JOURNAL OF THE SCIENTIFIC AGRICULTURAL SOCIETY OF FINLAND
Maataloustieteellinen Aikakauskirja

251

Vol. 54:251-262, 1982

Relationship between phosphorus intensity and capacity
parameters in Finnish mineral soils

II Sorption-desorption isotherms and their relation to
soil characteristics

HELINÄ HARTIKAINEN

Department of Agricultural Chemistry, University of Helsinki, 00710
Helsinki 71

Abstract. The relationship between P intensity and capacity parameters in 104 mineral soil samples was
studied by means of sorption-desorption isotherms of two types. In the isotherm A the P exchange was
expressed as a function of P concentration in the initial solution, in the isotherm B as a function of P
concentration in the final equilibrium solution. Both isotherms conformed to the equation y = a + bx,
where y stands for the amount of P sorbed or desorbed and x the P concentration in the solution.

In the isotherm A the constant a is the intensity factor expressing the amount of water soluble P at a
given soil-solution ratio. The term a in the isotherm B, on the contrary, was only poorly related to water

solubleP in soil. In both isotherms the slope h of the line seemed to be most effectively affected by oxalate
extractable Al. The relative importance of oxalate soluble Fe appeared to be greater in affecting the
effectiveness of sorption-desorption reactions than in affecting the buffer reactions. However, the slope b
of both isotherms was found to be a semi-intensive parameter: it was quite markedly dependent also on
soil characteristics which control the level of water soluble P in soil.

The ratio of the term —a to b (termed as EBS or EPC), expressing the zero point of net P exchange,
varied from 0.003 to 13.89 mg P per liter, the lowest values tending to be in the heavy clay soils and the
highest ones in the non-clay soils. The practical significance of this quantity was discussed.

Introduction

The applicability of various isotherms to studies dealing with agricultural
and environmental P problems has been subject to to many investigations. A
great deal of effort has been expended on trying to find the isotherm
parameters of essential significance in predicting e.g. the fertilizer require-
ment of plants or the P loading of surface waters induced by eroded soil
material.

The important soil factors controlling the P supply to plants are the
intensity, quantity and P buffering power factors (HELYAR and MUNNS
1975, HOLFORD 1976). The intensity refers to P concentration in the soil

https://www.c-info.fi/en/info/?token=XkhC1Jrd_zQNuRTp.mW-_VF3iTZlmbpmBxaNfjA.sVyj9tFNlGcsTcOlauKrw5KJZ3WXkNvE6En7IYcbWBTaV61UGbyhLDi3zL4oJhN7vJmWUFfgU_B6dpfDtlPX0MysaKyO_NIu6AbG4wEmosqQLmMKOHbqvr8qyDZeLaghUnu-OIzBgBJn6ug3Jx1womStMPfURIjO_I1LzykewTBjOtsDYg


252

solution and the quantity to total labile P in soil. The intensity parameter, e.g.
water soluble P, describes a transient situation in soil, but it does not exactly
inform about changes in P intensity occurring when P concentration in the
solution is reduced by P uptake or increased by P fertilization.

The purpose of the present study was to find out in more detail the
relationship between the P intensity and capacity parameters and soil factors
involved therein. The relationship was investigated by sorption-desorption
isotherms of two kinds. The results were assumed to give further information
about factors to be taken into account when developing methods for extract-
ing plant available P as well as methods for determining the requirements of P
fertilization. The isotherms were considered to give intimations also about
factors in lakes controlling P exchange between the sediment and overlying
water and, thus, being noteworthy when developing models for predicting
the ability of a lake to tolerate P loading or in polluted waters the ability of
bottom deposits to supply the water body with P.

Materials and methods
The material consisted of 104 mineral soil samples, some characteristics of

which are reported in Table 1. The means and range of other properties in
various soil groups as well as the methods of soil analyses are presented in a
previous study (HARTIKAINEN 1982 a). The method for preparing the
isotherms is described in the first part of this study (HARTIKAINEN 1982 b).
In some soil samples, however, no sorption was found to have taken place
even at the highest P concentration (1 mg/1) in the bathing solution. In these
cases, the sorption or desorption from and to a solution were investigated by
using standard solutions of higher P concentrations (up to 14 mg/1).

Results

The P exchange by soils was expressed in two ways: as a function of P in
the initial solution (isotherm A) and as a function of the P concentration in
the final equilibrium solution (isotherm B). The graphs being curved at high
P concentrations, calculations of regressions and correlations were carried
out after plotting of sorption or desorption versus concentration data in
order to determine the upper end of the concentration range for linearity. For
most soils this was 1.0 mg P per liter (in the initial solution), even if, on the
whole it ranged from 0.2 mg to 6.0 mg P per liter or it was not reached.
Points diverging from the linearity above this limit concentration were
excluded from the analyses and only the straight-line sections of lower P
concentrations, better simulating the circumstances in nature, were
examined. Thus, both isotherms conformed to the equation y = a + bx,
where y stands for the amount of P sorbed or desorbed, x the P concentration
in the solution.

