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Comparing simple methods for measuring phosphate sorption by soils.


Although the buffering capacity of a soil for phosphate has a large effect on optimum phosphate fertiliser rates, analytical laboratories are reluctant to measure sorption curves. They argue that the cost of the several analyses required can seldom be recovered. Simple indexes of the buffering capacity are therefore used. These may be divided into two categories. The first comprises

indirect measures such as the iron dissolved by ammonium oxalate solution which is used by the largest commercial laboratory in Western Australia. The second category comprises measures of buffering capacity derived not from a sorption curve but from a single measurement of sorption using just one initial phosphate concentration. The resulting pair of values for sorption and concentration may then be treated in different ways. In one method, the phosphate retention index (PRI) of Allen and Jeffery (1990) is obtained by dividing sorption by concentration. In another method, the phosphate sorption index (PSI) of Bache and Williams (1971), sorption is divided by log concentration, where concentration is expressed as [micro]g/L. In a third method (Barrow 2000) the pair of values is used to calculate the amount of sorption between solution concentrations of 0.25 and 0.35 mg P/L -- the Ozanne and Shaw (O&S) value.

When these methods are applied to soils that have previously received substantial applications of phosphate, the amount of phosphate sorbed from the solution underestimates the total amount of sorbed phosphate. It was found by Barrow (2000) that the phosphate extracted by 0.5 M sodium bicarbonate could be used as an index of the phosphate supplied by the soil during the sorption measurement. That is, the phosphate extracted by bicarbonate is added to the observed sorption. The indexes with this adjustment are expressed here as `[PRI.sup.*]', `[PSI.sup.*]', and `O&[S.sup.*]'.

To add further complexity, some measurements have been made using 0.01 M calcium chloride as a background solution, others using 0.02 M potassium chloride. We have designated these using the appropriate subscripts. This gives us 4 versions of each index, and these are designated, for example, as [PRI.sub.K], [PRI.sub.Ca], [PRI.sup.*.sub.K], and [PRI.sup.*.sub.Ca].

In this paper we compare these measures. We have used values derived from a sorption curve using at least 7 initial phosphate concentrations as the standard against which other indexes are measured. We also provide a calibration curve so that measurements made in potassium chloride can be converted into values appropriate for calcium chloride.

Materials and methods

Soil samples were collected from farmers' fields of the Western Australian cereal cropping region near Geraldton, Katanning, Merredin, Northam, Esperance, Tonebridge, Frankland, Moora, Wongan Hills, Corrigin, Pallingup, and North Stirlings. Samples of topsoil were taken to a depth of 5-18 cm following sampling procedures used for the soil survey conducted by Agriculture Western Australia. These procedures have been described by Beattie and Gunn (1988). The samples were bulked, air-dried, and sieved and the <2-mm fraction was retained for analysis. There was a total of 103 samples. Frequency distributions for some basic soil properties are given in Fig. 1.


Iron and aluminium soluble in ammonium oxalate were determined by a method similar to that of Schwertmann (1964). Soil was mixed with 0.3 M ammonium oxalate at pH 3.5 for 2 h at 23 [degrees] C on an end-over-end shaker at 10 rpm using a soil to solution ratio of 1:33.3. After centrifugation the concentrations of Al and Fe in the solution were measured by inductively coupled plasma-atomic emission spectrometry (ICP-AES).

Phosphate soluble in 0.5 M sodium bicarbonate was determined using the method of Colwell (1963, 1965). Phosphate sorption curves were obtained using both 0.02 M potassium chloride and 0.01 M calcium chloride as background solutions. For each of at least 7 initial P concentrations, 5 g of soil was mixed with 100 mL of background solution containing the appropriate phosphate concentration. The initial phosphorus concentrations ranged from 0 to 25 mg P/L and were chosen so that the observed concentrations were in the range 0.1-5 mg P/L. Mixing was for 16 h at 23 [degrees] C on an end-over-end shaker (10 rpm). Chloroform (0.25%, v/v) was added to inhibit microbial activity. After centrifuging for 15 min at 3500 rpm on a rotor with a diameter of approximately 30 cm, the concentration of phosphate remaining in solution was measured (Murphy and Riley 1962) and sorption calculated from the change in concentration. The sorption curves were described by a Freundlich equation modified as follows:

S + [P.sub.Bic] = a [c.sup.b]

where S is the measured sorption (mg P/kg soil), c the observed phosphate concentration in solution (mg P/L), [P.sub.Bic] is the phosphate extracted by sodium bicarbonate (mg P/kg soil), and a and b are coefficients. From the fitted curves, the P sorbed between 0.25 and 0.35 mg P/L was calculated and the values are designated as O&[S.sub.Fit, K] and O&[S.sub.Fit, Ca].

