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III. Phosphorus Fixation by Soils, Clay Minerals, and Hydrous Oxides
L. A. DEAN
the soil using plants or a reagent capable of removing the readily soluble
phosphorus. The assumption is made that the added phosphorus remaining in the soil is in a slowly soluble form and consequently is relatively
Radioactive phosphorus has been used in phosphate fixation studies
(Ballard and Dean, 1940; Neller and Comar, 1947). The obvious advantage in such procedures lies in being able to distinguish between the phosphorous added and that already in the soil. However, the validity of such
an assumption is subject to question. It has been shown that P3*04ions
added to soils will undergo an isotopic exchange with some of the native
soil phosphorus (McAuliffe et al., 1947). If the order of magnitude of
this exchange is appreciable under the experimental condition for measurement of phosphorus fixation it would be necessary to account for
losses by this mechanism.
What appear to be discrepancies arise through the use of different
methods for determining phosphorus fixation. For example, the effect
of the degree of saturation of soils with calcium on the fixation of phosphorus depends on the method of determination. Heck (1934~)concluded that “a low degree of base saturation tends to give a soil a
greater capacity for fixing phosphorus in difficultly available form than
if the soil is more fully saturated with bases.” The method used was
to add a phosphate solution to soil, evaporate to dryness, and extract the
readily soluble phosphorus with 0.002 N HzSOa a t pH 3, the phosphorus
remaining in t,he soil being considered fixed phosphorus. When the phosphate fixing capacity is measured by observing the decrease in concentration of phosphorus in a soil-water system, however, the fixation of
phosphorus is found to increase with increasing degree of saturation in
respect, to calcium (Benne et al., 1936; Davis, 1946).
The methods determining t.he phosphorus fixation by soils are empirical. It is necessary to rigidly control such factors as concentrations
of phosphorus added, time of reaction, temperature and pH in order to
obtain reproducible results. Hibbard (1935) stated “soils have no definite fixing power.” The resu1t.s of phosphorus fixation measurements are
frequently reported as of percent fixation: the percentage of added phosphorus that had been fixed. When considered as relative values these
results have shown interesting differences in the properties of soil and
Measurement of the penetration of phosphorus in soils has also been
used as an index of fixation. Fraps (1922) placed soils in tubes 2 inches
in diameter and 14 inches long, mixed 1 g. of superphosphate with the
top 3 inches of soil, added 100 ml. water and allowed the column to
stand 24 hours. Then water was percolated through the column until
FIXATION OF SOIL PHOSPHORUS
1 liter was collected. With soils having a high fixing capacity almost
no phosphorus was found in the percolate. Several refinements to this
basic procedure have been used in studying the effect of fixation and
fertilizer properties and the penetration of phosphorus (Conrad, 1939;
Heck, 1934a). Henderson and Jones (1941) demonstrated the excellent
possibilities of using radio phosphorus in movement studies. Field investigations (Midgley, 1931 ; Stephenson and Chapman, 1931) have
shown that little or no penetration takes place with heavy soils, but with
light. textured soils appreciable movement may be expected.
Early in the history of the study of phosphate fixation attention was
turned toward ascertaining what soil components had the property of
retaining phosphate ions. Fraps ( 1922) found a significant correlat,ion
between the amounts of iron and aluminum dissolved from soils by strong
acid and the phosphorus fixing capacity of Texas soils. Gile (1933) and
Scarseth and Tidmore (1934) found an inverse relationship between the
silica-sesquioxide ratio and the phosphate fixing capacity of soil colloids.
The naturally-occurring hydrated oxides such as goethite, limonite, diaspore, and bauxite were shown to have phosphate fixing power comparable
with that of soils (Dean, 1934; Ford, 1933; Weiser, 1933). Hematite
does not show a noticeable phosphate-fixing capacity. Such investigations have established the importance of iron and aluminum compounds,
especially the hydrous oxides, in the fixation of phosphorus by soils.