The values of the constants and the slopes of the lines obtained are
reported in Table 1. The constant and the tangent for the isotherm A are



Table 1. Chemical characteristics of soil samples, constants (aA and a B ) and tangents (bA and bB) for P
isotherms.

Soil H2 O-P Oxalate extr. Isotherm A Isotherm B
. ~ . mmol/kg aA b A aB bB EBS = EPC

samp e p mg g A 1 Fe mg/kg 1/kg mg/kg 1/kg mg/1

Heavy clays
43 4.2 2.4 134 74
20 4.6 6.1 118 118
36 4.7 0.5 255 30
38 4.7 2.5 95 109
18 4.8 4.4 86 148
22 4.8 4.7 98 82
24 4.8 4.7 98 81
35 4.9 2.8 137 116
37 4.9 0.2 65 77
41 4.9 1.7 75 108
42 4.9 5.7 110 58
40 5.0 3.2 78 108
23 5.1 4.2 79 114
21 5.2 8.6 71 102
44 5.3 2.3 72 99
45 5.4 4.3 61 73
48 5.5 10.4 45 88
19 5.8 20.0 67 100
39 6.0 2.9 69 102

- 2.8 41 -16.0 239 0.067
- 2.2 49 79.0 1715 0.046
- 1.0 49 -29.9 1521 0.020
- 1.6 40 - 7.9 207 0.038
- 2.5 44 -16.0 288 0.056
- 3.1 43 -21.5 297 0.073
- 2.9 42 -16.9 247 0.068
- 3.3 41 -17.9 221 0.081
- 0.2 47 - 3.4 751 0.005
- 1.9 41 - 9.5 214 0.044
- 5.7 41 -30.9 223 0.139
- 3.6 40 -18.7 210 0.089
- 5.2 43 -36.0 295 0.122
- 5.2 41 -27.0 214 0.126
- 4.3 36 -19.8 171 0.116
- 5.2 40 -23.7 182 0.130
-11.9 25 -24.0 51 0.469
-15.6 31 -40.3 80 0.505
- 2.0 43 -14.7 317 0.046

x 5.0 4.8
s 0.4 4.5

Coarser clays
15 4.3 4.9 94 171
73 4.4 4.0 85 65
13 4.5 6.7 74 71
59 4.5 1.7 64 144
61 4.5 5.5 82 150

109 4.5 3.5 77 56
72 4.6 9.3 76 61
47 4.7 1.3 81 102
54 4.7 5.0 88 75
80 4.7 3.2 59 43
82 4.7 0.2 76 49
49 4.8 3.1 76 59
62 4.96.7 111 71
63 4.941.3 68 83
66 4.94.7 73 24
74 4.98.3 50 58
83 4.95.9 45 64
89 4.95.9 59 59

7 5.08.9 57 67
56 5.05.8 73 97
64 5.04.2 50 54
93 5.08.4 62 53

108 5.05.1 32 56

95 94 - 4.2 41 -23.8 392 0.118
46 26 3.7 6 16.3 455 0.1363.7 6 16.3 455 0.136

- 4.2 45 - 43.9 473 0.093
- 4.1 41 - 23.5 240 0.098
- 5.7 37 - 21.5 140 0.154
- 0.7 44 - 5.5 373 0.015
- 5.4 45 - 49.2 404 0.122
- 4.0 41 - 21.3 214 0.099
- 9.7 33 - 28.1 95 0.296
- 2.5 43 - 14.7 278 0.053
- 4.3 40 - 22.2 209 0.106
- 2.4 40 - 11.5 187 0.061
- 0.9 45 - 7.3 415 0.018
- 4.5 33 - 16.5 125 0.132
- 7.8 36 - 27.7 129 0.214
-37.1 17 - 55.7 25 2.214
- 3.0 29 - 7.0 67 0.104
- 7.7 28 - 17.3 63 0.273
- 5.7 29 - 12.2 62 0.197
- 4.3 28 - 9.5 62 0.152
- 7.4 34 - 22.4 102 0.220
- 3.8 38 - 15.5 152 0.102
- 3.7 24 - 6.9 45 0.154
- 8.2 26 - 17.2 55 0.311
- 5.3 30 - 13.4 77 0.174

253



254

Table 1. Chemical characteristics of soil samples, constants (aA and aB ) and tangents (b A and b B) for P
isotherms.