Single-point measures of sorption were obtained by similar techniques again using both 0.02 M potassium chloride and 0.01 M calcium chloride but using in all cases one initial concentration of 10 mg P/L. The individual indexes thereby obtained are summarised in Table 1.


Feasibility of single-point measurements

Out of the 103 samples tested, there was only one for which the final concentration of phosphate was too low for accurate measurement (c <0.2 mg P/L, >98% of P removed from solution). This shows that single-point measurements are feasible and conditions can be chosen such that repeat analysis is seldom required. However, the initial P concentration and the solution:soil ratio used here may not be universally applicable. It was chosen as a consequence of local experience. In other regions, different initial P concentrations and/or solution:soil ratios may be preferred.

Choice of the b coefficient to be used in O&S calculations

In order to obtain an objective estimate of the value to be used for the coefficient represented as b in Table 1 in calculation of the O&S values, the single-point values were compared with the fitted values. For this comparison, the values from the Ca[Cl.sub.2] solutions were used in both cases and for the single-point values, the adjustment for bicarbonate P was used (i.e. O&[S.sup.*.sub.SP, Ca] was compared with O&[S.sub.Fit, Ca]). The optimum value of b was found to be 0.356. Because the single-point values are not sensitive to its value (Barrow 2000), it was approximated to 0.35 in further calculations.

Prediction of buffer capacity

In the following sections, the standard measure of buffer capacity is taken as the O&S value calculated from the fitted curve using Ca[Cl.sub.2] as the background electrolyte (O&[S.sub.Fit, Ca]).

The single-point estimates of the buffering capacity (O&[S.sub.SP] values) were all highly and linearly correlated with the values derived from the fitted curves (Fig. 2). The correlation coefficients obtained using Ca[Cl.sub.2] were larger than those using KCl (Fig. 2). As the values to be predicted were obtained using a Ca[Cl.sub.2] solution this was to be expected. For both the Ca[Cl.sub.2] and KCl solutions, the single-point values adjusted for bicarbonate P gave the larger correlation coefficients. For the Ca[Cl.sub.2] solution, the correlation was excellent and the slope was close to unity (Table 2).


Most of the soils had O&S values lower than 13.8 (Fig. 2). This value corresponds to an observed solution concentration of 1 mg P/mL. Therefore, most concentrations were higher and thus fell in the desirable range when the value of b is uncertain (Barrow 2000). However, the present work, plus the previous work (Barrow 2000), indicates that a value of 0.35 for b may be widely applicable to Western Australian topsoils. This gives greater flexibility in the concentrations that can be tolerated.

The PRI and the PSI values (Figs 3 and 4) were also closely related to the O&S values. This is to be expected as they are calculated from the same single-point observations. However, the relations are not linear. The PRI values curve to the right because calculation involves dividing by the observed concentration. For soils of high buffering capacity, this concentration is small and so the index becomes large. In contrast, the PSI values curve slightly upwards because sorption is divided by log(concentration) rather than by concentration. The coefficients of the fitted equations are provided in Table 2 to enable estimations of the O&S values for given values of PRI or PSI.


Iron extracted by oxalate was poorly related to the O&S values (Fig. 5a). Extracted aluminium was better related (Fig. 5b) but the relation was inferior to those obtained with the single-point methods.


Figure 6a shows that buffering was greater in the Ca[Cl.sub.2] solution than in the KCl solution. The slope of the regression line is 1.25. In this figure the indexes used are the O&S values. Because of the way that PRI values are calculated, they are more sensitive to the background electrolyte. A ratio of 1.25 for the O&S values is equivalent to a ratio of about 1.8 for the PRI values. These results are therefore consistent with the regression slope of 1.76 reported when PRI values measured in Ca[Cl.sub.2] were related to those measured in KCl (Barrow et al. 1998)


The potassium chloride solution was introduced in Western Australia because of an expectation that calcium phosphates might precipitate from calcium chloride solutions in some of the soils with high pH. If this were the case, we would expect positive deviations between the calcium and the potassium solutions at high pH. Figure 6b shows that there was no evidence for this. Hence precipitation was not important within the period of measurement.

Because many values for [PRI.sub.SP, K] have been distributed to farmers and to farm advisors in Western Australia, it may be helpful to provide a scheme by which these values could be converted to O&[S.sub.SP, Ca.] The following scheme can easily be programmed into, for example, a spread-sheet.

Step 1. Calculate observed concentration c.

Recall that [PRI.sub.SP, K] is calculated from the ratio S/c when a solution containing 10 mg P/L is mixed with soil at a soil solution ratio of 1:20. Then c is given by:

c = 200/([PRI.sub.SP, K] + 20) mg P/L

Step 2. Calculate corresponding sorption S.

