0038-075X/79/1272-0108$02.00/0 SOIL SCIENCE Copyright © 1979 by The Williams & Wilkins Co.

Vol. 127, No. 2 Printed in U.S.A.

S U L F A T E S O R P T I O N B Y T W O OXISOLS A N D A N ALFISOL O F T H E T R O P I C S 1

WALTER COUTO, D. J. LATHWELL, AND D. R. BOULDIN

Cornell University, Ithaca, New York 14853

Received for publication May 2, 1978

ABSTRACT

Sulfate availability in tropical soils is highly variable, and factors influencing its sorption in the soil profile are incompletely understood. The influence of pH and S04° concentration on the amount of S04

= sorbed and on negative charges of the soil was investigated through equilibration procedures using soil samples representing two Oxisols and one Alfisol of Sao Paulo, Brazil. These experiments revealed that the amount of S04

= sorbed was significant in subsoil (B2) samples. At a pH of about 4.0, the amount sorbed reached more than 2 me/100 g of soil in one sample, an amount that is high in relation to the low cation retention capacity of these highly weathered tropical soils. Sulfate sorption was clearly dependent on the pH of the equilibrium solution; the amount sorbed decreased as the pH increased with each soil. In the surface horizons with relatively high organic matter content, the S04

= sorption capacity was smaller, and the pH effect was less evident. When S04

= sorption occurred, the pH of the equilibrium solution increased, probably in part at least a result of S0 4

= substitution for OH - on the surface of oxides. Sorbed S0 4

= was desorbed only partially in some soil samples, while in others S04

= was desorbed in larger amounts than were actually sorbed, indicating a release of native S04

= . Potassium desorbed, considered as a measure of cation exchange capacity, suggested an increase in cation retention sites as a result of increasing levels of sorbed S04

= .

INTRODUCTION

Tropical soils and other soils with high iron and aluminum oxides or 1.1 type clay minerals sorb important amounts of S04

= and other an-ions (Harward and Reisenauer 1966; Fox, Han-son, and Jones 1971). The amount of S04

= that may be sorbed is important from the point of view of S04

= storage in the profile as a means of avoiding losses in soils of the humid areas or during the rainy season of subhumid areas and its effects on S04

= availability to plants, as well. Because of the large proportion of pH-depen-

dent charges of soil material generally present in these soils, S04

= sorption has been found to be highly pH-dependent (Kamprath, Nelson, and Fitts 1956; Chao, Harward, and Fang 1962;

1 Agronomy Paper Number 1163, Department of Agronomy, Cornell University, Ithaca, New York 14853. Acknowledgment is given to the Ford Founda-tion for financial support (to W. C.) and to the U.S. Agency for International Development, which sup-ported in part the research work under 211 (d) grant csd 2834 to Cornell University.

Atkinson, Posner, and Quirk 1967). According to Hingston et al. (1967, 1968), S04

= anions are specifically absorbed on the surface of iron ox-ides and are retained to a greater extent than would be expected from the composition of the equilibrium solution. The mechanism involves the exchange of an OH - coordinated with an Fe atom of the oxide structure, resulting in increas-ing pH at higher levels of sorption. Nevertheless, most S0 4" sorption studies have been done in uncontrolled pH conditions. The separate effect of both S04™ concentration and pH variable is difficult to understand and quantify in these conditions, as it is of interest to know the ability of the soil to sorb S04

= at different pH values. When S04" is specifically sorbed by goethite

or hematite, the negative charges on the surface increase, increasing the ability of the material to retain cations (Hingston et al. 1967, 1968). In tropical soils, this aspect of S0 4

= sorption could be of great importance, given the extremely low cation exchange capacity, at low pH, of many of these soils.

108

SULFATE SORPTION 109

The purposes of the present laboratory study were: (1) to determine the amount of S04~ sorbed by some Oxisols and Alfisols of the trop-ics; (2) to estimate the separate effect of pH and S04

= concentration of the equilibrium solution on S04

= sorption; and (3) to determine the effect of S04

= sorption on negative charges of soils.

