Directory UMM :Data Elmu:jurnal:S:Soil & Tillage Research:Vol53.Issue3-4.Feb2000:

Soil & Tillage Research 53 (2000) 255±273

Tillage, mineralization and leaching: phosphate
T.M. Addiscott*, D. Thomas
Soil Science Department, IACR, Rothamsted, Harpenden, Hertfordshire AL5 2JQ, UK
Accepted 28 July 1999

Abstract
Phosphate is usually the limiting nutrient for the formation of algal blooms in freshwater bodies, so tillage practices must
minimize phosphate losses by leaching and surface run-off from cultivated land. Mineral soils usually contain 30±70% of their
phosphate in organic forms, and both organic and inorganic phosphate are found in the soil solution. Some organic phosphates,
notably the inositol phosphates, are as strongly sorbed by soil as inorganic phosphates, and this decreases their susceptibility to
mineralization. The strength with which both categories are sorbed lessens the risk of their being leached as solutes but makes
it more likely that they will be carried from the soil on colloidal or particulate matter, and the greatest losses of phosphate from
the soil usually occur by surface run-off and erosion. Recent studies at Rothamsted have, however, shown substantial
concentrations of phosphate in drainage from plots that have long received more phosphate as fertilizer than is removed in
crops. These losses probably occurred because preferential water ¯ow carried the phosphate rapidly from the surface soil to
the ®eld drains. For lessening losses of phosphate by leaching and run-off, the prime requirement of tillage is that it should
encourage ¯ows of water through the soil that help it to retain phosphate. Primary and secondary tillage should ensure that the
surface roughness and porosity of the top-soil encourage the ¯ow of water into the soil matrix where it will move relatively
slowly and allow phosphate to be sorbed, thereby avoiding problems from run-off and preferential ¯ow. Inversion tillage can

be useful for lessening the loss of phosphate by run-off and erosion. Secondary tillage could be used to decrease the size of the
aggregates and increase the surface area for sorption. Although tillage will increase the mineralization of organic phosphate,
pulses of mineralization are unlikely to be so rapid or to lead to such large losses as with nitrate. The strength with which
phosphate is sorbed also lessens the problem. As with nitrate, the key to managing phosphate is basically good husbandry.
# 2000 Elsevier Science B.V. All rights reserved.
Keywords: Leaching; Surface run-off; Organic and inorganic phosphate; Erosion; Tillage; Mineralization

1. Introduction
Public concern about the effects of plant nutrients
lost from agricultural land to the wider environment
*

Corresponding author. Tel.: ‡44-1582-76-31-33; fax: ‡441582-76-09-81.
E-mail address: tom.addiscott@bbsrc.ac.uk (T.M. Addiscott).

has concentrated mainly on nitrate. This has resulted
from fears that nitrate in public water supplies can
cause methaemoglobinaemia in infants and stomach
cancer in adults, both of which have proved unjusti®ed
(Addiscott et al., 1991). These concerns, though

understandable, have tended to distract attention from
two facts: (1) the main problems caused by nutrient
losses from agriculture are to our environment rather

0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 9 9 ) 0 0 1 1 0 - 5

256

T.M. Addiscott, D. Thomas / Soil & Tillage Research 53 (2000) 255±273

than to our health, and (2) the main environmental
problem, algal blooms, often has phosphate rather
than nitrate as the limiting nutrient, particularly in
freshwater (e.g., Reynolds, 1992; Sharpley et al.,
1994; Ferguson et al., 1996). Whether nitrogen or
phosphate is the limiting nutrient depends on the type
of water body and the nature of the phosphate source,
which may be a point source (often sewage ef¯uent) or
a non-point source (usually agricultural land). We

clearly need to pay at least as much attention to the
behaviour of phosphate in soil as to that of nitrate and
to extract as much information as possible from
existing reports on the behaviour of phosphate. This
paper attempts to do so with particular reference to the
effects of tillage on the mineralization and leaching of
phosphate.
This paper begins by listing and discussing the
various chemical entities found in the soil that contain
phosphate. It then considers the processes that these
species may undergo and the interactions that occur
between them. It asks next how tillage affects these
processes and interactions and ®nally what its overall
effect on the leaching of phosphate is likely to be.

