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VI. Mobility of Soil Phosphorus

VI. Mobility of Soil Phosphorus

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The amount of movement by mass flow is the product of the concentration of phosphorus in the soil solution and the extent of liquid

flow. This latter may vary considerably, but a discussion of the factors

affecting it are outside the scope of the present paper. Movement by

mass flow may be of importance in bringing soil phosphorus to the

plant root and in causing leaching. Since the concentration of phosphorus

in the soil solution is generally low, the amount of movement will

normally be insignificant; for example, Barber (1962) showed that in

his soils mass flow could account for only a small fraction of the phosphorus taken up by the plants. Similarly, phosphorus is not normally

considered to be lost by leaching, although some loss must occur over

geological time since total soil phosphorus contents are generally lower

than parent materials.

Where the phosphorus concentration in solution is higher, movement by mass flow may be important. In soils of extremely low phosphorus adsorption capacity, for example, Ozanne et al. (1961) demonstrated that phosphorus could leach. Similarly Larsen and Sutton (1966)

showed that considerable phosphorus movement could take place in a

glasshouse soil when the adsorption complex had been satisfied by

heavy phosphorus applications. The phosphorus concentration in the

soil solution may also be raised by the activity of organisms. As Hannapel

et al. (1964b) pointed out, this may account for the large body of

evidence which shows that phosphorus penetration is greater in soils

which have received manure rather than inorganic phosphorus fertilizer.


Although the study of movement of phosphorus in soil by mass flow

dates back to Way’s classical studies in the middle of the last century,

the study of phosphorus movement by diffusion is of recent origin. It

has advanced only since the advent of 32P,which permits the precise

measurement of movement over short distances.

Diffusion is the process by which matter is transported from one

part of a system to another as a result of the thermal movement of

molecules or ions. This movement is continuous, but where the system

is at equilibrium there is no net transport. However, where differences

in concentration exist, transport will occur, tending to move the system

toward equilibrium. Transport of phosphorus through the soil will

cause chemical changes to occur both in the liquid and solid phases,

which will complicate the measurement of a diffusion coefficient. This

complication can be overcome by using carrier-free 32Pwhich permits



the measurement to be made in the absence of a concentration gradient.

Under these conditions adsorption and desorption will still be occurring,

so that the reactions between the liquid and solid phases must be taken

into account. Since diffusion occurs essentially in the liquid phase and

an individual phosphorus ion spends only a relatively short time in this

phase, the diffusion coefficient of phosphorus in the soil solution will be

different from that in free solution. The important factor is the fraction

of time a given ion spends in the solution phase. A term to account for

this was used by Lewis and Quirk (19sS), who suggested that

where D, is the diffusion coefficient which would have been obtained

if 32Phad not been adsorbed onto the solid phase and D, is the observed

(apparent) diffusion coefficient. In a given soil, the term solution

32P/total 32P and hence D,, will increase with increasing degree of

saturation of the phosphorus adsorption system. Thus Lewis and Quirk

(1965) showed that D, was directly related to the amount of added

phosphate. Comparable terms have also been introduced by other

workers. Olsen et al. (1962) used a capacity factor, related to the

phosphorus adsorption capacity of the soil, which he defined as the

slope of the line relating labile phosphorus to the concentration of

phosphorus in the soil solution.

The inclusion of factors of the type discussed above makes allowance

for only one aspect of phosphorus diffusion in soil. The observed diffusion coefficient will, however, differ from that in free solution in other

respects. An aspect which is of particular importance is the continuity

of the solution phase. This will be influenced by the solid particles

present, by the electrostatic forces in the solution adjacent to these

particles and by the moisture content of the soil. The significance of

these have been discussed by Porter et at. (1960) for chloride ions and

they introduced a “tortuosity factor” in their diffusion theory to allow for

solid impedance as well as factors for volumetric moisture content and

ionic interaction. Olsen et al. (1962) found that the “tortuosity factor”

determined for chloride could equally well be used for phosphorus,

provided that a further correction was made for the adsorption process.

Nye (1966) has pointed out that ions other than phosphate may

influence the rate of phosphorus diffusion. In his development of the

theory of self-diffusion and bulk diffusion in soil, he takes account also

of the small extent to which phosphorus diffusion may occur along the

surfaces of the solid phase.