MATERIALS AND M E T H O D S

Soil samples

The six soil samples used in this study were from the Ap and B2 horizons of three soils of the state of Sao Paulo, Brazil. The soils, classi-fied as Acrorthox, Acrohumox, and Tropudalf, are the same ones described and used by Van Raij and Peech (1972) in their study. Some of the characteristics of the soils, as presented by these authors, are summarized in Table 1. The samples were air dried, crushed to pass a 2) mm sieve, and stored in plastic containers.

Sulfate sorption

An amount of air-dried soil equivalent to 10 g of oven-dried soil was equilibrated with 50 ml of solution containing 0, 0.3, 0.6, 0.9, 1.2, 1.5, 1.8, 2.1, and 2.4 mM of S04

= per liter. All solutions also contained 20 mM of KC1 per liter as sup-porting electrolyte. The equilibration was made in 250-ml Erlenmeyer flasks by shaking for 24 h.

After this period, the final pH was measured potentiometrically using combined glass-calo-mel electrodes, the suspension was filtered through Whatman filter paper Number 42, and the amount of S04

= remaining in solution was determined turbidimetrically, as described be-low. The amount of S04

= sorbed was estimated from the amount initially present in the equilib-rium solution.

For S04= sorption estimation at different pH

values, the procedure was es antially the same,

but HC1 or KOH was added to adjust the pH to some desired value. Soil suspended in solutions containing different amounts of S04

= , as de-scribed above, were adjusted to either four or five pH values within the range 3.5 to 7.0, de-pending on the original pH of the soil. The suspensions were left standing for 3 h and the pH was measured and adjusted each half hour during this period. After the pH was adjusted again at the end of this period, the flasks were shaken overnight. The following day the pH was measured again and adjusted to the desired value. Shaking was continued to complete 24 h. All samples were shaken for at least 4 h after the last addition of HC1 or KOH. No attempt to reach the desired pH value was made during the last 4 h of shaking. As in the unadjusted pH studies, the S04

= sorbed was estimated from S04

= remaining in solution after 24 h of shaking. The final pH was measured and recorded as indicated on the suspensions before filtering. For each soil, three equilibrations and S04

= sorption estimations were replicated six times to give an estimate of the experimental error.

Sulfate and potassium desorption

After separating the supernatant (Solution A) following the procedure described above, the bottles were weighed immediately to estimate the amount of solution occluded by soil. The soils were then washed five times with 50-ml portions 0.5 N NH4N0.3, and the washings were collected in a volumetric flask. After the final washing, the volume was made up to 250 ml with 0.5 N NH4N03 . This solution (Solution B) was analyzed for S04

= and K+, and the amount of S04

= and K+ desorbed was calculated after correction for the amount of these ions remain-ing in the solution occluded by the soil. All equilibrations and desorptions were performed twice for each sample.

TABLE 1 Selected characteristics of the soils used in this study (Van Raij and Peech 1972)

Sample num-ber Soil horizon Organic

C, % pH in Mineralogical

composition, % Kaolinite H.O KC1 Fe-̂ Oi Gibbsite

1 Acrohumox-Ap 1.9 4.9 4.1 4 3 25 2 Acrohumox-B2 0.3 4.9 4.3 6 6 37 3 Acrorthox-Ap 2.5 5.3 4.7 19 37 9 4 Acrorthox-B2 0.7 5.9 6.0 18 33 11 5 Tropudalf-Ap 2.3 6.3 5.4 16 8 33 6 Tropudalf-B2 1.0 6.7 5.9 17 10 43

110 COUTO, LATHWELL, AND BOULDIN

Sulfate and potassium determination

The amount of S04= remaining in solution

after equilibration was measured by turbidime-try, according to the method described by the American Public Health Association, Inc. (1965) for water, sewage, and industrial wastes, except that the conditioning reagent was that described in a modification of the original procedure (1971). Potassium was determined by flame pho-tometer.