2. Different forms of phosphate in the soil
Those who refer to soil phosphorus or soil P need to
keep in mind that virtually all the phosphorus in the
soil is there as phosphate (strictly as orthophosphate),
PO4 (Frossard et al., 1995). Both organic and inorganic phosphates are to be found (Table 1), but neither

category is ever present to the exclusion of the other,
and neither could be said to be the dominant category
in soils worldwide. This is in contrast to nitrate, which

is found in the soil only as an anion and is never
combined chemically with organic matter. There is
also usually about 50 times as much organic nitrogen
in a soil as there is nitrogen as nitrate.
2.1. Inorganic phosphate
Orthophosphoric acid is tribasic, but the ®rst dissociation constant is very much greater than the
second or third. Durrant and Durrant (1962) reported
disagreement about their values but gave the following
approximations:
H2 PO4 „ 2H‡ ‡ H2 PO4 ÿ ;
H2 PO4 ÿ „ 2H‡ ‡ HPO4 2ÿ ;
HPO4

2ÿ

‡

3ÿ

„ 2H ‡ PO4 ;

K1 ˆ 9  10ÿ3
K2 ˆ 6  10ÿ8
K3 ˆ 1  10

ÿ12

(1)
(2)
(3)

The proportions of the three orthophosphate ions
depend on the pH of the solution but all are likely to be
present at the pH values likely to be found in most soils
(Aslyng, 1954). All the dihydrogen phosphates are
soluble in water, but of the other orthophosphates,

only those of the alkali metals (except lithium) are
water-soluble. Plants reportedly show a preference for
the dihydrogen phosphate (Moser et al., 1959), and
Aslyng (1954) gave a table showing the proportion of
the total orthophosphate present in this form at various
pH values. This proportion will be relevant to phosphate leaching where the dihydrogen phosphate is
sorbed or precipitated preferentially or when plant
uptake is likely to diminish leaching signi®cantly.
Inorganic phosphate is found in a variety of insoluble forms, of which the commonest in the earth's
crust is apatite (Frossard et al., 1995). This has the
general formula Ca10X2(PO4)6, where X is OHÿ or Fÿ,

Table 1
Some categories of phosphate in the soil
Category
Inorganic
Organic

Subcategory

Examples

Ionic
Mineral

PO43ÿ,

Monoesters
Diesters

Inositol hexaphosphate
Phospholipids
Nucleic acids
Microbial P
Adenosine triphosphate

Biomass P
Humic P

HPO42ÿ,

References
H2PO4ÿ

Apatite, tinticite

Aslyng (1954)
Frossard et al. (1995)
Anderson et al. (1974)
Newman and Tate (1980); Hawkes et al. (1984)
Newman and Tate (1980); Hawkes et al. (1984)
Brookes et al. (1982, 1984)
Jenkinson et al. (1979)
Tiessen et al. (1994)

T.M. Addiscott, D. Thomas / Soil & Tillage Research 53 (2000) 255±273

giving hydroxyapatite or ¯uoroapatite, respectively.
Calcium may be substituted by sodium or magnesium
and phosphate by carbonate. Monocalcium, dicalcium

and octocalcium phosphates are also found in soils in
which calcium predominates over aluminium and
iron. Where the latter metals dominate the system,
the phosphate compounds formed are not usually well
crystallized (Frossard et al., 1995). Reaction with
aluminium oxides may give an amorphous phosphate
or an organized phase such as sterrerite, (Al(OH)2)3HPO4H2PO4, while iron oxides may give tinticite,
Fe6(PO4)4(OH)6
7H2O or griphite, Fe3Mn2(PO4)2(OH)2.
2.2. Organic phosphate
The organic component usually comprises 30±70%
of the phosphate in mineral soils (e.g., Dormaar, 1972;
Hedley et al., 1982; Tiessen et al., 1984). It is found in
a wide range of forms in the soil (Table 1), which is not
surprising, given its role in metabolic energy transfer
and other life processes. Some forms are clearly
de®ned from a chemical point of view, others less
so. Organic phosphate is often fractionated using
chemical extractants, giving fractions that are de®ned
in terms of the extractant (e.g., Hedley et al., 1982;
Sharpley, 1985a), but there does not seem to be a

generally accepted procedure. We discuss two chemically identi®able forms and two with less speci®c
identities.
2.2.1. Monoesters
The term monoester-phosphate is used to describe
compounds with the general structure ROH2PO3, of
which the commonest in soils is inositol hexaphosphate. Inositol is essentially a hexane ring on which
each carbon atom carries a hydrogen atom and a
hydroxyl group. Its hexaphosphate, also known as
phytin or phytic acid, results from the esteri®cation
of each hydroxyl group. It has been known to soil
scientists for many years (Anderson, 1955; Arnold,
1956), and Arnold (1956) studied its hydrolysis, ®nding that the ester linkages were not all broken at the
same time. The presence of the organic group in the
molecule does not prevent the phosphate group from
being sorbed by the soil, and this plays an important
part in the compound's behaviour in the soil (Anderson et al., 1974).

