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V. Fixation of Phosphorus by Surface Reactions
FIXATION OF SOIL PHOSPHORUS
The amount of phosphorus taken up by soils is proportional to the
concentration. It has been pointed out by Russell and Prescott (1916)
and others (Davis, 1935; Kurtz et al., 1946) that the adsorption of phosphorus by soils can be described by the equation of Freundlich
Fisher (1922) pointed out that compliance of data with this equation
cannot be considered as a criterion of adsorption. Nevertheless it can be
considered as corroborative evidence.
It is well known that some of the phosphorus associated with the
solid phase of soils which have been brought to equilibrium with phosphate solutions is water soluble. A dist.inction is frequently made between this phosphorus and the phosphorus ions more tightly associated
with the solid phase. Mattson and Karlson (1938) have distinguished
colloid-bound phosphate as ions that have become a nondiffusable structural unit in the colloidal aggregate, and saloid-bound phosphate as ions
in the diffusable ionic atmosphere held as compensation to ions of opposite charge. These two forms of binding are named micellar binding in
contrast to extra-micellar binding, which is precipitation of phosphate
by another ion, both being outside of the soil micelles.
The fixation of phosphorus as a function of pH by sodium and calcium-saturated bentonite was measured by Scarseth (1935). Calcium
ions greatly increased the phosphorus fixing capacity of the clay. The
Fig. 2. Phosphorus retention by sodium and calcium-soil colloids (Allison, 1943).
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theory proposed to explain this observation was that the phosphate ions
were held by the calcium present as exchangeable calcium. Similar experiments were undertaken by Allison (1943)and are illustrated in Fig. 2.
It may be seen that there is always a greater fixation of phosphorus by
the calcium saturated clay. Between p H 4 and 6 there is a tendency
for the Ca and Na curves to be parallel, but at about p H 6 the curves
break in opposite directions. This is the point where precipitation of
calcium phosphate probably becomes an effective mechanism in phosphate fixation. The role of calcium in the fixation of phosphorus within
the range pH 4 to 6 has not been adequately clarified. This is probably
what Mattson and Karlsson (1938) have termed saloid-bound phosphorus, the mechanism being that the calcium ions share valences with
the clay micelles and phosphate ions. Davis (1945) also concludes that
some of the H20-insoluble phosphate retained by soils may be in the
form of a double layer or a soil micelle-calcium-phosphate linkage.
2. Metathetical Reactions and Anion Exchange
Metathesis may be defined as the substitution of one ion for another.
In a broad sense it may be considered as a chemical react.ion without the
stigma of the laws of mass action and stoichiometry. It describes a
reaction involving chemical forces and affinities by which an ion in solution may become associated with the solid phase.
The fixation of phosphorus by the exchange or substitution of phosphate ions for hydroxyl ions has been suggested by Mattson (1930),
Demolon and Bastisse (1934), Scarseth (1935), Toth (1937),Stout
(1939),Kelly and Midgley (1943),and Coleman (1944). When a clay
suspension and a phosphate solution of similar pH are mixed, an increase
in pH accompanies the disappearance of phosphate ions from solution.
This can be t.aken as evidence that phosphate ions are replacing hydroxyl
ions associated with the solid phase. Quantitative estimates of the hydroxyl ion release with phosphorus retention have also been reported.
Other corroborative evidence of this phenomenon is the decrease in phosphorus retention with increasing pH and the release of phosphorus from
soils extracted with dilute sodium hydroxide.
The exchange of phosphate for hydroxyl ions can be accounted for
in several ways. It can be postulated that it entails an exchange for the
hydroxyl ions associated with surface of the clay minerals. Since the
order of magnitude of the phosphate fixing capacity and the inorganic
base exchange capacity are similar, all of the phosphate retention can be
accounted for on the basis of a surface reaction. This exchange of surface hydroxyl ions is a rateher plausible mechanism when considering the
fixation by minerals such as kaolinite and montmorillonite. However,
FIXATION OF SOIL PHOSPHORUS
evidence of silica release (Low and Black, 1947; Toth, 1937) accompanying phosphorus retention is not explained by this mechanism.
The fixation by the hydrated oxides of iron and aluminum can also
be ascribed to a simple substitution of phosphate for hydroxyl ions.