TABLE 2 Estimated values of b and k parameters of

Langmuir equations for soil samples

Sample Soil horizon b k R2

1 Acrohumox-Ap 0.2468 0.6359 0.82 2 Acrohumox-B2 1.1324 9.8193 0.99 3 Acrorthox-Ap 0.2653 10.1130 0.51 4 Acrorthox-B2 0.7505 10.9327 0.99 5 Tropudalf-Ap 0.1071 2.6776 0.20 6 Tropudalf-B2 0.3770 8.7897 0.90

R E S U L T S AND DISCUSSION

Sulfate sorption at several S04" concentrations and pH values of the

equilibrium solution In subsoils S04

= sorption varied from about 0.4 me to about 1.1 me/100 g soil, while in surface soils the values were much lower, rang-ing from about 0.2 to 0.4 me/100 g soil at maxi-mum sorption of S04". The amount of S04

=

sorbed can be predicted by the Langmuir equa-tion of the form:

m = kbc 1 + kc

(1)

which can be expressed also in its linear form:

1 1 = 1 m b bkc

(2)

where m is the amount of sorbed S04= in me/

lOOg of soil, b is the sorption maximum, c is the S04

= concentration of the equilibrium solution, and k is a constant.

Table 2 shows the estimated b and k values for the six samples. The R2 values correspond to the linear form of the Langmuir equation from which b and k values were estimated. R2 values show that a good fitting to the data was obtained for the B2 horizons of each soil that sorbed more S04

= . Data for surface horizons showed a poorer fit of the data, possibly due to greater experi-mental error at low levels of sorption. The quan-titative relationship between the amount of S04

= sorbed and the final concentration of the equilibrium solution, as predicted by the Lang-muir equation and the observed values, are in close agreement. There was an apparent trend, however, of overestimation of S04

= sorption at low concentrations and underestimation at high S04

= concentrations for some samples. The increase in pH was significant in those

samples that sorbed the most S04= . If S04

=

sorption occurs as a result of nonspecific sorp-tion, a basic premise of the Langmuir equation, exchange with another anion already sorbed on the surface must occur, and little change in pH would be observed. If, on the other hand, S0 4" is specifically sorbed as proposed by Hingston et al. (1967) an O H - should be released from an iron oxide surface for each S04

= sorbed. A con-stant increase in pH per unit of S04

= sorbed was not observed, however, for the different samples used in this study. Adams and Rawajfih (1977) have suggested that S04

= is precipitated as in-soluble basic Al and Fe sulfates in these kinds of soils.

Since the amount of S04= sorbed was affected

by both pH and S04= concentration, where pH

of the reacting solution was not controlled, S04=

sorption was measured at several pH values of the reacting solution. To estimate the effect of pH and S04

= concentration, the amount of sorbed S04

= was measured at several pH values and was regressed on pH values and S04" con-centrations according to the following model:

Y ^ B o + BASOr) + B2(pH)

+ fl„(S(V)I/2 + fi,2(S04-) (pH) (3)

where Y is S04= sorbed in me/100 g soil, S04

= is the concentration of the reacting solution, pH is the observed final pH of the equilibrium solu-tion, and Bo, Bu Bu, and Bn are constants. Sulfate concentration varied between 0 and 2.4mM/liter, and pH values varied between 4 and 7 for all samples. The observations used in this analysis include those collected at unad-justed pH, as well as those where the pH was controlled. Between 42 and 51 observations per soil were made. The sulfate concentration of the initial reacting solution as well as the final con-centration of the equilibrium solution were used in Eq. (3) in fitting the data. Table 3, giving the R2 values of both regression models, shows that

SULFATE SORPTION 111

a larger part of the total sum of squares was explained by a model including initial concentra-tion of the reacting solution. This finding is explained by the fact that in many observations an equal or similar final concentration was found after equilibrating the same soil with solutions containing different initial concentrations. In these cases, the amount sorbed is obviously dif-ferent for the same final concentration. This suggests that the amount sorbed is affected by

' S04= initially present in the solution and that at

least part of S04= sorbed is not desorbed in the

$ presence of a less concentrated final solution. From the regression equation, Eq. (3), using

the initial concentration of the reacting solution, S04

= sorption at selected constant pH values were estimated for each sample. The results are

shown graphically in Figs. 1 to 3. It can be seen that Ap horizons sorbed small amounts of S04

= , and the effect of pH on S04

= sorption was not marked. B2 horizons sorbed high amounts of S04

= , and sorption clearly decreased as the pH of the equilibrium solution increased. With the B2 horizon of the Tropudalf, S04

= desorption resulted at high pH values.

The difference between the horizons is due probably to the higher organic matter content of the Ap horizons, since no clear changes in mineralogical composition were observed. Blocking of positive charge sites may occur as a result of occupation of these sites by organic anions or coating of oxide surfaces by organic matter.