An evaluation of the fundamental soil properties which are likely



to influence each of the modifying factors would enable predictions of

the rate of phosphorus diffusion to be made. However, the present stage

of development of the diffusion theory relevant to movement of phosphorus in soil is not advanced enough to allow a more complete assessment of these.

Studies on phosphorus diffusion in crumbs, which will yield results

which are relevant to soil in its natural condition, have been conducted

by Gunary ( 1963, 1964) and Gunary et al. (1965). He found that the

rate of diffusion was related to the degree of saturation of the phosphorus adsorption capacity. Thus addition or removal of phosphorus or

treatments which brought about changes in the phosphorus adsorption

capacity of a given system brought about changes in the rate of diffusion.

Complications may arise when the chemical environment within soil

crumbs is considered. In a study of isotopic exchange in a solution in

equilibrium with soil crumbs, Gunary (1963) showed that the 32P:31P

ratio decreased with time despite the absence of diffusion of 32Ptoward

the center of the crumbs. Larsen and Gunary (1965) explained this

observation by a release of 31P to the solution brought about by

anaerobic conditions in the center of the crumbs, which caused a phosphorus concentration gradient between the center and the surface. A

process of this type could be important even at normal soil moisture

levels in that it causes phosphorus to move to a more favorable position

for absorption by plants.


Agronomic Considerations

Where soil phosphorus is considered purely as a source of phosphorus

for plants, some simplifications are possible. The precise form in which

the phosphorus exists is then of little significance, and all the emphasis

can be placed on its behavior. Using an extension of Schofields (1955)

analogy, the behavior of phosphorus in soil can be likened to the behavior of water in a well system.

In Fig. 15, the central well is immediately surrounded by highly

porous material, beyond which there is an infinite extent of less porous

material. The level of water in the well will be the same as that in the

high porosity material through which water will flow freely. In contrast

the water in the material of low porosity will flow so slowly that its

level may differ from that in the high porosity material. As can be seen

from the diagram there is a central trough in the bed rock and until

this is full all the water will be confined to the highly porous material.

In the analogy, phosphorus in the soil solution is represented by the free

water in the well, and labile phosphorus by mobile water in the highly






FIG. 15. Well analogy depicting the behavior of phosphorus in soil.

porous material. Nonlabile phosphorus is equivalent to nonmobile

water in the outer zone of low porosity material. The phosphorus

adsorption capacity of the soil will be represented by the amount of

high porosity material, and the width of this zone will consequently

vary when the analogy is applied to different soils.

The parallel between the behavior of water in the well system and

phosphorus in soil can be seen when the two systems are considered

side by side.



Phosphorudsoil system

When only a small quantity of

phosphorus is present it will be strongly

adsorbed, the concentration in solution

will be below that for the precipitation

of any mineral and all the inorganic

phosphorus will be labile (Larsen,

1964 ) . The concentration of phosphorus

in the soil solution will here be controlled simply by the amount of inorganic phosphorus present and the size

of the adsorption system.

Waterlwell system

When only a small amount of

water is present it will all be in the

central trough, the level being too low

for seepage into the low-porosity material. In this situation, the level of water

in the highly porous material and in

the well will be controlled by the

amount of water which is present in

the zone of highly porous material and

the width of this zone.

If phosphorus is added, the concentration in solution will rise ,until

an equilibrium level corresponding to

the solubility product of some phosphorus mineral is reached; a crystalline

phase will then precipitate and the

phosphorus within its lattice will no

longer be labile. By further phosphorus

addition, it is possible to raise the concentration above the equilibrium value,

but in time the level will fall until

equilibrium is reattained.

If water is added, the level in the

well and highly porous material will

rise until eventually an equilibrium will

be reached at the lip of the trough.

Seepage of water will then occur into

the low porosity material and nonmobile water will begin to accumulate.

If addition is more rapid than the rate

of seepage, a temporary enriched state

will exist where the level is above the

lip of the trough. Seepage will continue

however, and in time the level in the

well will fall back to its stable equilibrium position.