Soil H2O-P Oxalate extr. Isotherm A Isotherm B
. mmol/kg a A bA aB bB EBS = EPC

samp e p mg g A 1 p e mg/kg 1/kg mg/kg 1/kg mg/1

6 5.1 25.3 37 63 -21.2 22 - 38.0 40 0.951
60 5.1 78.2 68 86 -85.4 17 -131.2 27 4.894
75 5.1 6.9 76 94 - 5.8 39 - 25.6 171 0.149
92 5.1 22.1 42 62 -19.5 18 - 30.1 27 1.112
14 5.2 9.5 47 71 - 7.8 31 - 20.5 82 0.252
17 5.2 6.3 69 91 - 5.3 39 - 22.0 164 0.134
53 5.2 0.4 49 86 - 0.2 47 - 2.1 634 0.003
58 5.2 7.6 59 60 - 3.8 28 - 8.7 66 0.132
69 5.2 6.5 55 60 - 3.7 17 - 16.4 75 0.220
81 5.2 11.8 39 64 -11.9 21 - 19.4 37 0.532
51 5.3 9.8 75 80 - 8.4 35 - 27.1 113 0.240
76 5.3 8.4 46 59 - 7.2 22 - 12.9 40 0.324
78 5.3 0.2 28 31 - 0.4 44 - 6.4 574 0.011
12 5.4 13.9 39 73 - 8.5 18 - 12.1 25 0.483
16 5.4 4.3 161 92 - 3.6 41 - 20.8 228 0.091
52 5.4 15.0 79 81 -14.5 25 - 29.2 50 0.579
55 5.6 3.8 69 69 - 2.4 37 - 8.6 136 0.063
50 5.7 32.7 47 46 -35.4 13 - 44.9 16 2.819
79 5.7 64.7 38 68 -50.5 6 - 57.7 7 8.186
95 5.7 8.3 45 49 - 8.5 23 - 15.5 41 0.379

8 6.0 38.2 43 79 -33.7 23 - 63.1 44 1.434
46 6.0 5.0 50 64 - 5.2 26 - 10.6 54 0.197
57 6.0 11.9 40 72 -10.0 21 - 12.4 26 0.478
77 6.1 17.1 106 68 -16.7 33 - 49.7 99 0.504
71 6.2 5.4 53 49 - 5.1 34 - 15.6 103 0.151
68 6.3 48.5 36 52 -46.0 9 - 56.9 11 5.337
84 6.5 35.7 50 63 -33.2 13 - 45.3 18 2.481
67 6.6 0.8 56 61 - 1.0 46 - 13.0 587 0.022

x 5.2 12.8 63 71 -11.7 30 - 25.2 147 0.736
s 0.6 16.3 23 26 15.8 11 21.6 158 1.533

Non-clay soils

90 3.8 1.9 95 84
96 4.2 1.7 36 44
99 4.5 9.4 61 56

113 4.5 4.5 38 80
3 4.6 0.5 141 50

91 4.6 1.2 138 57
110 4.6 5.0 61 85
111 4.6 0.9 59 112

4 4.7 0.8 133 63
114 4.7 1.5 17 84
100 4.8 6.6 53 63
102 4.8 22.9 72 57

5 4.9 12.0 66 80
106 4.9 4.5 78 50

- 1.7 47 - 24.2 660 0.037
- 2.7 34 - 8.4 109 0.077
- 7.9 30 - 20.2 77 0.261
- 5.7 26 - 11.9 53 0.222
- 0.3 49 - 16.6 2921 0.005
- 1.2 48 - 36.0 1519 0.024
- 6.0 40 - 30.1 199 0.151
- 3.0 40 - 14.4 191 0.075
- 0.6 47 - 11.1 875 0.012
- 2.3 30 - 5.4 72 0.075
- 8.1 29 - 18.8 66 0.282
-25.1 21 - 42.8 35 1.206
-10.4 37 - 41.3 148 0.279
- 5.7 33 - 16.5 96 0.171



255

Table 1. Chemical characteristics of soil samples, constants (aA and a B) and tangents (b A and bB ) for P
isotherms.