The high capacity for S04" sorption developed

TABLE 3 Multiple determination coefficients of regression equations containing the variable initial or final sulfate

concentration of the equilibrium solution

Initial concentration Final concentration Number of

observations Sample Soil horizon Number of variables R2 Number of

variables R2 Number of

observations

1 Acrohumox-Ap 4 0.68 4 0.59 42 2 Acrohumox-B2 4 0.92 5 0.87 51 3 Acrorthox-Ap 5 0.89 5 0.80 42 4 Acrorthox-B2 5 0.97 5 0.68 51 5 Tropudalf-Ap 4 0.80 4 0.73 42 6 Tropudalf-B2 4 0.95 5 0.82 42

FIG. 1. Sulfate sorption as a function of concentration at selected pH values, Acrohumox. Lines are regression equations.

112 COUTO, LATHWELL, AND BOULDIN

equations.

FIG. 3. Sulfate sorption as a function of concentration at selected pH values, Tropudalf. Lines are regression equations.

SULFATE SORPTION 113

by the B2 horizons at low pH values could be explained by protonation of hydroxyl groups on the surface of iron and aluminum oxides, as well as other oxides. When nonspecific sorption oc-curs, S0 4" would be attached to positive charge sites generated in this way. Where specific sorp-tion occurs, an OH~ would be displaced by an SO4" after protonation according to the mecha-nism proposed by Hingston et al. (1967). The increase in pH at increasing levels of sorbed SO4" observed in the former part of the experi-ment could be explained by this mechanism.

Desorption of sorbed sulfate

Sulfate desorbed by 0.5 N NH4NO3 solution includes the S04

= sorbed from the equilibrium solution, as well as any native S04

= already present in the soil before equilibration that is desorbed. The amount of S04

= desorbed after equilibration with solutions containing no S04

=

is a measure of the amount of native S04= that

can be desorbed by 0.5 N NH4NO3. The amounts of S04" recovered under these conditions were small, ranging from 0 to 0.4 me/100 g of soil.

Table 4 summarizes S0 4" sorption and de-sorption for those samples equilibrated with so-lutions at approximately pH 4.0 containing two levels of S04

= . The highest amount recovered after equilibration was 1.81 me/100 g of soil from the Acrohumox B2. In the surface horizons the amount of desorbed SO4" after equilibration with solutions of 1.2 mM/liter and 2.4 mM/liter was somewhat less and somewhat more, respec-

tively, than the total sorbed. In these Ap hori-zons with higher organic matter contents, lower quantities were sorbed, and S04

= was readily desorbed by the procedure used.

Qn the other hand, the Acrorthox B2 and Tropudalf B2 horizons consistently showed S04

= desorption below the amount sorbed for all levels of sorbed S04

= . The amounts desorbed were 46 percent and 64 percent of the amount sorbed by the Acrorthox B2 and 57 percent and 73 percent of the amount sorbed by the Tropu-dalf B2 for the lowest and highest levels of sorbed S04

= , respectively. Considering the pro-cedure followed for desorption (washing five times with 0.5 N NH4NO3), the reduced amount of S04

= desorbed in relation to sorbed S04* indicates a strong retention of sorbed S04

= by the B2 horizons of these two soils. If the sorption of S04

= were not specific, washing five times with 0.5 N NH4NO3 should desorb most of the sorbed S04

= . The results suggest a strong reten-tion of specifically sorbed S04

= , which cannot be desorbed by CI - or N 0 3 (Hingston et al. 1968).

The higher percent desorption observed (128 and 122 percent) by the Acrohumox B2 horizon, however, suggests a high amount of native S04

=

in this sample and possibly a difference in the mechanism of S04

= sorption. This sample (Ac-rohumox B2) is relatively lower in Fe203 com-pared with the other B2 horizons (Acrorthox B2 and Tropudalf B2). This could explain the weaker retention of sorbed S04

= on the Acro-humox B2, in agreement with the results pre-sented by Aylmore, Karim, and Quirk (1967),

TABLE 4 Desorption of adsorbed sulfate by 0.5 N ammonium nitrate"