S = 20 (10 - c) mg P/kg

Step 3. Calculate O&[S.sup.*.sub.SP, K] assuming the Freundlich b coefficient to be 0.35.

Recall that O&[S.sup.*.sub.SP, K] is calculated from the difference in sorption between solution concentrations of 0.25 and 0.35 mg P/L. When b is 0.35, then [0.35.sup.b] - [0.25.sup.b] = 0.0769. Further, the a term of the equation is estimated from the calculated S and c values as (S + [P.sub.Bic])/[c.sup.0.35] where [P.sub.Bic] (mg P/kg) is the amount of P extracted by sodium bicarbonate solution. Thus:

O&[S.sub.SP, K] = 0.0769 (S + [P.sub.Bic])/[c.sup.0.35] mg P/kg

Step 4. Calculate O&[S.sub. SP, Ca] from the regression of O&[S.sup.*.sub.SP, Ca] on O&[S.sup.*.sub.SP, K] (data not shown).

O&[S.sub.SP, Ca] = 0.266 + 1.10 O&[S.sub.SP, K] ([r.sup.2] = 0.954)

Figure 7 shows the outcome of applying this calculation. The prediction is good, but imperfect. This is to be expected because the effect of changing from a divalent salt to a monovalent one depends on the pH of the soil relative to the point of zero salt effect (Barrow 1984).



This work shows that single-point indexes of phosphorus buffering were measured with just one analysis on all but one of the Western Australian soils tested. The resulting indexes were better related to measures derived from sorption curves than indirect measures such as the iron or aluminium dissolved in oxalate solutions. A further advantage is that measurement of phosphate concentration would be routine in soil testing laboratories. Such methods would therefore seem to be a better way to characterise sorption.

This work also supports the suggestion of Barrow (2000) that for fertilised soils phosphate extracted by bicarbonate can be used to adjust the measure of sorption. As bicarbonate P is routinely measured in many Australian laboratories this does not pose any extra analytical burden. It is beyond the scope of this work to test whether other extractants could be used for this purpose. Some discussion of other methods was provided by Barrow (2000)

The question which then arises is which of the single-point indexes is to be preferred. As they are calculated from the same data, they are closely correlated, and all 3 (PRI, PSI, and O&S) are closely correlated with buffering capacity as measured from a sorption curve. Any of the three could therefore be used to adjust fertiliser recommendations. The O&S values have the advantage that fertiliser requirement increased linearly with O&[S.sub.Fit, Ca] values of up to 17 in field trials and up to about 67 in pot trials (Ozanne and Shaw 1967). This index would therefore have the advantage of providing a simple adjustment.
Table 1. Definitions of the symbols used for the several indexes of
buffering capacity and of their method of calculation

A solution containing 10 mg P/L in 0.02 M KCl or 0.01 M Ca[Cl.sub.2]
was mixed with the soil for 16 h and the concentration (c mg P/L) of P
was measured. From the change in concentration, the P sorbed (S mg
P/kg) was calculated. [P.sub.Bic] (mg P/kg) indicates the P dissolved
in sodium bicarbonate as separately measured. PRI is the phosphate
retention index of Allen and Jeffery (1990), PSI is the phosphate
sorption index of Bache and Williams (1971), and O&S is the P sorbed
between solution concentrations of 0.25 and 0.35 mg P/L (Ozanne and
Shaw 1967). Definitions are for the KCl solutions. When Ca[Cl.sub.2]
was used, the K in the symbol was replaced by Ca, e.g. [PRI.sub.SP, K]
became PR[I.sub.SP, Ca]

[PRI.sub.SP, K] = S/c [PRI.sup.*.sub.SP, K] =
[PSI.sub.SP, K] = S/ln(1000c) [PSI.sup.*.sub.SP, K] =
O&[S.sub.SP, K] = (S/[c.sup.b]) O&[S.sup.*.sub.SP, K] =
 [gamma] where [gamma] = ((S+[P.sub.Bic])/[c.sup.b])
 [0.35.sup.b] - [0.25.sup.b] [gamma] where [gamma] =
 [0.35.sup.b] - [0.25.sup.b]
Table 2. Coefficients of the equations used in Figs 2-4 to describe the
relations between indexes of buffering capacity and the measure of
buffering capacity derived from fitted sorption curves
(O&[S.sub.Fit, Ca])

 Index Equation Coeffi- Without bicarbonate P
 cient adjustment

 0.02 M 0.01 M
 KCl Ca[Cl.

 Figure 2

O&[S. O&[S.sub.Fit, Ca] = [a.sub.1] 1.23 0.93
 sub.SP] [a.sub.1] + [b.sub.1] 1.19 1.06
 [b.sub.1] [r.sup.2] 0.895 0.934

