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III. Phosphorus Fixation by Soils, Clay Minerals, and Hydrous Oxides

III. Phosphorus Fixation by Soils, Clay Minerals, and Hydrous Oxides

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394



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

unavailable.

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

related material.

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



395



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



396



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-



- - ------



Bentonite Colloid



Upshur Colloid

Miami H Colloid



---



b



1



1



3



I



I



I



I



5



7



9



II



PH

Fig. 1. Phoshorus retention by bentonite and soil colloids as a function of pH

(Steele, 1936).



FIXATION OF SOIL PHOSPHORUS



397



erties of this material may not be analogous necessarily to natural

kaolinite.

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.



IV. CHEMICALLY

PRECIPITATED

PHOSPHORUS

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.



398



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



399



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

of kaolinite.

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.



400



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.



PzOa

1881 sample

1944 sample



per cent



F

per cent



0.44

2.80



0.032

0.250



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.



V. FIXATION

OF PHOSPHORUS

BY SURFACE

REACTIONS

1. Adsorption

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.



401



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



$=kCn.



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



Frederick Colloid



Miami Colloid

-5



2

2



4



6



PH



8



u



2



6



4



8



PH



Fig. 2. Phosphorus retention by sodium and calcium-soil colloids (Allison, 1943).



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III. Phosphorus Fixation by Soils, Clay Minerals, and Hydrous Oxides

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