Soil H2 O-P Oxalate extr. Isotherm A Isotherm B
. . mmol/kg a A bA aB bB EBS = EPC

samp e p mg g Al p e mg/kg 1/kg mg/kg 1/kg mg/l

65 5.0 4.0
97 5.0 2.1
88 5.1 16.1
87 5.2 11.4

101 5.2 14.0
112 5.2 0.7

33 53 - 3.3 22 - 5.7 37 0.152
- 2.4 38 - 10.0 160 0.063
-16.5 29 - 38.4 66 0.578
- 7.0 15 - 9.7 20 0.480
-14.8 21 - 25.0 35 0.719
- 2.8 39 - 12.5 171 0.073
-10.9 21 - 18.7 36 0.518
-15.2 15 - 22.0 22 0.986
-97.0 7 -112.6 8 13.870
- 4.0 27 - 8.6 60 0.144
- 4.8 37 - 17.8 137 0.130
-18.1 20 - 29.9 33 0.917
-18.5 22 - 33.3 40 0.836
-14.9 9 - 17.8 11 1.687
-20.1 22 - 24.2 26 0.920
-17.7 19 - 28.3 30 0.937
- 0.8 45 - 7.6 448 0.017
-11.0 17 - 16.8 26 0.645
-31.3 19 - 51.8 32 1.608
-20.7 17 - 31.8 27 1.190

40 73
70 69
43 33
54 50
67 45

1 5.3 14.0
10 5.3 17.9
86 5.3 117.8

104 5.3 2.4
105 5.3 2.9

47 54
29 46
47 73
52 81
79 64

9 5.5 21.9
103 5.5 18.7

11 5.6 28.7
85 5.7 18.5

48 55
44 69
40 47
69 64

2 6.0 21.6
98 6.0 0.3

115 6.1 10.2
107 6.3 32.2
94 6.4 22.4

23 50
26 64
38 44
66 50
44 60

X 5.1 13.3
0.6 20.6

60 62 -12.1 29 - 24.1 248 0.863
30 16 17.0 12 19.5 561 2.347s 17.0 12 19.5 561 2.347

expressed by a A and b\, respectively, and those for the isotherm Bby and
£b, respectively. Both graphs intersect the x-axis on the same point, termed
equilibrium bathing solution (EBS) for isotherm A and, according to TAY-
LOR and KUNISHI (1971), equilibrium phosphate concentration (EPC) for
isotherm B. Also the intersecting points are presented in Table 1.

In the isotherms A, expressing the retention or removal of P as a function
of Pin the initial solution, the absolute values of the constants aa describe the
solubility of soil P in pure water. When comparing them with the quantities
of water extractable P, obtained in an earlier study (HARTIKAINEN 1982 a),
close correlations were found:

r

Heavy clays (19)
Coarser clays (51)
Non-clay soils (34)

o.92***
0 99*»»
0 99»»»

Table 1 shows that the absolute values of the constant tended to be
somewhat lower than the amounts of P extracted by water, because the soil-
solution ratio 1:50 used in the present study was a little higher than that used
in water extraction (1:60).



256

The correlation analyses showed that, contrary to aK, the constant was
quite poorly related to water soluble P. The following correlation coefficients
were found for the relation between water soluble P and the logarithm of the
absolute value of the term <zß :

r

Heavy clays (19)
Coarser clays (51)
Non-clay soils (34)

0.51»
0.72*»
0.56»»»

The values of the slopes of the lines ranged widely; those of b\ from 49 to
6 and those of b$ from 2921 to 7. On the average, the values of bK for the
isotherms of the type A, which expresses the P exchange as a function of P
addition, were highest in the heavy clay soils and lowest in the non-clay soils.
There was 33 samples with b\ values exceeding 40: 15 heavy clays, 12 coarser
clays and 6 non-clay soils. In this group the correesponding value of b%,
describing the P buffering power of the soil, ranged from 2921 to 199.

It was observed that the correlations found for the relation between the
slope b\ and the soil characteristics were linear and those for the relation
between the slope b$ and soil characteristics were logarithmical (Table 2).

Table 2. Total correlation coefficients for the relation between soil characteristics and the slope f>A as well
as the logarithm value of slope f> B.

Heavy clays Coarser clays Non-clay soils All samples
b A log b B bA log bB b A log b B bA log bB

pH -0.53» -0.48* -0.44** -0.42*» -0.56»»* -0.41* -0.49**» -0.47»»»
Oxal. extr. Al 0.52» 0.62»» 0.50*»» 0.45*»* 0.63*»* 0.73»»* 0.57»»» 0.62»»»

Fe ns ns 0.36*» 0.36*» 0.35» ns 0.41*»» 0.34*»»
ns -0.64*»» -0.63»»» -0.64*»* -0.58*»* -0.66*»» -0.59*»»NH4F-P/A1 ns

NaOH-P/Fe nsns ns -0.63*»» -0.63**» ns ns -0.36**» -0.27»»
-0.64»» -0.53» -0.71»»» -0.66*»» -0.62*»* -0.57»»* -0.67»»» -0.63»»»h 2o-p

ns = not significant

This was due to the fact that distribution of the values of £
B appeared to

deviate markedly from the normal distribution, the Pearson’s coefficient of
skewness S being 0.94. The utilization of the logarithms of the b% values
decreased this coefficient to 0.07. Nevertheless, all correlation coefficients for
the relation between the b parameters and soil properties were relatively low,
indicating that the sorption-desorption system of the soil is of multi-
component nature.