Conversely, if phosphorus is removed from a soil which has reached the

equilibrium level, the solution and labile

phases will be depleted and nonlabile

phosphorus will be slowly mobilized to

restore the status quo.

Conversely, if water is removed

from a system that has reached equilibrium, the level in the well and high

porosity material will drop and nonmobile water will flow back slowly to

restore the status quo.

This picture of the behavior of phosphorus in soil can be summarized

by the reaction:

solution P


labile P

nonlabile P

in which it will be remembered that the reaction between solution

phosphorus and labile phosphorus is rapid, but that between labile and

nonlabile phosphorus is slow.



The immediate source of phosphorus for plants is the small amount

that is in the soil solution. As this is removed, the equilibrium is disturbed

and phosphorus in the IabiIe fraction will be drawn upon. Nonlabile

phosphorus is not likely to contribute to the supply over a period as



short as one growing season since its rate of release is too slow. The

supply of phosphorus to the plant then depends directly on the concentration in solution and indirectly on soil factors which maintain this.

The factors responsible may be better appreciated by reference to

the well analogy. The concentration of phosphorus in the soil solution

(the level of water in the well) is a function of the amount of labile

phosphorus (amount of mobile water) in relation to the phosphorus

adsorption capacity (quantity of high-porosity material), that is, the

extent to which the sorption capacity is filled which can be expressed

as the percentage saturation. If phosphorus is removed from the solution, it will be replenished from the solid phase labile phosphorus and

the system will readjust to a lower level. If this readjustment occurs

slowly, a temporarily larger drop in the phosphorus concentration in

solution will result. The new level which is eventually attained depends

on the adsorption capacity, since soils with a large adsorption system

will have a greater quantity of labile phosphorus for a given level in

solution. Thus the initial phosphorus level is controlled by the percentage saturation while the buffering of this level is controlled by the

quantity of labile phosphorus.

When appreciable uptake occurs, it will substantially lower the

phosphorus level in the solution immediately adjacent to the roots.

Maintenance of phosphorus supply to the plant will then depend on the

movement of phosphorus to replenish this. Barber (1962) showed that

this movement is primarily a diffusion process, the rate of which is

related to the concentration in solution.

The important factors in phosphorus supply to the plant are therefore the intensity, kinetic, and capacity factors of Wiklander (1951))

and the diffusion factor. The intensity factor is a measure of the concentration of phosphorus in solution; the kinetic factor describes the

rate at which the solution is replenished from the solid phase; the

capacity factor is the quantity of phosphorus capable of replenishing

the solution (the labile phosphorus), and the diffusion factor is the rate

at which the absorption zone is replenished from nearby soil solution.

The supply of phosphorus to plants could be limited by any of these

four factors, and it is therefore of interest to consider their relative


The intensity factor is of direct importance, but account must also

be taken of the extent to which the concentration in solution is buffered. This buffering depends on the quantity of labile phosphorus

present, that is, the capacity factor. For soils with similar adsorption

capacities, the level of phosphorus in solution will be directly related

to the quantity of labile phosphorus so that either the intensity or



capacity factor on its own may be well correlated with plant uptake.

Where a wider range of soils is considered, the adsorption capacities

will vary so that both factors must be taken into account. Thus Gunary

and Sutton (1967) were able to account for 80 to 85 percent of the

variation in phosphorus uptake from a range of soils when intensity

and capacity factors were considered together. This high degree of correlation does not allow for much improvement from further introduction of kinetic and diffusion factors, However, these latter factors have

both been shown to be closely correlated with the concentration of

phosphorus in solution and so were already taken into account by the

intensity factor used by Gunary and Sutton.

That the intensity and capacity factors together can generally fully

describe phosphorus supply in soils can be appreciated from the well

analogy, Here the level of water in the well and the amount of mobile

water with which it is in equilibrium are all that are necessary to

completely describe the short-term water supply for any well system.

For a specific system, the change of status brought about by water

addition or removal can be monitored by following changes in either

one of these parameters.

Similarly, for a particular soil only one parameter need be followed,

and in the subsequent section on the maintenance of phosphorus status,

only changes in the amount of labile phosphorus are considered.




