el a[., Soil Science Society of America Proceedings, 25, 214-218
(1961). The period of cropping ranged from 30 to 48 years at the different locations in the
Great Plains region of the United States.
similar increase in extractable inorganic phosphorus (Thompson et al., 1954; Van
Diest and Black, 1959). The decrease in organic phosphorus when virgin soils are
brought under cultivation is possibly due to mineralization, for Haas et al.
(1961) observed that the decrease in phosphorus from the soil on cropping was
approximately equal to the loss of organic phosphorus (Table XI). Further
evidence that soil organic phosphorus could be mineralized is that phosphatase
activity is directly related to organic phosphorus content (Gavrilova et al.,
1974). Another form of evidence for organic P mineralization is provided by
analogy with the organic nitrogen and carbon mineralization. Thompson et al.
(1 954) showed that the rate of organic phosphorus mineralization was positively
correlated with the rates of nitrogen and carbon mineralization. The amounts of
organic phosphorus, nitrogen, and carbon mineralized were proportional to these
constituents present in the soil organic matter, both in virgin and in cultivated
soils (Table MI),although this relationship does not always exist (Williams and
The mineralization of organic phosphorus in soil is largely due to the combined activities of the soil microorganisms and the free enzymes, phosphatases
Regression of Mineralized Nitrogen and Organic Carbon on Mineralized Organic Phosphorus
in Virgin and Cultivated Soilsa
Regression coefficient Constant term Significant at P =
Organic carbon Virgin soils
aTaken from Thompson et al. (1954). 63 1954 The Williams & Wilkins Co., Baltimore.
The regression equation is: N (or C) = a P + b, where a is regression coefficient (which did
not significantly differ between virgin and cultivated soils) and b is a constant term (which
differed significantly between virgin and cultivated soils).
(exo- as well as intracellular phosphatases released following the lysis of microbial cells), present in soil. The factors that regulate the activity of microorganisms thus mainly govern the mineralization of organic phosphorus in soil.
Since the optimum temperature for growth of most bacteria is between 30"
and 45°C (Stanier et at., 1971), mineralization of organic phosphorus increases
with increasing temperature, particularly above 30°C (Thompson and Black,
1949; Van Diest and Black, 1959; Acquaye, 1963). Eid et al. (1951) observed
that the optimum temperature for soil organic phosphorus mineralization
was 35°C. Thus Floate (1970) did not observe phosphorus mineralization from
plant material below 30°C;in fact net immobilization of phosphorus occurred at
such temperatures (Table XIII). Similarly, Eid et al. (1951) showed that organic
phosphorus was not available to plants at 20°C. Saunders and Metson (1971)
and Dormaar (1972) observed that the soil organic P increased during winter and decreased during spring. From these observations, Williams (1967)
concluded that in tropical soils, mineralization of organic phosphorus may
contribute significantly to the phosphorus nutrition of plants, whereas in temperate soils the contribution of organic phosphorus may be much smaller.
In fact, Williams and Lipsett (1961) found that only about 15 kg P per hectare
or 17% of the total organic P was mineralized during 50-60 years of wheat
cultivation in (temperate) New South Wales. Another consequence of the effect
on P mineralization is that in temperate soils organic phosphorus is mineralized
Effect of Temperature and Moisture on Mineralization of Phosphorus from Plant Material
and Sheep Feces Incubated for 12 Weeks'
Percentage of original P mineralized
Temperature ("C) WHC at 10°C (%)
'Adapted from Floate (1970).
bA and B plant materials were Festuca-Agrostis and Nardus type, respectively.
'Feces from sheep fed with A and B type of plant material, respectively.
SOIL ORGANIC PHOSPHORUS
more slowly than carbon, nitrogen, or sulfur when the soil is cultivated, causing
an increase in the proportion of organic phosphorus in soil organic matter
(Thompson et al., 1954; Williams and Lipsett, 1961); in contrast it is mineralized
at approximately the same rate as carbon and nitrogen in tropical soils (Thompson et al., 1954; Paul, 1954). The knowledge of the chemical nature of organic
phosphorus in such soils should enable the processes involved in the
mineralization of soil organic phosphorus to be elucidated.
Adequate moisture is essential for mineralization of organic phosphorus from
soil organic matter and decomposing plant residues. Feher et ~ l (1939)
that citric acid-soluble phosphorus (organic phosphorus) in soils was greater after
incubation at 50 to 75% of the water-holding capacity (WHC) than at either 25
or 90% WHC. Floate (1970) observed that the mineralization of organic phosphorus from plant material and sheep feces was enhanced at 25% WHC compared with that at 50 or 100%WHC (Table XIII). Considerable mineralization of
inositol hexaphosphate has been found to occur when soils are submerged
(Furukawa and Kawaguchi, 1969; Islam and Ahmed, 1973). Mineralization of
organic P in laboratory experiments was found to be greater in flooded soils than
in soils maintained at field capacity (Campbell and Racz, 1975). It was suggested
that this might have been due to anaerobic conditions occurring in flooded soils.