257

2.2.2. Diesters

Diesters have the general structure (RO)(R0O)HPO3, but this simpli®ed structure covers a wide range
of compounds that include fragments of RNA (Anderson, 1970; Newman and Tate, 1980), phospholipids
(Newman and Tate, 1980) and teichoic acid (Anderson, 1980), a compound which consists of sugar units
linked by phosphate groups and which may originate
from bacterial cell walls (Ward, 1981).
2.2.3. Microbial biomass phosphate
The term microbial biomass has evolved as a collective term for the bacteria, fungi and small soil
animals that between them effect the turn-over of
organic matter in the soil (Jenkinson and Powlson,
1976). Phosphate is a constituent of phospholipids,
DNA and RNA in these organisms and is also involved
in their metabolic energy transfers. Brookes et al.
(1984) estimated the microbial biomass phosphate
in six arable soils to be between 6 and 24 kg haÿ1,
i.e., about 3% of the organic phosphate, and that in
eight grassland soils to be between 18 and
101 kg haÿ1, about 14% of the organic phosphate.
2.2.4. Humic phosphate
The term humic phosphate is used here to describe
phosphate associated with dead organic matter that
does not fall into either of the ester categories. In terms
of the extraction procedure described by Hedley et al.
(1982), this would be phosphate left after treatment
with chloroform and sodium bicarbonate. Some of it
will be susceptible to mineralization by microbes in
the soil and some inert.

3. Processes
3.1. Inorganic and physical chemical processes
3.1.1. Dissolution and precipitation
The mineral apatite is the primary source of phosphate for plant life. It dissolves in natural systems
when roots or microbes release hydrogen ions or when
the soil is naturally acid. Apatite-containing rock
phosphate is most useful as a fertilizer when the soil
has a pH of 6.2 or less. Superphosphate fertilizer is
made by acidifying rock phosphate with sulphuric
acid, which makes it a source of sulphur as well as

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T.M. Addiscott, D. Thomas / Soil & Tillage Research 53 (2000) 255±273

phosphate, but it has tended to be replaced by more
modern compound fertilizers containing monoammonium or diammonium phosphate.
Phosphate precipitation is of interest in the present
context because it lessens the vulnerability of phosphate to leaching, but in this respect it is usually less
important than sorption. The process has to be discussed in terms of the dominant metal oxides and
hydroxides in the soil. In calcium systems, it can occur
following the sorption of the phosphate on to a calcite
surface (Cole et al., 1953; Freeman and Rowell, 1981),
which leads to the precipitation of monocalcium
phosphate, Ca(H2PO4)2. This changes to dicalcium
phosphate dihydrate, CaHPO4
2H2O, and thence to
octacalcium phosphate, Ca8H2(PO4)6
5H2O, and
hydroxyapatite, Ca10(OH)2(PO4)6. The last compound
is least soluble in water and should in theory control
the phosphate concentration in the soil solution. In
practice, apatite is not usually among the calcium
phosphates found in the soil after application of
fertilizer, possibly because other ions in the soil
solution interfere with the formation of its crystals
(Frossard et al., 1995). In soils dominated by aluminium or iron the precipitation cannot be described with
the same detail and leads to the less well crystallized
compounds discussed in Section 2.1.
3.1.2. Sorption and surface reaction
Phosphate sorption is again of interest in the present
context because it lessens the vulnerability of phosphate to leaching. It is a complex process that cannot
be considered as independent from other physicochemical processes in the soil. Physical chemists
sometimes make the distinction between physical
sorption, in which the sorbed substance is simply
attracted to the sorbing surface, and chemical sorption,
in which there is a surface reaction. The sorption of
phosphate is best seen as a continuum, with some ions
loosely held but most of them strongly (chemically)
sorbed. Ions move within the continuum, and equilibrium will be reached if the system is left undisturbed.
Both inorganic and organic phosphates are sorbed.
Because of the heterogeneous nature of the sorbing
surfaces in most soil, detailed studies of the mechanisms of sorption are often made on soil minerals rather
than whole soils. Freeman and Rowell (1981) studied
the sorption of inorganic orthophosphate ions by
calcite and showed by scanning electron microscopy