In virgin soils, presumably near to equilibrium, the amount of labile

phosphorus present will be controlled by the solubility product of some

phosphorus mineral. For slightly acid, neutral, and calcareous soils, the

relevant mineral is likely to be hydroxylapatite, so that the concentration of phosphorus in solution will be low.

If phosphorus is removed, the equilibrium level will still tend to be

maintained, since in time, mobilization of nonlabile phosphorus will

occur, as shown for example by Larsen and Sutton (1963) and Vaidyanathan and Talibudeen ( 1965). These latter authors removed phosphorus from soil by means of anion and cation exchange resins. This

treatment brought about an initial decrease in the readily isotopicaIIy

exchangeable phosphorus and they followed the recovery of this fraction

during incubation periods of up to 9 weeks. They were not able to

study in detail the rate at which this recovery occurred, but it appeared

to have been completed within the experimental period. From their

data, the ‘%alf-life”of the process can be estimated to be about 10 days

in one soil rich in isotopically exchangeable phosphorus, and twice as

long in a soil of a lower phosphorus status. This mobilization of pre-



viously nonlabile phosphorus is of agronomic importance in the maintenance of phosphorus levels under extensive agricultural conditions.

For intensive agriculture, the equilibrium level of labile phosphorus

is likely to be far too low for maximum crop growth. The phosphorus

status has to be raised and this may be achieved either by adding more

phosphorus or by reducing the total adsorption capacity. The effect of

reducing the adsorption capacity can be visualized from the analogy,

where reducing the amount of highly porous material without altering

the quantity of mobile water will have the effect of raising the level.

Until more is known of the mechanism of phosphorus adsorption in

soil, progress in reducing the adsorption capacity is bound to be slow.

However, the beneficial effects of organic matter, silicates, and lime on

phosphorus uptake must at least in part be due to blocking or eliminating adsorption sites.

The commonest way of increasing the phosphorus status is by the

addition of phosphorus in manure or fertilizer. Where the initial phosphorus status is very low or the sorption capacity is very high, the

amount of phosphorus required to reach a satisfactory level will be

prohibitive, Under these circumstances it is necessary by fertilizer placement to restrict the amount of soil that the fertilizer actually contacts,

in order for at least part of the growing medium to reach a satisfactory

level. It is well known that the quantity of phosphorus removed by

crops is small in comparison with normal fertilizer additions (recoveries

as low as 10 percent are common). The use of repeated applications

should thus lead to an enriched state.

However, an enriched state is metastable and there will be a gradual

loss of labile phosphorus to a nonlabile form. An exponential rate of loss

of labile phosphorus was suggested by Larsen et al. (1965) and an

example of their results is shown in Fig. 16. In the pH range 5.5 to 7.5

they found half-lives for the rate of fall of labile phosphorus content in

their (mineral) soils to vary from 1 to 6 years, the more rapid loss being

associated with the soils of higher pH. This suggested that the mechanism for the loss of lability could be a slow formation of crystalline

calcium phosphate, presumably hydroxylapatite. The results of Eanes

et al. (1965) suggest that in the formation of pure hydroxylapatite there

is a spontaneous autocatalytic change from the initial amorphous

product to a well crystalline material. It may be that the exponential

curve for loss of labile phosphorus was due to this spontaneous change

occurring at random in the isolated spots where labile amorphous

phosphorus compounds had been formed from the added fertilizer.

The maintenance of an enriched level requires account to be taken

of loss by conversion to nonlabile forms. This loss has been treated



theoretically by Larsen and Probert ( 1968). They considered the

situation where phosphorus that was fully and immediately labile was

added repeatedly in a regular pattern. The loss of labile phosphorus

between -additions would initially be less than that added, and the

phosphorus status would rise. As the status rose the amount lost



o 5001R Pfilacra






* \o,





















FIG. 16. Rate of loss of labile phosphorus after enrichment. Asterisk (left

ordinate): millimoles of P per kilogram of soil. (From Larsen et al., 1965.)

between applications would increase until eventually the loss would

be exactly equal to the amount added. They also considered the situation where the added phosphorus was not immediately labile, for

example, water-insoluble phosphorus. The level attained could still be

predicted provided that the rate of conversion to a labile form (the rate

of dissolution) was known.