The effect of the removal of iron from soils or clays on their ability
to fix phosphorus has been one approach to the consideration of the role
of iron compounds in the fixation of phosphorus (Allison and Scarseth,
1942; Black, 1942; Chandler, 1941; Coleman, 1942, 1944; Metzger, 1940,
1941; Romine and Metzger, 1939; and Toth, 1937). Invariably, treatments which remove a part of the iron result in a reduction in phosphate
fixation. Such treatments, however, do not completely eliminate significant fixation by the residual materials. For example, Toth (1937) removed the free iron oxides from a Cecil colloid by the method of Drosdoff
and Truog (1935), thus reducing the fixation of phosphorus from 0.370
to 0.205 m.e. per g., and with a Sassafras colloid from 0.275 to 0.125 m.e.
per g. This approach does not completely clarify the role of iron in the
fixation of phosphorus. Some of the free iron oxides in soil colloids exist
as coatings on the clay particles. The relation of this coating to the
properties of clays such as kaolinitjr is not known. It, is not improbable
that. the iron coatings mask the phosphate fixing properties of the clays
themselves. Black (1942) discussed the role of the kaolinite and iron
oxide components of Cecil clay, and suggests that kaolinite is only of
importance in fixing phosphorus from solutions of high concentrations.
There is little information available on the phosphorus fixing capacity
L. A. DEAN
of the mechanical soil separates or the relation of the surface area of
specific materials to the fixing capacity. Perkins, et a2. (1942)measured
the phosphorus fixation of separates from a Wabash soil. As the particle
size decreased phosphate fixation decreased when expressed as PzOafixed
per square meter of surface but increased when expressed on a weight
basis. Dean and Rubins (1947) show the anion exchange capacity of
Sassafras soils in millimols per 100 g. to be roughly proportional to the
specific surface when expressed as square meters per g.
The clay minerals common to soils such as kaolinite, halloysite, montmorillonite and illite all exhibit phosphorus fixation. It can be questioned,
however, whether the order of magnitude is sufficient to materially contribute to the fixation capacity of many soils. Studies have shown that
particle size, pH, concentration of phosphate solutions, and time of contact all have effects on the fixation of phosphorus by the clay minerals
(Black, 1942; Murphy, 1939; Scarseth, 1935; and Stout, 1939).
The amount of phosphorus fixed by natural unground kaolinite is
relatively low. Ball mill grinding, however, greatly increases this capacity. In light of recent studies by Laws and Page (1946) on the effect
of grinding kaolinite, it would be reasonable to conclude that the prop-
- - ------
Miami H Colloid
Fig. 1. Phoshorus retention by bentonite and soil colloids as a function of pH
FIXATION OF SOIL PHOSPHORUS
erties of this material may not be analogous necessarily to natural
Experiments with finely ground kaolinite have shown very high retention of phosphate with a maximum a t about p H 4. Fixation decreases
rapidly as the p H approaches neutrality. When natural kaolinites are
used t.his does not necessarily hold. Black (1942) has shown natural
kaolinites to have much lower fixing capacity. A change in the type of
fixation was noted when the time and concentration of phosphate solution
are varied. With dilute solutions there was no evidence of maximum
fixation a t p H 4, but as the concentration and time of contact are increased greater fixation occurs in the acid range. Similar results were
obtained by Coleman (1944) using kaolinite isolated from the C horizon
of an Orangeburg soil.
Fixation studies with bentonite by Scarseth (1935) and Stout (1939)
have shown a maximum retention a t p H 6. Black (1942) concurs; however, when using high concentrations the point of maximum fixation
shifted to pH 5. The curves by Steele (1935) shown in Fig. 1 illustrate
the relation between pH and phosphate retention by bentonite and two
soil colloids. Bentonite shows characteristic maximum fixation a t about
p H 6. Near neutrality the behavior of the soil colloids appears similar
to bentonite. However, they also show a point of maximum fixation in
the acid range. Presumably this is attributable to the iron oxides associted with these materials.