that the sorption involved a precipitation reaction
which produced hemispherical coral-like crystalline
growths on the surface of the calcite. These underwent
the sequence of chemical changes described earlier in
the section.
The oxides and hydroxides of iron and aluminium
play an important part in the sorption of phosphate
(e.g., Yuan and Lavkulich, 1994).The role of goethite
(FeOOH) has been studied in detail by Ognalaga et al.
(1994) and Frossard et al. (1995). When it comes into
contact with orthophosphate ions in aqueous solution
there is a very rapid reaction involving exothermic
ligand exchange between the ions and the reactive
surface groups. A hydroxyl ion or a water molecule is
released from the surface, and a phosphated surface
complex is formed (Par®tt, 1978; Goldberg and Sposito, 1985; Torrent et al., 1990). The sorption of two
monoester phosphates, inositol hexaphosphate and
glucose-1-phosphate, on goethite was found by Ognalaga et al. (1994) to show a very similar mechanism to
that of the orthophosphate. Indeed, plotting the quantity of phosphate sorbed, the pH and the zeta-potential
against the equilibrium concentration gave very similar curves for all three sorbates. This ®nding accords
with the observation by Anderson et al. (1974) that the
sorption sites for inositol hexaphosphate and inorganic
orthophosphate in acid soils were the same. Ognalaga
et al. (1994) proposed a conformation for sorbed
inositol hexaphosphate in which the hexane ring
was parallel to the surface of the goethite, with four
of the six phosphate units attached to it and the other
two oriented in the other direction. This would be a
very stable conformation and it would account for the
observation of Anderson et al. (1974) that inositol
hexaphosphate was sorbed in preference to inorganic
orthophosphate in soils.
Studies on calcite and goethite provide useful
insights into the mechanisms of phosphate sorption,
but we are concerned ultimately with what happens in
soils and have therefore to consider the contribution of
clays and organic matter to the process. Most clay
minerals sorb less phosphate than goethite and the
other hydrous oxides. They generally offer two categories of sorption site, those at the edges of aluminium
layers and those on negatively charged surfaces. The
broken edges of aluminium layers are variable-charge
surfaces containing ±Al(OH) groups which function in
the same way as aluminium hydroxide surfaces. There

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T.M. Addiscott, D. Thomas / Soil & Tillage Research 53 (2000) 255±273

are considerable differences between the various clays
in their ability to sorb phosphate, and ranking the clays
and the iron and aluminium hydrous oxides for their
capacity to sorb phosphate gives the following order
(Sollins, cited by Frossard et al., 1995):
Montmorillonite < Kaolinite ˆ Hematite
< Gibbsite < Goethite
< Ferrihydrite < Allophane
Organic matter does not sorb phosphate directly in
all circumstances, but it can have a strong in¯uence on
the sorption or desorption of phosphate by other soil
components. Carboxylates originating from organic
matter may improve the availability of soil phosphate
to crops (Staunton and Leprince, 1996) and presumably its vulnerability to leaching, but other reactions
and reaction sequences involving organic matter may
enhance its sorption. These reactions were covered
recently by Frossard et al. (1995) in a comprehensive
review of the reactions of phosphate in soils.
The relative importance of the various sorbing
surfaces for phosphate in the soil will obviously vary
greatly between soils. In the highly weathered soils of
the tropics the oxides and hydroxides of iron and
aluminium are likely to dominate phosphate sorption.
Many African and South American soils, e.g., support
only very small concentrations of phosphate in the soil
solution and supply very little to crops unless fertilizer
is applied (Le Mare, 1981, 1982; Warren, 1992). The
strong sorption of inositol phosphate might have
serious implications in such soils, because many farm-

ers are not easily able to afford mineral phosphate
fertilizers. If they depend on organic fertilizers whose
phosphate is mainly in organic forms, the sorption of
such phosphate may limit its availability to crops.
Allophane also sorbs phosphate very strongly and is
likely to dominate phosphate sorption in soils in which
it is present in appreciable amounts.
Calcareous soils are fairly widespread, so we need
to assess the relative importance for phosphate sorption in these soils of the sorbing surfaces associated
with calcium, iron and aluminium. Holford et al.
(1974) developed a Langmuir two-surface equation
to describe phosphate sorption in soils in which differing bonding energies were to be found, and Holford
and Mattingly (1975) applied this to eight calcareous
soils in UK, plots of which had received three differing
amounts of superphosphate for at least 2 years. They
inferred from the results the high-energy and lowenergy Langmuir sorption capacities for the soils and
computed multiple regressions of these capacities on
the dithionite-extractable iron (Bascombe, 1968), the
pH, the surface area of calcium carbonate (Talibudeen
and Arambarri, 1964) and the percentage of organic
matter in the soils. The high-energy sorption capacity
was related highly signi®cantly to the dithioniteextractable iron but not to the other three variables
(Table 2), suggesting that it was associated with the
hydrous oxides of iron. The low-energy sorption
capacity was highly signi®cantly related to both the
surface area of calcium carbonate and the percentage
of organic matter (Table 2), suggesting that both types
of surface contributed low-energy sites. The relation-

Table 2
The main categories of site for phosphate sorption in soilsa
Category

Hydrous oxides
Ironc
Aluminiumd
Clay
Calcium carbonate
Organic matter
a

Calcareous soils

Acid soils

High energy

Low energy

Britishb

Tropical

***
*
nte
±
±

±
±
nte
***
***

***
***
*
±
**

**
***
*
±
**

Importance shown by the number of asterisks from Holford and Mattingly (1975) and Lopez-Hernandez and Burnham (1974a,b).
Relationships for British soils depended on drainage. See text.
c
Dithionite-extractable Cof®n (1963); Bascombe (1968).
d
McLean et al. (1958).
e
ntˆnot tested.
b