The relationship between the soil properties and the parameters describing
the effectiveness of the desorption or sorption (ba) as well as the buffer
power of soil (&B) was investigated by the regression analysis. The coeffi-
cients of multiple determination R 2 were calculated for the equations with
the following variables:

X] = oxalate extractable AI mmol/kg
x 2 =

” ” Fe ”

Xj = molar ratio NH,F-P/A1 ( ■ 102 )

x, =
” ” NaOH-P/Fe (• 102 )



In 104 samples studied the relationship between the parameter bK (1/kg) =

y and these soil characteristics was found to conform to the following
regression equation:
y = 0.169x, + 0.094x2 - 0.845x 3

- 0.483x4 + 23.459 (F = 58.89***) R 2 = 0.66
S = 6.17
Sk, = 0.022
sb2 = 0.025
su = 0.178
sM = 0.206

In the corresponding equation, calculated for the relationship between the
parameter log b% and soil characteristics, the molar ratio NaOH-P/Fe was
not statistically significant: it explained only 2 % of the variation in log
Thus, the regression equation was:

y = 0.0078x, + 0.0035x2
- 0.0488x3 + 1.519 (F = 63.62***) R 2 = 0.70

S = 0.312
s b, = 0.00096
sb2 = 0.00123
sb3 = 0.00621

The relative importance of these soil factors affecting the parameter bp, and
log b% may be compared on the basis of P-coefficients (P A and Pb, respectively)
which were as follows:

Pa Pb
A 1 0.50 0.49
Fe 0.22 0.17

NH4F-P/AI -0.39 -0.48
NaOH-P/Fe -0.20

Oxalate extractable A 1 had the highest values of p-coefficients, but in the
equation for log bB the role of NH4F-P/A1 seemed to be nearly as appreci-
able. Further, it can be seen that the oxalate extractable Fe is of importance,
affecting relatively more the parameter b\ than the parameter log b%.

The zero point of net P exchange (EBS of EPC) expresses the P concentra-
tion in a solution where y = 0. Table 1 shows that these points ranged
widely: 0.003 - 13.89 mg P per liter. In all samples the average value was
0.665 mg P per liter. The non-clay soils tended to have the highest values and
the heavy clay soils the lowest ones. The magnitude sequence of average EBS
(= ECP) values of the soil groups was the same as that of water soluble P and
high values of correlation coefficients were found for the relation between
water soluble P and EBS (=EPC) values.

The calculation of these correlations is, however, questionable. This is due
to the fact that e.g. the EBS is determined by dividing the constant -<r A which
represents water soluble P by the slope b\. Thus, the water soluble P is
included in the EBS values.

As expected on the basis of the associations stated above, the EBS (=EPC)
values correlated moderately with the soil characteristics found by HAR-
TIKAINEN (1982 a) to regulate the extractability of soil P into water (Table 3).
Like in water soluble P, the relationship between the EBS and the molar ratio

3 257



258

Table 3. Total correlation coefficients for the relation between the EPC (EBS) values and soil characteris-
tics.

Heavy clays Coarser clays Non-clay soils All samples
(19) (51) (34) (104)

pH 0.57» 0.35» ns 0.26»»
NH4CI-P 0.79»»» 0.95»»» 0.96»»» 0.95»»»
NH 4F-P ns 0.56»»» 0.72»»» 0.57»»»
NH 4F-P/A1 0.52» 0.80»»» 0.86»»» 0.79»»»
NaOH-P ns 0.55»»» 0.51»» 0.43»»»
NaOH-P/Fe ns 0.77»»» 0.47»» 0.43»»»

NH4F-P/AI tended to become the closer and that between the EBS and soil
pH the poorer the coarser the soil material was. However, the values of the
correlation coefficients for the EBS were somewhat lower, but in NH4F-P/
Al, statistically significantly lower than those found for the water soluble P
(tested by the z-transformation test according to SNEDECOR and COCHRAN
1972, P = 0.05). This indicates the role of the factor h. In general, the EBS
and EPC increased with a decrease in b A and bB , respectively.

Discussion

The equations obtained differ from the Langmuir and Freundlich as well as
Temkin isotherms often used to describe the interaction between soils and
phosphate solutions (OLSEN and WATANABE 1957, KAILA 1963, BACHE and
WILLIAMS 1971, MEAD 1981, etc.). Also these isotherms, generally applied to
sorption studies, normally fit the sorption data only within a limited range of
phosphate concentrations in solution (MEAD 1981). These equations are
hardly used in sorption-desorption studies, because quite low phosphate
concentrations are needed in experiments of this kind.

