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V. Organic Phosphorus Turnover in Soil

V. Organic Phosphorus Turnover in Soil

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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.



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




Increase in Soil Organic Phosphorusunder Irrigated Pastures"



organic P

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




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



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.



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




Changes in Organic and Inorganic Phosphorus upon Cropping'

Percent change from virgin soil



Organic P

Inorganic P

-3 5

-3 I


+ 2

'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

Lipsett, 1961).

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 =

Virgin soils

Cultivated soils


2.6 2





Organic carbon Virgin soils

Cultivated soils

20.6 1







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.

1. Temperature

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 (%)

Material incubated

Plant material







C/P ratio























- 2





















'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.



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.

2. Moisture

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

of aeration.

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



on various components (such as carbon, nitrogen, and phosphorus) of the soil

organic matter needs to be investigated (Birch and Friend, 1961).

3. Aeration

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.



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.



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

cultivated soils.

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

immobilized (-)b


Experiment no.




7 days

21 days











-1 0



















+3 3

6 7

+ 7


'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,.



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.

Z Cultivation

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

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