 Figure 3

[PRI. O&[S.sub.Fit, Ca] = [a.sub.2] 111 152
 sub.SP] [a.sub.2] - m 19.8 27.5
 [a.sub.2]/ c 0.00641 0.00355
 {[(1 + m c [r.sup.2] 0.896 0.937

 Figure 4

[PSI. O&[S.sub.Fit, Ca] = [a.sub.3] 0.936 0.771
 sub.SP] [a.sub.3] + [b.sub.2] 0.180 0.138
 [b.sub.2] [b.sub.3] 0.00121 0.00164
 [PSI.sub.SP] [r.sup.2] 0.891 0.935
 + [b.sub.3]

 Index Equation Coeffi- With bicarbonate P
 cient adjustment

 0.02 M 0.01 M
 KCl Ca[Cl.

 Figure 2

O&[S. O&[S.sub.Fit, Ca] = [a.sub.1] 0.205 0.044
 sub.SP] [a.sub.1 + [b.sub.1] 1.13 0.97
 [b.sub.l] [r.sup.2] 0.953 0.985

 Figure 3

[PRI. O&[S.sub.Fit, Ca] = [a.sub.2] 82 91
 sub.SP] [a.sub.2] - m 10.7 12.7
 [a.sub.2]/ c 0.00543 0.00392
 {[(1 + m c [r.sup.2] 0.933 0.978

 Figure 4

[PSI. O&[S.sub.Fit, Ca] = [a.sub.3] 0.503 0.483
 sub.SP] [a.sub.3] + [b.sub.2] 0.110 0.0856
 [b.sub.2] [b.sub.3] 0.00192 0.00187
 [PSI.sub.SP] [r.sup.2] 0.934 0.949
 + [b.sub.3]


We thank Jennifer Harbord who conducted most of the chemical analyses. We also thank Mr Noel Schoknecht of the Natural Resource Assessment Group, Agriculture Western Australia, for provision of the soil samples. The project was funded by the Grains Research and Development Corporation (project CHC17).


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Barrow NJ (1984) Modelling the effects of pH on phosphate sorption by soils. Journal of Soil Science 35, 283-297.

Barrow NJ (2000) Towards a single-point method for measuring phosphate sorption for soils. Australian Journal of Soil Research 38, 1099-1113.

Barrow NJ, Bolland MDA, Allen DG (1998) Effect of previous additions of superphosphate on sorption of phosphate. Australian Journal of Soil Research 36, 359-372.

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Campbell NA, Keay J (1970) Flexible techniques in describing a range of response curves of pasture species. In `Proceedings of the 11th International Grasslands Congress'. Surfers Paradise. (Ed. MJT Thomas) pp. 332-334. (University of Queensland Press: St Lucia, Qld)

Colwell JD (1963) The estimation of phosphorus fertilizer requirements of wheat in Southern New South Wales by soil analysis. Australian Journal of Experimental Agriculture and Animal Husbandry 3, 190-197.

Colwell JD (1965) An automatic procedure for the determination of phosphorus in sodium hydrogen carbonate extract of soil. Chemistry and Industry 1965, 893-895.

Murphy J, Riley JP (1962) A modified single solution method for the determination of phosphate in natural waters. Analytica Chimica Acta 27, 31-36.

Ozanne PG, Shaw TC (1967) Phosphate sorption by soils as measure of the phosphate requirement for pasture growth. Australian Journal of Agricultural Research 18, 601-612.

Schwertmann U (1964) Differenzierung der eisenoxide des bodens durch photochemische extraktion mit sauer ammoniumoxalate-losung. Zeitschrift fur Pflanzenernahrung und Bodenkunde 105, 194-202.

Walkley A, Black IA (1934) An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method. Soil Science 37, 29-38.

Manuscript received 18 September 2000, accepted 5 April 2001

D. G. Allen (A), N. J. Barrow (B), and M. D. A. Bolland (C)

(A) Chemistry Centre (WA), 125 Hay Street, East Perth, WA 6004, Australia.

(B) 22 Townsend Dale, Mt Claremont, WA 6010, Australia.

(C) Agriculture Western Australia, PO Box 1231, Bunbury, WA 6230, Australia; and Plant Science, Faculty of Agriculture, The University of Western Australia, WA 6907, Australia.
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Author:Allen, D.G.; Barrow, N.J.; Bolland, M.D.A.
Publication:Australian Journal of Soil Research
Article Type:Statistical Data Included
Geographic Code:1USA
Date:Nov 1, 2001
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