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V. Fixation of Phosphorus by Surface Reactions

V. Fixation of Phosphorus by Surface Reactions

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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).



402



L. A. DEAN



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



403



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

probably similar.

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

capacity.

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



404



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

participate.

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

being formed.

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

TABLE 2.

Phosphate and Arsenate Adsorbed by Soils and Displaced by Anion Exchange a

Results in millimols per 100 grams



Soil type



Total

phosphorus

adsorbed



Phosphorus

displaced by

arsenate



Total

arsenate

adsorbed



5.4

15.8

25.7



4.1



2.3



9.1

13.8

15.0



4*Q

9.1

112



Atwood h e sandy loam

Sassafras loam

Chester clay loam

Davidson clay loam

a



Dean and Rubim (1947).



32.0



Arsenate

displaced by

phosphate

3.8

3.9

10.0



10.0



405



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.



-



Pszo4f ps104

solution surface



PS204 f



surface



Ps104

solution



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



1



0



I



50



I



I

I50



100



I



200



Time, Hours



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



L. A. DEAN



406



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

TABLE 3.

Relationship between the Surface P Readily Exchangeable with P and

that Soluble in NaOH'

____~



Fraction

Soil type

Psssurface/

and number F soh.

Moyock 451586

Caribou 451588

Caribou 451589

Caribou E969

Davidson E787

a



11



71

220

100

210



P3'soil

solution

ppm P

2.6

0.27

0.029

0.026

0.005



PS1

surface

lbs. P per

2,000,000

Ibs.soi1



Na OH-soluble

phosphorus

lbs. P per

2,000,000

lbs. soil



surface/

NaOH-P

x 100 = yo



290

190

64

26

10



1,800

2,700

1,700

1,150

520



16.0

7.O

3.8

2.3

2.o



Pa



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.



VI. BIOLOGICAL

FIXATION

OF PHOSPHORUS

IN SOILS

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



407



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-



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