Probably the oldest theory pertaining to the mechanism of phosphate
fixation is that phosphate ions in solution are precipitated, thus becoming
a part of the solid phase. For the purposes of this discussion the term
chemically precipitated phosphorus will be limited to those compounds
which are formed as chemically homogeneous particles from ions in solution. Such a definition is intended t o eliminate from consideration
chemically precipitated layers on surfaces of the soil constituents.
Quite obviously no single mechanism will account for the decrease in
concentration of phosphate that takes place when all soils are brought in
contact with phosphate solutions. Bradfield et al. (1935) postulate three
separate mechanisms which possibly overlap. A t p H 2 t o 5 the retention
is chiefly due to the gradual dissolution of iron and aluminum oxides
which are then reprecipitated as phosphates. At p H 4.5 to 7.5 phosphates
are fixed on the surface of clay particles, and a t pH 6 to 10 phosphate
is precipitated by divalent cations if present.
L. A. DEAN
1 . Acid Soil Systems
I n acid soil systems iron and aluminum appear to be the most likely
soil constituents to fix phosphorus by chemical precipitation. The precipitation of phosphorus by iron and aluminum has been the subject of
a systematic and comprehensive study (Gaarder, 1930; Gaarder and
Graehl-Nielson, 1935). This work has shown that when iron and phosphorus are combined in equivalent quantities minimum solubility occurs
between p H 2 and 3. In the presence of an excess of iron, however, there
is a tendency to extend the range of minimum solubility to about p H 4.
When aluminum and phosphorus are combined in equivalent quantities
minimum solubility occurs at about pH 4 but when an excess of aluminum is present the range of minimum solubility extends from p H 4 t o 7.
Considering these possibilities there is no reason to dispute the possibility
of the formation of chemically precipitated iron and aluminum phosphate
in soils. There is considerable evidence available, however, which indicates that such compounds do not exist in soils in important quantities.
The availability for plant growth of precipitated iron and aluminum
phosphates (Marais, 1922; McGeorge and Breazeale, 1932; Truog, 1916)
suggests that these materials are more available to plants than much of
the fixed phosphorus in soils. Electrodialysis (Dean, 1934) and acid
extraction (Heck, 1934a) studies have indicated that precipitated iron
and aluminum phosphates are more readily extracted from soils than
much of the fixed phosphorus. These same studies did indicate that
dufrenite, a naturally-occurring basic iron phosphate, had analogous
properties t o much of the fixed phosphorus in soils. Mineral specimens
of dufrenite are usually in the form of hard nodules, however, and intensive grinding is necessary to prepare samples having a surface area
similar to the precipitated iron and aluminum phosphates. No indicat.ion
is given about the relative surface of the dufrenite used in these studies.
The concept that basic iron and aluminum phosphates are precipitated
in soils still persists.
The amounts of water soluble iron and aluminum in soils are very low
in comparison to the amounts of phosphorus that soils are capable of
fixing. Bear and Toth (1942) in discussing phosphate fixation by a Colts
Neck soil show the following: this soil has a phosphorus fixing
capacity of 1.2 g. P205per 100 g. yet prolonged electrodialysis of the
soil only removed 5.6 mgm. of iron and 3.4 mgm. of aluminum. On the
basis of these amounts of iron and aluminum it is hard to conceive how
any appreciable part of the fixing capacity of this soil can be accounted
for on the basis of the formation of chemical precipitates. On the other
hand, Metzger (1940, 1941) presents evidence to support the conclusion
FIXATION OF SOTL PHOSPHORUS
that the phosphorus fixing capacity of acid prairie soils can be accounted
for largely by precipitation phenomena. These conclusions were based
on the amounts of iron extracted from soils with 0.002 N H2S04 and the
reduction in phosphorus fixing capacity observed when soils were extracted with this reagent.