For a particular soil the ultimate level attained for any source

depended on the amount of phosphorus added, the interval between

applications, and the rate of loss of labile phosphorus. In the example

shown in Fig. 17, it was assumed that the rate of loss of labile phosphorus and the rate of dissolution of the slow-acting source both obeyed

first-order kinetics. The mean status eventually attained would be virtually independent of source, although the time taken to reach this level

would increase as the rate of dissolution decreased. In the example

quoted it would take many more applications of the slow acting source

than of the fully labile source to reach this situation.

Once a stable situation had been reached the essential difference

between the sources was in the variation which occurred around the

mean status. The water-soluble source resulted in high peaks at the

time of application with subsequent low troughs, whereas the slowrelease source showed smaller oscillations.



Thus when it is required to raise the soil status, it is obvious that the

immediate effect of the water-soluble source is essential. Once a high

status has been reached, its maintenance at a relatively constant level

would require frequent small applications of a water-soluble source,




Water-sobble P source









P source







FIG. 17. Theoretical variation in phosphorus status with time. Equilibrium

situation reached after repeated triennial applications of phosphorus. (Half-life of

immobilization, 2 years; half-life of dissolution of slow-acting source, 1 year.)

whereas less frequent, larger additions of a slow-release source may be

tolerated. However, the contrasting requirements of the various crops

in a rotation require attention, and applications of the water-soluble

source could be phased with advantage so that the peaks coincided

with the most demanding crop.

On such a basis, it should be possible to predict the most suitable

amount, timing, and source of phosphorus for particular agronomic

situations, provided that the relevant soil parameters are known.


The determination of soil phosphorus as a nutrient source for plants

should ideally yield information on the behavior of the phosphorus. It

was concluded in Section VII, A that the intensity and capacity factors

together can describe phosphorus supply with considerable precision.

Measures of these two factors are thus required.



The simplest measurement of the intensity factor is the phosphorus

concentration in the soil solution. However, the determination of this is

complicated. For example the phosphorus concentration is affected by

the soi1:solution ratio and the ionic strength of the soil solution. As

suggested by Schofield, both complications can be reduced by standardizing the soi1:solution ratio at l : l O , using 0.01 M CaCl? as extractant.

The phosphorus in 0.01M CaCI, solution can be expressed in various

ways: ( a ) total Concentration; ( b ) concentration of individual phosphorus ions; ( c ) activity of individual phosphorus ions; ( d ) activity

products of calcium and individual phosphorus ions, e.g., % pCa


The choice of parameter will to some extent depend on the purpose

of the investigation. For plant uptake, the total concentration generally

gives a better measure than the activity (Wild, 1964), and this may be

improved by expressing it logarithmically ( Gunary and Sutton, 1967).

The capacity factor, the quantity of phosphorus that is capable of

replenishing the soil solution, can only be measured by isotopic dilution

analysis. The various methods for doing this have already been discussed in Section V, B, and the practical details for methods used in

this laboratory have been given by Gunary and Sutton (1967). They

found that of the capacity factors studied, the L value gave the best

correlations with plant uptake of phosphorus. These authors also found

that combining their best measure of the intensity factor (log P concentration) with the L value, 80 to 85 percent of the variation in phosphorus uptake by ryegrass grown in pots could be explained.

A practical method, suitable for routine laboratory analysis, which

gives a combined measure of the relevant factors, is to use an anion

exchange resin as extractant (Cooke and Hislop, 1963; Hislop and

Cooke, 1967).

This method causes a minimum of chemical change in the soil, and

it is well correlated with phosphorus uptake by plants.

As discussed in the preceding section the amount of phosphorus

required to maintain a particular level can be predicted if the following

are known: ( a ) the level of phosphorus required, ( b ) the rate of loss

cf labile phosphorus, ( c ) the rate of dissolution of the phosphorus


The critical level of labile phosphorus required will depend on

many agronomic factors. With present knowledge this can only be

determined initially under practical conditions using conventional field


The rate of loss of labile phosphorus may be measured in the field


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VI. Mobility of Soil Phosphorus

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