The results in Table 1 and the correlation analyses show that the constant
aA in the isotherm A which expresses the P exchange as a function of P
concentration in the initial solution, really corresponds to the quantity of
water soluble P at the soil-solution ratio used and is thus the intensity factor
in the isotherm.

As stated in the first part of this study (HARTIKAINEN 1982 b), the
physical meaning of the constant #B cannot be interpreted as exactly and
explicitly as the meaning of the constant a A . The equation of the isotherm B
presented in this study is to some extent similar to the Freunlich sorption
isotherm modified by FITTER and SUTTON (1975) into the form AP bCk

a, where C stands for the P concentration in the final solution and A P the
amount of sorbed P. According to the authors, the third term, a, theoretically
represents the phosphate which must be removed in order to reduce P
concentration in the final solution to zero, since AP = —a when C = 0. When
the equation was fitted to the sorption data, a close relationship was found



259

between a and the resin-P values, with the calcareous and acid soils falling
into two distinct groups.

However, in the present study there existed quite a poor association
between the water extractable Pin the soil and the term <tB corresponding to
the term a of FITTER and SUTTON (1975). This result supports the concept
discussed by HARTIKAINEN (1982 b) that, with increasing solution-soil ratio
the isotherm expressing the P exchange as a function of final P concentration
can be assumed in practice to converge the y-axis without intersecting it. This
is due to the fact that the intersecting point on the y-axis would express the
equilibrium P concentration x = 0 which, in turn, does not allow any net
phosphate exchange.

As can be seen in Table 2, water soluble P and some soil properties
involved were correlated to some extent also with the slope b\ and the
logarithm value of the slope bs. In addition, on the basis of the P-coefficients,
the soil characteristics controlling the level of water soluble P in soil seem to
be important factors in both equations, which indicates the factor £A as well
as the factor b$ to be semi-intensive parameters.

Water extraction has been shown to illustrate the P status determined by
the quantity and quality of sorption components in soil, soil pH, and the
content of organic carbon (HARTIKAINEN 1982 a). These factors seemed not
to affect the amounts of P dissolved in water directly, but indirectly, by
controlling the nature of P bonding which, in turn, seemed to be of decisive
importance in the extractability of P into water.

The importance of various P fractions differed in the equations for £A and
log h#. It can be concluded that, with an increase in the molar ratio NH 4F-P/
Al, the buffer power {b%) is decreased more markedly than the effectiveness
of sorption-desorption reactions (b A). On the other hand, the molar ratio
NaOH-P/Fe, an insignificant variable in the equation for the buffer power
parameter, was of importance in affecting the effectiveness of sorption-
desorption reactions. These facts give intimations also about the role of
sorption components. As anticipated, Al seemed to play a more appreciable
role than Fe, but some difference was really found between the isotherms.
The relative importance of Fe appeared to be greater in affecting the
effectiveness of sorption reactions than in affecting the buffering reactions.
Thus, it is obvious that in the rapid P exchange reactions (i.e. in P buffering)
Al and P bound by it are more decisive factors, because Al forms a weaker
bond with phosphate than Fe (see AURA 1980). If these assumptions are
valid, the result is of interest also as far as limnological studies are concerned
in which the role of Al generally fails being noticed.

The ratio of the term -<tA to b\, or -«B to £B , expressing the zero point of
net P exchange, tended to be lowest in the heavy clay soils and highest in the
non-clay soils. This gives reason to suppose that the heavy clay soils in
general were able to sorb P from more diluted phosphate solutions than the
coarser soils.

TAYLOR and KUNISHI (1971) observed the EPC values of fertilized soils
somewhat to change with changing soil-solution ratio. Thus, the EPC does
not necessarily correspond to the actual pre-existing P equilibrium level in



260

the soil solution, but is probably closely related to it. Taking all this into
consideration, the EPC value may be a practical index. It can be used, for
instance, in predicting the direction and extent of P exchange reactions
between suspended soil material and recipient water. The data in Table 1 give
reason to suppose that, under suitable conditions, some samples may be able
to sorb P even from very pure surface waters. The water conducted from
Lake Päijänne to Helsinki, for instance, used to making drinking water,
contains about 10 /u,g P04-P per liter (data obtained from the Water Works
of Helsinki) which is more than the EPC of the samples 3, 37 and 53. A
greater number of the samples studied would have been able to retain P from
the polluted water of Lake Tuusulanjärvi, containing 14—280 /ig P0 4-P per
liter in the hypolimnion in 1981 (analyzed by the Water District of Helsinki).
On the other hand, many of the soil samples would cause P load, if carried
into watercourses.