Heck (1934a) discussed the fixation of phosphorus in terms of it being
unnecessary for iron and aluminum to be in solution in order to have iron
and aluminum phosphates formed. It is not clear whether the iron or
aluminum phosphates formed are chemical precipitates or surface phosphates. Possibly this distinction is overemphasized. I n any event the
chemical bonding between the iron or aluminum and phosphate ions is
Both Mattson (1930) and Davis (1935) foresaw that the fixation of
phosphorus could increase the base exchange capacity of soils. Ample
evidence is now available to demonstrate this increase in base exchange
capacity. This increase in base exchange capacity is not inconsistent
with the theory that phosphate ions are fixed by an exchange with surface
hydroxyl ions, and would be applicable to both the hydrous oxides and
the silicate minerals. However, Toth (1939) has shown that deferrated
colloids may adsorb phosphorus without altering the base exchange
Anion exchange may be defined as the reversible substitution of one
anion for another. The term anion exchange immediately brings to mind
an analogy with t.he well-known base exchange reactions. As compared
with cations, only a limited number of anions having rather specific properties are considered as taking part in anion exchange reactions. Since
the nitrates, chlorides, and sulfates are only sparingly retained by soils,
any concept of anion exchange must exclude these ions.
Phosphate ions which are fixed by soils can be displaced by hydroxyl,
fluoride, silicate, arsenate, and possibly other anions (Dean and Rubins,
1947; Dickman and Bray, 1941; Stout, 1939; Weiser, 1933). If this exchange involves a reversible equilibrium of ions in solution with surface
ions the possibility of an anion exchange reaction presents itself. Kolthoff (1936) describes an exchange adsorption between lattice ions in the
surface of precipitates and foreign ions from the solution. It was shown
that less restricted ions a t the corners and edges of ionic crystals may
enter into exchange reactions with ions in the surrounding solution.
When crystalline barium sulfate was placed in a solution containing
chromate ions, there was an equivalent exchange between sulfate and
chromate ions. Dean and Rubins (1947) have postulated that a similar
reaction takes place when soils are placed in a solution containing phosphate ions. I n this instance ph0sphat.e ions in solution exchange for the
hydroxyl ions associated with the surface of the clay mineral or hydrous
L. A. DEAN
oxides. Thus a reversible equilibrium would be established between
hydroxyl ions of the soil surface and phosphate ions in solution and vice
versa. If chemical forces and affinities are involved in this reaction,
nitrate, chloride, and sulfate ions would not necessarily be expected to
Piper (1942) has suggested a method for determining the anionexchange capacity of soils. It is essentially a counterpart of the usual
methods for determining the base exchange capacity. Soils are saturated
with phosphate by treating with N ammonium phosphate at pH 4,
washed with alcohol to remove the excess, the adsorbed phosphate displaced by sodium hydroxide and subsequently determined to give the
anion-exchange capacity. Dean and Rubins (1947) sought to determine
the anion exchange-capacity with respect to several different oombinations of anions. For example, soils were saturated with arsenate by
treating with 0.5 M sodium arsenate p H 5.7, the excess removed with
alcohol, the adsorbed arsenate dispIaced by 0.5 M sodium phosphate p H
5.7 and subsequently determined to give the anion-exchange capacity.
Furthermore, it was also possible to proceed one step further and with
the same soil sample displace the phosphate with arsenate. This ability
of soils to become alternately saturated with phosphate and arsenate ions
has been used as evidence of anion exchange. However, this does not
completely preclude the possibility that chemical precipitates were not
When soils are saturated with arsenate and phosphate by using 0.5
M solutions a t pH 5.7, more total phosphorus is adsorbed than arsenic.
Also, all of the adsorbed arsenate is displaced by phosphate but all of
the adsorbed phosphate is not displaced by arsenate (see Table 11). It
would be interesting to know whether the differential adsorption was
simply attributable to ionic size or whether other ionic properties are
Phosphate and Arsenate Adsorbed by Soils and Displaced by Anion Exchange a
Results in millimols per 100 grams
Atwood h e sandy loam
Chester clay loam
Davidson clay loam
Dean and Rubim (1947).
FIXATION OF SOIL PHOSPHORUS
involved. There is also no indication as to whether mono- or divalent
anions are involved.
Another approach to the exchange equilibrium of phosphate and hydroxyl ions between surface and solution is through the use of isotopic
exchange with Ps204 (McAuliffe et al., 1947). If a soil is suspended
in water an equilibrium should be established between the phosphate ions
in solution and those associated with the solid phase. Then, if Ps204
ions are introduced into the system without materially altering the total
phosphorus concentration it should be possible to measure the extent of
the equilibrium between the phosphate ions in solution and on the surface
in accordance with the following reaction.
This is a simple isotopic exchange and the equilibrium constant should
be equal to 1. Thus, knowing the total Ps204and measuring the P3204
and P3'04 in solution, it is possible to calculate the P3l04 on the surface
that is in equilibrium with the P3lo4in solution. Such experiments have
been performed. The rate of equilibration of PS2o4in solution wit,h
Fig. 3. Changes in the ration P% surface to €%
solution with time (McAuliffe
e t al, 1948).