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T.M. Addiscott, D. Thomas / Soil & Tillage Research 53 (2000) 255±273

ship with the percentage of calcium carbonate, rather
than its surface area, was not signi®cant, and Bakheit
Said and Dakermaiji (1993) found the same in some
Syrian soils. Holford and Mattingly (1975) calculated
that phosphate was sorbed much less strongly on the
calcium carbonate in the soils than it was in Jurassic
limestone and suggested that this was because organic
anions occupied many of the sites that would otherwise have sorbed phosphate. This conclusion seems to
be supported by the effects of such anions reported by
Staunton and Leprince (1996) in a calcic luvisol.
In acid soils, no contribution to phosphate sorption
is expected from calcium carbonate, and extractable
aluminium emerged as the factor dominating sorption
in some acid Scottish soils (Williams et al., 1958).
Lopez-Hernandez and Burnham (1974a) examined the
behaviour of phosphate in 20 tropical and 20 British
acidic soils, using `anion exchange capacity' of Piper
(1942) and phosphate sorption index of Bache and
Williams (1971). They related these indices to soil pH,
percent clay, percent carbon, free iron oxides (dithionite-citrate extraction) and extractable aluminium
(acidi®ed ammonium acetate), ®nding no differences
between the British and the tropical soils (Table 2).
Sorption of phosphate was well correlated with extractable aluminium and free iron oxides, the correlation
with free iron oxides being the stronger in the freely
drained British soils but not in the poorly drained ones.
Sorption also correlated well with percent carbon in
the poorly drained British soils and in the tropical soils
when sorption was estimated using a large phosphate
concentration. The relationships with pH and percent
clay were not strong. However, when Lopez-Hernandez and Burnham (1974b) examined a group of pedologically similar soils differing mainly in pH, they
found a highly signi®cant decrease in phosphate
retention with increasing pH. This was associated to
some extent with decreases in exchangeable and
acetate-extractable aluminium. This accords with previous studies on hydrous oxides, such as that of Bache
(1964).
Ions not discussed in other contexts may in¯uence
phosphate sorption through their contribution to the
ionic strength of the soil solution. Ryden and Syers
(1975) found that increasing the ionic strength
enhanced sorption. This they attributed to the effect
of ionic strength on the surface charge of sorbing
surfaces and the thickness of the diffuse double layer,

but this effect depends on the ions contributing to the
ionic strength (Choudhary et al., 1993). The dominant
cation on the cation exchange complex also in¯uences
phosphate sorption. Curtin et al. (1992) reported that
sodium-saturated soil sorbed less phosphate than calcium-saturated soil and that the sorption was also
in¯uenced by the pH of the soil. They could not
explain the difference between the cations in terms
of precipitation or surface reactions involving calcium
phosphate, but they were able to deduce that it
occurred during the rapid initial sorption rather than
the time-dependant sorption discussed below.
The parent material from which the soil is formed is
another in¯uence on phosphate sorption. Toreu et al.
(1988) examined phosphate sorption on highly weathered soils derived from four parent materials in tropical Queensland. They found that the sorption
capacity was greatest on those formed from basalt
and lowest on those from granite, with metamorphic
and alluvial material intermediate between them.
Whether the soil is in a virgin or cultivated state
may also be important. Mehadi and Taylor (1988)
found that virgin soils sorbed more phosphate than
those which had been cultivated, apparently because
they had a lower pH and contained more exchangeable
aluminium and free iron oxide. The effects of cultivation on the mineralization of phosphate are discussed
in Section 4.2.
3.1.3. Time dependence of sorption
The sorption processes described above all occur
rapidly, but in many soils there is an additional longer
term slow reaction (Barrow, 1983; Van Riemsdijk et
al., 1984). One consequence of this was identi®ed by
Aharoni et al. (1991), who found that the Freundlich
and Elovich equations and an apparent ®rst-order
treatment of phosphate sorption were each valid for
only a limited range of reaction times. The slow
reaction has some of the characteristics of diffusion
and Barrow (1983), Van Riemsdijk et al. (1984), and
Van der Zee et al. (1989) interpreted it as an interparticle diffusion process. The diffusion would be very
slow, because the effective diffusion coef®cient for a
sorbed species depends (Nye, 1966) on the slope of the
sorption isotherm, which is very small for phosphate
in most soils. Given that the sorption of phosphate on
calcite or goethite involves a chemical reaction, it may
be more appropriate to think of the process as the

T.M. Addiscott, D. Thomas / Soil & Tillage Research 53 (2000) 255±273

movement of a reaction front. Barrow (1983) developed a computer model in which sorption was speci®cally dependant on time. Another approach (Van
Riemsdijk et al., 1984; Van der Zee et al., 1989) treats
the slow sorption as a function of the `exposure
integral'. This is the integral of phosphate concentration with respect to time, the area under the curve
obtained by plotting the concentration to which the
soil is exposed against time (Fig. 1a). The fractional
phosphate saturation of the soil is then related (Fig.
1b) to this exposure integral by a sigmoid curve (e.g.,
Freeze et al., 1995). This approach is useful because it
allows parameters describing the slow sorption to be

Fig. 1. (a) The exposure integral is the area under the curve
obtained by plotting the phosphate concentration (c) to which the
soil is exposed against time (t), and (b) plotting the fractional
phosphate saturation (Fs) against the logarithm of the exposure
integral (I) gives a characteristic sigmoid curve (e.g., Freeze et al.,
1995).