Evidently information about the effects of moisture on the mineralization of
phosphorus from soil organic matter or plant materials is too scarce to draw any
firm conclusions; more work, therefore, is needed to evaluate the effects of
moisture on organic phosphorus mineralization. Further, some of the reported
results on the effect of moisture on soil organic P are confounded with the effect
Alternate wetting and drying of soil enhances phosphorus mineralization. For
example, Birch and Friend (1961) found that the phosphorus in an organic soil
was completely mineralized as a result of 204 wetting-incubationarying cycles.
This was in marked contrast to organic carbon and nitrogen; 37% of organic
carbon and 54% of nitrogen remained in organic form after such cycles. The
mechanism of the alternate wetting and drying effect has not been established,
but it has been suggested that an important part of the organic matter which
decomposes during the drying process is the one that disperses into the soil
solution on wetting (Skyring and Thompson, 1966; Russell, 1973). It is also
possible that since wetting and drying breaks up water-stable soil aggregates, the
humic matter that has been inaccessible to the soil microorganisms becomes
exposed for decomposition, for Soulides and Allison (1961) showed that multiple wetting and drying cycles caused a greater reduction in water-stable aggregates than did a single cycle. The differential effect of wetting and drying cycles
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on various components (such as carbon, nitrogen, and phosphorus) of the soil
organic matter needs to be investigated (Birch and Friend, 1961).
The effect of aeration on phosphorus mineralization is complex (Wdliams et
al., 1960; Basak and Bhattacharya, 1962) and depends on the nature of the
organic material (Fabry, 1963). In general, if aeration becomes poorer, the rate
of decomposition of organic matter becomes smaller, an effect which becomes
significant at oxygen levels below about 1% of the partial pressure of oxygen in
the atmosphere (Greenwood, 1961). Thus Jackman (1964) observed that net
immobilization of phosphorus occurred in the first 7.5 cm of the grassland soil
and net mineralization of organic phosphorus occurred between 7.5 and 15 cm
of soil depth. Apparently the effect was not due to the oxygen content of the
soil but to the nature of the organic material in these layers. However, Campbell
and Racz (1975), from their observations on greater organic P mineralization in
flooded soils, suggested that anaerobic conditions increased mineralization rate.
It would be interesting to investigate the effect of moisture and aeration on
organic P mineralization separately and concurrently.
4. Soil p H
The rate of mineralization is enhanced by adjusting the pH such that it is
optimum for general microbial metabolism. Mineralization of soil organic phosphorus increases following liming of acid soils (Pearson et aZ., 1941;Goring and
Bartholomew, 1952; Thompson et aZ., 1954; Halstead et al., 1963; Islam and
Ahmed, 1973). The explanation for this phenomenon is that as the soil pH is
increased, microbiological activity is markedly increased (Halstead et al., 1963),
with concomitant increases in organic carbon and nitrogen mineralization. In
addition, it may also be due to less organic matter being fixed by clays and the
increase in the solubility of organic compounds. It has been observed that liming
does not always increase mineralization of organic phosphorus (Kaila, 1961).
Some of the variations in the effect of lime may be produced by the Ca:Mg ratio
effect on mineralization and turnover of P in soil (Fabry, 1963).
The rate of mineralization of organic phosphorus has been shown to be
positively correlated with the mineralization rates of carbon and nitrogen.
However, Thompson et al. (1954) showed that the rate of mineralization of
organic phosphorus increased with increasing soil pH whereas the mineralization
rates of carbon and nitrogen did not. A consequence of this is that in alkaline
soils the ratios of organic carbon:organic phosphorus and total nitrogen:organic
phosphorus should be greater than in acidic soils.
SOIL ORGANIC PHOSPHORUS
5. Addition of Inorganic Phosphorus
It was indicated earlier that addition of inorganic phosphorus (e.g., superphosphate) to soils under pasture results in increasing immobilization of phosphorus
(Jackman, 1964; Rixon, 1966). However, there are data (Table XIV) showing
that the addition of inorganic phosphorus results in increased mineralization of
organic phosphorus (McCall et d.,1956; Kaila, 1961; Acquaye, 1963; Fabry,
1963; Enwezor, 1966).
The increase in the mineralization of organic phosphorus in the presence of
inorganic phosphorus may be due to the increase in solubility of organic
phosphorus; hence its susceptibility to mineralize because inorganic phosphorus
competes with organic phosphorus for Fe, Al, and Ca that keep a part of organic
P in a sparingly soluble form (Wier and Black, 1968). Some investigators,
however, have observed that addition of inorganic phosphorus does not affect
phosphorus mineralization. Thus Hofman and Teicher (1 964) found that additions of inorganic phosphorus as fertilizer had essentially no effect on the
content of organic phosphorus in soil, Similarly, Wier and Black (1968) and
Ghoshal (1975) could not find any evidence of enhanced mineralization of
organic P due to the additions of inorganic phosphorus.