On the basis of the idea introduced by BECKWITH (1965), OZANNE and
SHAW (1967) developed a method for predicting the phosphate requirements
of pasture plants. The method is based on the measurement of phosphate
sorption by soil at a standard equilibrium concentration. In the experiments
performed by the authors, the variations in phosphate sorbed accounted for
over 80 % of the variation found in the phosphate requirement of plants.

The determination principle of OZANNE and SHAW (1967) presupposes the
use of an isotherm corresponding to the graph of type B, presented in this
study. However, more accurate information may be obtained, if the isotherm
of type A is used simultaneously. As can be concluded from the paper of
HARTIKAINEN (1982 b), the sorption data derived from the isotherm B give
no information about P quantities to be added to the soil. The following
example elucidates this fact. The heavy clay soil sample 20 and 39 have the
same EPC value, i.e. 0.046 mg/1 (see Table 1). If the equilibrium concentra-
tion of 0.100 mg/1 is to be achieved, it can be calculated from the equation B
that 92.5 and 17.0 mg/kg should be sorbed by the sample 20 and 39,
respectively. In experimental conditions (soil-solution ratio 1:50) the initial P
concentration in the solution to be added (as calculated from the correspond-
ing equation A) should be 1.933 and 0.442 mg/1, respectively. These calcula-
tions show that, in order to achieve the same P concentration in the
equilibrium solutions, sample 20 has to retain 5.44 times more P than sample
39, but the P addition needed is only 4.37 times greater. If the P added were
distributed evenly in the surface soil, 20 cm in thickness, and if one liter of
soil were assumed to weigh one kilogram, these additions would correspond
to 193.4 kg (soil 20) and 44.2 kg (soil 39) per hectare.

Further, it can be concluded from the equations of the isotherms B that a
given desorption decreases the P concentration in a solution the more
markedly the lower the buffer power of the soil is. Also HOLFORD and
MATTINGLY (1976) have emphasized the importance of the buffering factor.
They claim that, in P requirements of plants, the critical level of P quantity in
soil increases and P intensity decreases as the buffer power of soil increases.

An ideal extractant would account for the P intensity as well as the P
supplying capacity in a wide range of soil types. The results obtained in the



261

present study show the water soluble P in soil to be closely related to
parameters describing the interaction between soil material and surrounding
P solution. As compared to other soil characteristics studied, it seems to be of
a relatively greater importance because it is related to both the term a and the
term b in the equations of the isotherms. This supports the conclusion
presented in some earlier studies (e.g. AURA 1978, SCHACHTSCHABEL and
BEYME 1980) that the water soluble P gives intimations about P intensity as
well as P capacity in soils.

However, simple water extraction seems not to indicate adequately the P
exchange properties of a soil, but the amount of Al and Fe must also be taken
into account. Thus, the use of sorption-desorption isotherms, including all
the parameters discussed above, is justified. By applying simultaneously the
isotherms of both types investigated in this study, the lines can be used as a
dynamic model to describe P exchange. The advantage of this method is that
the data on both isotherms are obtained by the same analysis and no
additional measurements are needed.

Acknowledgement. The author wishes to thank the Maj and Tor Nessling Foundation for supporting
this study financially.

References

AURA, E. 1978. Determination of available soil phosphorus by chemical methods. I. Scient. Agric. Soc.
Finl. 50: 305-316.

AURA, E. 1980. Oxygen as an exchangeable ligand in soil, J. Scient. Agric. Soc. Finl. 52: 34—44.
BACHE, B. W. & WILLIAMS, E. G. 1971. A phosphate sorption index for soils. J. Soil Sci. 22; 289—301.
BECKWITH, R. S. 1965. Sorbed phosphate at standard supernatant concentration as an estimate of the

phosphate needs of soils. Aust. J. Exp. Agric. Anim. Husb. 5; 52—58.
FITTER, A. H. & SUTTON, C. D. 1975.The use of the Freundlich isotherm for soil phosphate sorption

data. J. Soil Sci. 26: 241-246.
HARTIKAINEN, H. 1982 a. Water soluble phosphorus in Finnish mineral soils and its dependence on

soil properties. J. Scient. Agric. Soc. Finl. 54; 89-98.
1982 b. Relationship between phosphorus intensity and capacity parameters in Finnish mineral
soils. I Interpretation and application of phosphorus sorption-desorption isotherms. J. Scient.
Agric. Soc. Eini. 54; 245-250.

HELYAR, K. R. & MUNNS, D. N. 1975. Phosphate fluxes in the soil-plant system: a computer
simulation. Hilgardia 43: 103 130.