Ps204 ions on the surface is indicated in Fig. 3. Apparently there are
two distinct reactions. The first rapid reaction reaches equilibrium in
approximately 32 hours whereas the second reaction shows no indication
of a change in rate at the end of 200 hours. Thus two easily separable
steps were observed in the phosphate interaction with soil surfaces. The
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extent of the reaction taking place in 32 hours was taken as the first step
and the amount of surface P31 involved was calculated. This readily
exchangeable surface P31 was compared with the NaOH soluble phosphorus (see Table 111). Only a small fraction of the phosphorus, of these
Relationship between the Surface P Readily Exchangeable with P and
that Soluble in NaOH'
and number F soh.
lbs. P per
lbs. P per
x 100 = yo
McAuliffe et al. (1947).
soils, that was presumed to be held as exchangeable anions appears to be
in direct equilibrium with the phosphorus in the soil solution. These
experiments make it difficult to conceive of any close analogy between
base exchange and anion exchange of soils, since the exchangeable bases
of soils are all in equilibrium with the bases in the liquid phase.
Soil phosphorus adsorbed by plants is, in part, converted into organic
compounds of phosphorus. When the plant products are returned to the
soil they provide a source of energy for the soil microflora which in turn
synthesize organic compounds of phosphorus. Thus, on the basis of
prima facie evidence, i t may be concluded that the organic phosphorus
of soils is contained in compounds and derivatives of compounds syn€hesized by plants and microorganisms, biological fixation of phosphorus
being the process by which these compounds are formed. The presence
of relatively large amounts of organic phosphorus compounds in soils is
now well established. Until relatively recently, however, there was insufficient evidence to substantiate this assertion.
1. Distribution of Organic Phosphorus in Soils
The methods which have been proposed for the determination of the
organic phosphorus in soiIs are indirect procedures. Two systems have
FIXATION OF SOIL PHOGPHOPUS
been applied, namely; the treatment of soils with hydrogen peroxide to
oxidize the organic matter, followed by measurement of the increase in
acid soluble phosphorus (Dickman and DeTurk, 1938; Peterson, 1911) ,
and the extraction of the organic with alkalies. The organic phosphorus
is assumed to be the difference between the total and tGheinorganic phosphorus in the extract (Dean, 1938; Pearson, 1940; Potter and Benton,
1916; Schollenberger, 1918; and Wrenshall and McKibben, 1937). Certain objections could be raised pertaining to the accuracy of these methods; however, their use does permit useful generalization concerning the
overall distribution of organic phosphorus in soils.
Schollenberger (1920) studied the organic phosphorus contents of
virgin and cultivated soils representative of 12 soil types of Ohio. When
the organic phosphorus was expressed as per cent of the total phosphorus,
the virgin and cultivated soils contained very nearly the same ratio of
organic to total phosphorus. The range in organic phosphorus found in
the surface soils was 18 to 52 per cent of the total phosphorus. Dean
(1938) measured the organic phosphorus content of 34 surface soils from
widely separated parts of the world. The organic phosphorus content of
these soils was correlated with their carbon content and varied from 8 to
50 per cent of the total phosphorus. A study of the distribution of
organic phosphorus in seven Iowa soil profiles by Pearson and Simonson
(1939) has shown the amounts to range from 205 to 393 p.p.m. in surface
soils to as low as 8 p.p.m. in the C horizons. The ratios of organic phosphorus t o organic carbon and nitrogen varied considerably within the
individual profiles and from one soil type to another. Wrenshall and
Dyer (1939) found 75 to 85 per cent of t.he total phosphorus in black
muck soils and approximately 50 per cent of the phosphorus in podsol
soils to be in organic combination.
2. Jdentification of the Organic Phosphorus Compounds in Soils
The efforts to identify and characterize the organic phosphorus in
soils have centered about nucleic acids, phytin, and t,heir derivatives.
The ether-soluble phosphorus fraction accounts for only about 1 per cent
of the total organic phosphorus (Wrenshall and McKibbin, 1937). The
common approach to characterizing the organic phosphorus compounds
of soils has been to isolate phosphorus-rich fractions of the soil organic
matter and study the properties of these fractions. Virtually complete
extraction of the organic phosphorus can be facilitated by leaching soils
with dilute hydrochloric acid to remove the calcium, followed by an extraction with hot sodium or ammonium hydroxide. When these alkali
extracts are made slightly acid, the alpha humus precipikates and may
be removed by filtration. Yoshida (1940)) working with sodium hy-