It would be advantageous if the changes in different organic phosphorus
fractions were studied following the addition of inorganic phosphorus rather
than the changes in total organic phosphorus that may occur. It may also be
helpful in estimating the labile organic phosphorus turnover in soil. Moreover,
the concurrent changes in organic carbon, organic nitrogen, and organic sulfur,
along with organic P upon the addition of inorganic phosphorus with and
without the addition of nitrogen and sulfur that occur in soil organic matter
The Average Effect of Ca(H, PO,), *H, 0 on the Mineralization of Organic Phosphorus in
Eight Organic Soils'
Added phosphorus (ppm)
Organic P mineralized b (%)
aCalculated from McCall e t aL, Soil Science Society of America Proceedings 20, 81-83
%oils were incubated for 4 months at moisture equivalent and at 267°C. Organic P
mineralized (%) = 100 (organic P without added P - organic P with added P)/organic P
without added P.
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should provide a better understanding of the effect of the addition of inorganic
phosphorus on the organic phosphorus mineralization in pasture as well as in
6. Fertilizers Other Than Phosphorus
Since organic matter contains carbon, nitrogen, phosphorus, and sulfur in
fairly definite proportions (Williams et al., 1960), the deficiency of carbon,
nitrogen, or sulfur in soil should result in smaller amounts of organic matter
buildup even if inorganic phosphorus in soil is in sufficient amounts. This
proposition has not been widely tested in cultivated soils but in legume-grass
pastures, where atmospheric nitrogen is fured by legumes and phosphorus is
supplied in superphosphate, the buildup of organic matter, hence organic phosphorus, may be limited by sulfur supply (Section V, A).
In the nonlegume cropping system, the application of nitrogen results in
greater immobilization of phosphorus (Barrow, 1960). Conversely, when nitrogen fertilizer is not applied and plants derive nitrogen from the mineralization of
organic nitrogen, organic phosphorus is also mineralized since mineralization of
organic nitrogen and organic phosphorus are similar processes (Section V, B and
Table XII). The similar effect of sulfur supply or withdrawal on phosphorus
immobilization or mineralization could occur (Table XV). However, from the
Effect of Varying Nitrogen, Sulfur, and Phosphorus Supply on Phosphorus Mineralization'
Percentage of P
mineralized (+) or
'Calculated from Barrow (1960).
'Incubated at 27°C for 7 and 21 days. In experiments 1,2, and 3, N, S, and P supply were
varied by varying the proportion of glycine, cysteine, and sodium pglycerophosphate,
respectively. In experiments 1 and 2, P was supplied as K, H PO,.
SOIL ORGANIC PHOSPHORUS
limited data available at present, it is not possible to postulate the mechanisms
of the processes of immobilization and mineralization of phosphorus as influenced by the application of nitrogen and sulfur fertilizers. There is an urgent
need to investigate this phenomenon in cultivated as well as in pasture soils.
It has been observed that cultivated soils generally contain less organic phosphorus than virgin soils (Thompson et al., 1954). It has been suggested that
cultivation increases the aeration of the soil which in turn stimulates microbial
activity and subsequently greater decomposition of organic matter (Clements
and Williams, 1964). But not all the components of organic matter are mineralized to the same extent. Thus Williams and Lipsett (1961) found that 50-60
years of wheat cultivation in New South Wales resulted in the loss of 30% of
organic carbon compared with 17% of organic P. Further, not all the organic
phosphorus fractions mineralize at the same rate, for Williams and Anderson
(1968) observed that the decrease in organic phosphorus resulted from cultivation was mainly a decrease of inositol phosphates. The cultivation of a virgin
thick chernozem resulted in the decrease of phytate, phospholipids, and nucleic
acid-P compounds (Levenets and Krivonosova, 1974) and in increase in the
organic phosphorus content in the nonhydrolyzable residues of the organic
matter (Krivonosova, 1972; Batsula and Krivonosova, 1973).
The land use also has an effect on the mineralization of organic phosphorus in
soil, possibly by manipulating temperature, moisture, and aeration effects.
Generally, the clean cultivated crops, such as maize, result in greater reduction in
organic phosphorus compared with pastures.
8. Soil Microorganisms
The rate of mineralization of organic phosphorus depends largely upon the
population as well as the activity of microorganisms in soil. Microorganisms
capable of extensively decomposing organic insoluble phosphorus compounds
have been reported to be present in soil. Species of Aspergillus, Penicillium,
Mucor, Rhizopus (Casida, 1959; Irving and Cosgrove, 1972) and BacilZus and
Pseudomonas (Kobus, 1961;Cosgrove, 1970) produce phosphatases that degrade
glycerophosphates, nucleic acids, and phytin. Up to 100% of the phosphorus in
phytin and 50% in nucleic acids and certain phospholipids is ultimately dissolved
by many bacterial strains (Kobus, 1961). Carbohydrate is apparently required as
an energy source for degradation of organic phosphates in soil by microorganisms (Hannapel et a]., 1964b); hence, organic phosphorus can be mineralized rapidly near plant roots (Greaves and Webley, 1965). Moreover,
phosphorus deficiency causes a 4- to 12-fold increase in phosphatase in many