261

derived from experimental data and used to simulate
the outcome of the process at various times.
3.1.4. Desorption
Desorption is usually of interest because it is a
process that makes phosphate available to plants. In
the present context, we are concerned with it as one of
the processes that make it vulnerable to leaching.
Desorption usually occurs in response to a lessening
of the phosphate concentration in the soil solution,
most commonly as a result of uptake by plants. The
concentration will also be lessened when rainfall
percolates through the soil, and if appreciable desorption results, phosphate leaching may occur. In many
soils the fact that the concentration of phosphate in
solution has to be small for desorption to occur implies
that phosphate leaching following desorption is unlikely to be appreciable. The concentration of phosphate in solution supported by a soil can be estimated
by shaking it with 0.01 M calcium chloride solution.
Johnston (1969) measured by this method the phosphate concentration supported by the soils of the Broadbalk Experiment at Rothamsted. He also extracted these
soils with 0.5 M sodium bicarbonate solution (Olsen
et al., 1954), which is regarded as a good measure of
phosphate available to plants. A plot of the concentration in the calcium chloride solution against the
amount extracted by the method of Olsen et al. (Fig. 2)
showed two interesting features: (a) the phosphate
concentration in solution was very small when the
bicarbonate-extractable phosphate was less than about
40 mg kgÿ1 but increased sharply when it was greater,
and (b) soils from the two plots that receive farmyard
manure supported larger phosphate concentrations than
those from plots getting mineral fertilizer. Brookes
et al. (1997) showed that data from soils from several
experiments on broadly similar soils at Rothamsted
could be combined in a similar relationship.
Three factors seem likely to be important in both
sorption and desorption:
 The proportion of the soil through which the water
passes and, in particular, whether there is preferential flow.
 The residence time of the water in the soil.
 The quantity of phosphate can be desorbed rapidly,
as opposed to phosphate that can be desorbed only
after a diffusion process.

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T.M. Addiscott, D. Thomas / Soil & Tillage Research 53 (2000) 255±273

Fig. 2. The phosphate concentration supported in 0.01 M CaCl2 solution plotted against the amount of phosphate-extractable with 0.5 M
NaHCO3 (Olsen et al., 1954). Data from the top soils of the Broadbalk Experiment at Rothamsted (Johnston, 1969). In®lled circles represent
plots receiving farmyard manure.

Measurements of phosphate sorption often involve
shaking the soil with more than its own volume of
solution, so that a large proportion of the possible
sorption sites are exposed to the phosphate in the
solution. Similarly, in desorption studies, a large
proportion of the phosphate in the soil is exposed to
the solution. When water, with or without dissolved
phosphate, percolates through the soil, it makes contact with only a limited proportion of the sorption
sites. The extent of contact will depend on the nature
of the soil (Section 3.3). The duration of contact is also
important, bearing in mind the exposure integral discussed above, so the residence time of the water in the
soil becomes a relevant factor, and this again is
in¯uenced by preferential ¯ow.
The third of the factors listed above, the quantity of
phosphate that can be desorbed rapidly was shown by
Heckrath et al. (1995) to be very important. They
measured the phosphate concentrations in water draining from the plots of the Broadbalk Experiment at
Rothamsted and related them to the amount of phosphate extracted from the soils by the extractant of
Olsen et al. (1954). The concentrations in drainage
were very small up to a `break point' above which they
increased sharply with the amount of phosphate

extracted by the reagent (Fig. 3). This break point
was at about 60 mg kgÿ1 of extractable phosphate, and
phosphate seemed to be desorbed much more readily
above it than below it. The relationships for the total
and molybdate-reactive phosphate were similar, but
the total phosphate concentrations were greater
because some phosphate was transported on particulate matter. Whether the phosphate extracted by the
Olsen reagent corresponds exactly with the rapidlydesorbable phosphate is open to question, but it is
clearly a useful measure of it.
Hawkes et al. (1984) used 31P Nuclear Magnetic
Resonance to detect changes in phosphate fractions in
the soil. They too found that phosphate from diesters
was the most susceptible to mineralization, and that
when old grassland was ploughed and left bare for
about 25 years, the proportional decrease in diester
phosphate was much greater than that of monoester
phosphate.
3.2. Microbiological processes
3.2.1. Mineralization
Mineralization is not usually as signi®cant a contributor to inorganic phosphate in the soil as it is to