The solubility of the iron and aluminum associated with montmorillonitic and kaolinite clays separated from soils was measured by Coleman
(1944). Samples of clay were shaken for a month with solutions prepared by adjusting a dilute solution of hydrochloric acid to different reactions with ammonium hydroxide. An appreciable amount of iron and
aluminum was dissolved a t tshe more acid reactions, but not sufficient to
account for the total fixing capacity of the clays. It is not improbable
that if the iron and aluminum were precipitated immediately upon
entrance into solution this would enhance the dissolving of these substances from soil colloids. Such a mechanism is suggested by Low and
Black (1947) on the basis of studies on phosphate induced decomposition
I n considering the fixation of phosphorus by acid Hawaiian soils
Davis (1935) dismissed the possibility of any large part of the phosphate
fixation being by the formation of double decomposition precipitates because for any given equilibrium phosphate concentration the amount of
phosphate fixed varies very nearly a? the ratio of soil to solution.
2. Calcium-Soil Systems
Considering the complexity of the system H20-C02-Ca0-P205soil
scientists have spent little time on the stable forms of calcium phosphates
that are formed and persist in soils. It is not improbable that a part of
the calcium phosphate combinations that exist are of uncertain composition. This is in line with the early suggestion of Cameron and Bell
(1907) that phosphoric acid and lime exist as a series of solid solutions.
Bassetk (1917) suggested hydroxy apatite as the only stable compound
that can exist under soil conditions. McGeorge and Breazeale (1931)
have concluded that in calcareous soils the phosphate of low availability
is a carbonate-phosphate compound in which one mol of calcium Carbonate is combined with three mols of tricalcium phosphate.
MacIntire and Hatcher (1942) discuss evidence to support. a theory
that some of the superphosphate incorporated into limed soils will ultimately be reverted to a fluorphosphate similar in characteristics to raw
rock. This is an interesting theory in that it provides a mechanism by
which it would be possible for soil components to alter a fertilizer in situ.
One usually envisions fixation taking place by the phosphate ions diffusing away from the fertilizer particles and reacting a t the soil surfaces.
L. A. DEAN
This theory, however, could provide for the basic soil constituents surrounding a superphosphate particle inducing reduction in solubility without the phosphate ions ever leaving the fertilizer particle.
The continuous fertilizer plots a t the Rothamsted Experimental Station
have provided excellent material for obtaining actual evidence of the
formation of calcium fluorphosphates in soils which received long continued applications of superphosphate. Prior to the time the differential
fertilizer treatments were started (over a century ago) the Broadbalk
field had received a heavy application of chalk. Many of the particles
were large and some of the original material still persists. Nagelschmidt
and Nixon (1944) selected chalk fragments (0.5-2 mm. diameter) from
the superphosphate plot and determined phosphorus and fluorine.
Analyses of samples taken 63 years apart are given below.
Fragments from the 1944 sampling were heated to 800'C. and the calcium
oxide removed with sucrose solution. The residue gave the X-ray powder
diagram of apatite.
The tendency for phosphorus to concentrate a t the interface between
the liquid and solid phase of the soil system is the phenomenon of
adsorption. This term supplies no implication which alludes to the
nature of the binding forces contributing to the phenomenon. Adsorption
is a sufficiently general term to include several kinds of surface reactions.
The simple statement that the phosphorus fixation is an adsorption frequently leads to confusion since this can hardly be construed as defining
a specific mechanism. A distinction between the kinds of adsorption is
of as much interest as t,he amounts of phosphorus involved.
Davis (1935) considers the retention of phosphorus a t the surface as
involving forces akin to those studied in organic chemistry. H e envisions
the phosphate ions penetrating the liquid-solid interface to form new
compounds with the hydrated minerals, and that these compounds are
in equilibrium with the hydrated minerals. Such a theory is not inconsistent with many of the experimental observations pertaining to the
fixation of phosphorus.
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).