HOLFORD, I. C. R. 1976. Effects of phosphate buffer capacity of soil on the phosphate requirements of
plants. Plant and Soil 45: 433—444.
& MATTINGLY, G. E. G. 1976. Phosphate adsorption and plant availability of phosphate. Plant
and Soil 44; 377-389.

KAILA, A. 1963. Dependence of the phosphate sorption capacity on the aluminium and iron in Finnish
soils. J. Scient. Agric. Soc. Eini. 35; 165—177.

MEAD, J. A. 1981. A comparison of the Langmuir, Freundlich and Temkin equations to describe
phosphate adsorption properties of soils. Aust. J. Soil Res. 19: 333—342.

OLSEN, S. R. & WATANABE, F. S. 1957. A method to determine a phosphorus adsorption maximum
of soils as measured by the Langmuir isotherm. Soil Sei. Soc. Amer. Proc. 21: 144-149.

OZANNE, P. G. & SHAW, T. C. 1967. Phosphate sorption by soils as a measure of phosphate
requirement for pasture growth. Aust. J. Agric. Res. 18; 601—612.



262

SCHACHTSCHABEL, P. & BEYME, B. 1980. Löslichkeit des anorganischen Bodenphosphors und
Phosphatdungung. Z. Pfl. ern. Dung. Bodenk. 143: 306—316.

SNEDECOR, G. W. & COCHRAN, W. G. 1972. Statistical methods. 593 p. Ames.
TAYLOR, A. W. & KUNISHI, H, M. 1971. Phosphate equilibria on stream sediment and soil in a

watershed draining an agricultural region. J. Agric. Food Chem. 19: 827—831.

Ms received June 2, 1982

SELOSTUS

Fosforin intensiteetti- ja kapasiteettiparametrien välisestä suhteesta
suomalaisissa kivennäismaissa

II Sorptio-desorptioisotermit ja niiden riippuvuus maan
ominaisuuksista

Helinä Hartikainen
Helsingin yliopiston maanviljelyskemian laitos, 00710 Helsinki 71

Maan fosforin helppoliukoisuutta sekä maan kykyä ylläpitää tiettyä fosforin pitoisuutta
maanesteessä selvitettiin kahden erityyppisen fosforin pidättymistä ja vapautumista kuvaavan
isotermin avulla 104 kivennäismaanäytteellä. Fosforin pidättyminen tai vapautuminen esitet-
tiin isotermissä A lisätyn liuoksen ja isotermissä B saadun tasapainoliuoksen fosforin
pitoisuuden funktiona. Molemmille isotermeille saatiin yhtälö y = a + bx, jossa y vastaa
pidättynyttä tai vapautunutta fosforin määrää ja x liuoksen fosforin pitoisuutta.

Isotermissä A vakion a todettiin ilmaisevan maan vesiliukoisen fosforin määrää käytetyssä
uuttosuhteessa ja kuvaavan fosforitilan intensiteettitekijää. Sen sijaan isotermissä B vakio a
näytti kytkeytyvän varsin heikosti vesiuuttoisen fosforin määrään. Suoran kulmakerroin h
kuvaa isotermissä A pidättymis- ja vapautumisreaktioiden tehokkuutta ja isotermissä B maan
fosforinpuskurikykyä eli kykyä vastustaa fosforin pitoisuuden muutoksia maanesteessä.
Molemmissa tapauksissa b:n arvo näytti voimakkaimmin riippuvan maan oksalaattiuuttoisen
aluminiumin määrästä. Oksalaattiin liukenevalla raudalla näytti olevan suhteellisesti suurempi
merkitys isotermissä A kuin isotermissä B. Tekijä b on ilmeisesti yhteydessä maan fosforitilan
intensiteettitekijöihin, sillä b:n arvoon vaikuttivat huomattavasti myös vesiliukoisen fosforin
pitoisuutta säätelevät maan ominaisuudet.

Isotermien yhtälöistä ratkaistiin fosforinvaihdon nollapistettä vastaavan liuoksen fos-
forikonsentraatio (-a/h), ts. fosforin pitoisuus liuoksessa, josta ei sitoudu fosforia maahan ja
johon ei myöskään maasta vapaudu fosforia. Tästä pisteestä käytettiin lyhennettä EBS tai EPC
ja se vaihteli koko aineistossa 0.003 mg:sta 13.89 mg:aan litrassa. EBS:n (=EPC) arvoista
voitiin päätellä, että aitosavimaat pystyivät pidättämään fosforia keskimäärin ottaen laimeam-
mista liuoksista kuin muiden ryhmien maanäytteet. Tulosten tarkastelussa kiinnitettiin
huomiota fosforinvaihdon nollapistettä kuvaavan suureen merkitykseen lannoitus- ja vesis-
tötutkimuksen kannalta.