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263

Fig. 3. Phosphate concentrations in drainage from plots of the Broadbalk Experiment plotted on various dates against the amount of
phosphate-extractable from the soil of the plot with 0.5 M NaHCO3 (Olsen et al., 1954): (a) total phosphate, and (b) molybdate-reactive
phosphate (Heckrath et al., 1995).

mineral nitrogen. Chater and Mattingly (1980) found
that mineralization released between 0.5 and
8.5 kg haÿ1 per year of P as phosphate, the largest
amounts coming from soils recently ploughed out of
permanent grass or regularly given large dressings of
farmyard manure and the smallest from soils long in
arable cultivation without organic amendments. They
calculated that the largest releases by mineralization
represented about one-half of the annual phosphate
uptake by an average cereal crop (in 1980, a crop
yielding 5 Mg haÿ1 of grain). They added, by contrast,
that mineralization could supply rather more than the
whole nitrogen requirement of such a crop. This
suggests that mineralization probably contributes less
proportionally to phosphate losses from arable land
than it does to nitrate losses. Sharpley (1985a) concluded that mineralization contributed about as much
phosphate during the growing season as was supplied
by fertilizer.
The various forms of organic phosphate found in
soils were discussed in Section 2.2 and it is important
to know which categories are most susceptible to
mineralization. Condron et al. (1990) investigated

the amounts of monoester and diester phosphates
and teichoic acid in three soil environments in Saskatchewan, Canada. They found the greatest proportion of diesters in the part of the landscape least
favourable for mineralization, and both diesters and
teichoic acid were found only in native, uncultivated
soils. In soils cultivated for 70±80 years, only monoesters were found. They concluded that diester phosphates and teichoic acid are more readily mineralized
than monoester phosphate. Monoesters such as inositol phosphate are probably protected from mineralization by the strength with which they are sorbed by
mineral components in the soil (Anderson et al.,
1974). The distinction between organic and inorganic
phosphates is probably not as sharp as for nitrogen
compounds because of the stability of the monoester
phosphates and because their sorption mechanism on
goethite is so similar to that of inorganic phosphate.
The diester phosphates include fragments of RNA
and phospholipids which probably originate from
dead microbial biomass. Bowman and Cole (1978)
incubated various nucleotide phosphates in soil,
including those of adenine, guanine, cytosine and

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T.M. Addiscott, D. Thomas / Soil & Tillage Research 53 (2000) 255±273

uracil, a mixture of nucleotides, and RNA. Sodium
phytate, the sodium salt of inositol hexaphosphate,
was incubated in the same way to provide a monoester
phosphate for comparison. The net mineralization of
phosphate from the nucleotide phosphates was
between 60 and 75%, but from the inositol phosphate
it was 3%.
A substantial proportion of the component
described above as humic phosphate must also be
mineralized if, as Sharpley (1985a) concluded, mineralized phosphate contributes as much available
phosphate for crops as fertilizer. Bowman and Cole
(1978) concluded that diesters alone did not constitute
a large proportion of the organic phosphate pool, and
even if inositol phosphate and other monoesters are
present in appreciable quantities, the strength with
which they are sorbed by the soil limits their mineralization. A substantial contribution from the humic
component can also be inferred from the observation
that where tillage has caused a decline in organic
matter, the concentrations of carbon, nitrogen and
phosphate have declined to similar extents (e.g.,
Bowman et al., 1990). Hedley et al. (1982) concluded
that phosphate was released from both `extractable'
and `stable' forms of organic phosphate. Tiessen
et al. (1984) found conversely that the accumulation
of organic matter depended of the availability of
phosphate.
3.2.2. Immobilization
It is clear from the dependence of organic matter
accumulation on phosphate that the latter must be
immobilized in some circumstances, but the literature
on the immobilization of phosphate is not extensive.
Part of the reason must be that mineralization and
immobilization occur simultaneously and both show
considerable spatial variability. Hedley et al. (1982)
found evidence of immobilization when soil was
incubated, both when cellulose and nitrogen were
added and when they were not. Rewetting and incubating an air-dried Rhodesian (now Zimbabwean) soil
also immobilized phosphate (Salmon, 1965). However, when Addiscott (1969) incubated a rewetted
Tanzanian hillsand soil for 10 days, phosphate seemed
to be immobilized during the ®rst 4 days and then rereleased during the next 6 days, so that the ®nal
concentration of phosphate differed little from that
at the start.

3.2.3. Rapid changes in phosphate concentration
Mineralization and immobilization are slow processes when compared with another category of phosphate transformation. All living cells, including those
in the soil's microbial biomass, use the ADP±ATP
energy shuttle to link energy-supplying and energyrequiring reactions in their metabolism (Lehninger,
1965). The turn-over time of the terminal phosphate
group on an ATP molecule is measured in fractions of
a second (Lehninger, 1965), suggesting that phosphate
concentrations could change rapidly during periods of
intense microbial activity. White (1964) examined the
changes in phosphate concentration and microbial
respiration when soils were shaken in 0.01 M calcium
chloride solution, and concluded that microbial uptake
of phosphate became signi®cant after 2 h. Addiscott
(1969), using the same extractant, measured a two- to
three-fold decline in the phosphate concentration
supported by a Tanzanian hillsand within 3 h. Because
of the very small concentrations involved, between
10ÿ6 and 10ÿ7 M, the quantity of phosphate involved
was small, about 0.1 mg P kgÿ1 soil or 0.2 kg P haÿ1.
3.3. Transport processes
We found rather few papers concerned with phosphate leaching from cultivated soils. This probably
re¯ects the widespread perception that phosphate is a
strongly sorbed species that is not vulnerable to
leaching. This is a perception that had to be revised
when Heckrath et al. (1995) found the relation shown
in Fig. 3 and discussed above. It moves by leaching,
surface run-off, erosion and diffusion. The last of these
processes is so slow for phosphate in soil that it is of
interest only at the scale of the soil aggregate or the
cylinder of in¯uence around a root (Rowell et al.,
1967). Its main interest in the present context is as a
part of sorption. With the convective processes, we
need to consider both the pathways for water over or
through the soil and the in¯uences that determine the
carrying capacity of the water for phosphate.
3.3.1. Water pathways
Whether rain falling on the soil passes through the
soil or runs off the surface depends on whether its
intensity exceeds the in®ltration capacity of the soil
(Hillel, 1977). Any rain that is not able to in®ltrate the
soil will accumulate on a level surface or become

T.M. Addiscott, D. Thomas / Soil & Tillage Research 53 (2000) 255±273

surface run-off on a sloping one. Accumulation causes
water-logging and anaerobicity, while surface run-off
carries nutrients and soil particles with it, leading to
erosion in severe cases. Sharpley (1985b) found that in
surface run-off the rainfall interacted with a thin layer
of surface soil (10±25 mm) before leaving the ®eld.
The fate of rain that in®ltrates the soil depends greatly
on the type of soil. Water moves fairly uniformly
through homogeneous sandy soils, although waterrepellent organic coatings on the soil particles can
cause `®ngering' of the water and thence non-uniform
¯ow. Silty soils generally show more distinction than
sandy soils between mobile and immobile water in the
soil, as evidenced by the patterns of chloride movement observed by Barbee and Brown (1986), but this
does not usually extend to preferential ¯ow. Mobile
and immobile categories of water are also found in
many types of soil because of aggregation.
In clay soils patterns of ¯ow become more complicated in two ways. One form of ¯ow not usually
found in the other soil types is the horizontal movement that occurs at the base of the plough layer. This
results from the fact that the unploughed soil is far less
permeable than the ploughed soil above it, and this
effect can be intensi®ed by the compaction resulting
from the vertical pressure exerted by the mouldboard
of a plough. Clay soils are also far more likely than the
others to show preferential ¯ow (Bouma and Dekker,
1978; Beven, 1981). This may arise because many
clays develop cracks as a result of their propensity for
swelling and shrinking. Continuous channels left by
worms and roots also tend to be more durable in clay
soils than in others. Clay subsoils are frequently very
impermeable and need to have arti®cial drainage for
the land to be usable for agriculture. If the preferential
¯ow pathways through the soil connect with the
drainage system, a very ef®cient conduit is established
from the soil surface to the point where the drain
discharges into a ditch or stream. Thus, heavy rainfall
can carry solutes and particulate, colloidal or organic
material very rapidly from the soil surface to surface
waters causing substantial phosphate losses (e.g.,
Duxbury and Peverly, 1978; Miller, 1979).
3.3.2. The carrying capacity of the water for
phosphate
We saw in Section 3.1 that for phosphate to desorb
from the soil surface the concentration in the soil

265

solution usually needs to be very small, implying that
phosphate concentrations in water ¯owing through the
soil are likely to be small. If, however, the sorbing
phase is carried in the water much more phosphate can
be transported. Matter varying considerably in size,
composition and sorption capacity may be carried,
depending on the type of soil and the intensity of the
rainfall causing the ¯ow.
Particulate matter (>0.45 mm). Substantial amounts
of large particulate matter may be carried in both
horizontal and vertical ¯ows during heavy rain, but
its relatively large size compared with other material
carried and its consequently relatively small surface
area to volume ratio may mean that it is not a
particularly important carrier of phosphate.
Clay (