Soil horizon Solution S07 mM/1

SOT Adsorbed S04 Desorbed

me/100 g of soil SOT Desorbed as percent of S07

adsorbed

Acrohumox-Ap 1.2 0.32 0.25 77 Acrohumox-Ap 2.4 0.40 0.47 117 Acrohumox-B2 1.2 1.02 1.31 128 Acrohumox-B2 2.4 1.48 1.81 122 Acrorthox-Ap 1.2 0.71 0.67 95 Acrorthox-Ap 2.4 0.92 0.98 106 Acrorthox-B2 1.2 1.12 0.51 46 Acrorthox-B2 2.4 2.31 1.47 64 Tropudalf-Ap 1.2 0.50 0.46 91 Tropudalf-Ap 2.4 0.63 0.83 131 Tropudalf-B2 1.2 1.14 0.65 57 Tropudalf-B2 2.4 1.94 1.42 73

" Sulfate desorbed after equilibration with solutions containing the indicated amount of SO4 at pH approxi-mately 4.0. Average of two observations.

114 COUTO, LATHWELL, AND BOULDIN

Koter, Panck, and Wojnowska (1973), and Hague and Walmsley (1973).

Desorption of sorbed potassium

The amount of K+ initially present in the equilibrium solutions varied from 20 to 24.8 mM/liter when the S04

= concentration in-creased from 0 to 2.4 mM/liter. Under these conditions a comparatively large proportion of the negative sites on the surface of soil particles will be occupied by K+.

The amount of K+ desorbed by 0.5 N NH4N03 after equilibration with solutions containing 0, 1.2, and 2.4 mM of S04

= per liter can be consid-ered as an index for estimation of negative charges already present in the soil material, as well as those developed as a result of S04~ sorption, if any.

Table 5 summarizes the average values of desorbed K+ for each level of sorbed S04

= and the final pH values of the equilibrium solutions. There is a trend for increasing K+ desorbed with increasing S04* sorption within each sample. The trend is clearer in the B2 horizons of the Acrohumox and Acrorthox, respectively. In these samples, the final pH of the equilibrium solution increased when S04~ sorption in-creased. Under these conditions the increase in sorbed K+ could be the result of S04

= sorption directly or the result of the increase in pH of the

equilibrium solution and its effects on charge development.

To determine if the increases in K+ retained by the soils were directly related to increasing levels of sorbed S04

= , the same experiment was performed at a constant level of pH, approxi-mately 4.0. The results are summarized in Table 6.

When pH was controlled there also was an increase in K+ retained by the soil with increas-ing levels of sorbed S04". The amount of K+

desorbed by 0.5 N NH4NOj, however, was smaller in all cases showing a high pH depen-dence of cation retention in all samples. With pH controlled at approximately 4.0, the amount of desorbed K+, expressed as percent of the amount desorbed at zero level of sorbed S04

= , was considerably higher when compared to the results observed at higher pH values. This is due to higher absolute increases in K+ desorbed at increasing levels of sorbed S04

= , as well as to the lower amount of K+ desorbed at zero level of S0 4" sorption.

The differences in the amount of K+ retained by the samples after equilibration with solutions containing different amounts of S0 4" could hardly be explained by the small change in K+

concentration in the equilibrium solution. It is more likely due to the effect of sorbed S04

= on the exchange properties of the oxide surface

TABLE 5 Potassium desorbed at each level of adsorbed sulfate in nonadjusted pH conditions (averages of two

observations)

Soil horizon Solution SOT mM/1 Final pH Adsorbed S07,

me/100 g Desorbed K*,

me/100 g AK+,

me/100 g Relative

desorption

Acrohumox-Ap 0.0 4.65 -0.04 1.16 — 100 Acrohumox-Ap 1.2 4.65 0.10 1.15 -0 .01 99 Acrohumox-Ap 2.4 4.70 0.60 1.25 0.09 108 Acrohumox-B2 0.0 4.32 -0 .02 0.86 — 100 Acrohumox-B2 1.2 4.50 0.85 1.04 0.18 121 Acrohumox-B2 2.4 4.65 1.01 1.16 0.30 135 Acrorthox-Ap 0.0 5.10 -0.04 1.42 — 100 Acrorthox-Ap 1.2 5.17 0.39 1.55 0.13 109 Acrorthox-Ap 2.4 5.15 0.77 1.65 0.23 116 Acrorthox-B2 0.0 5.77 -0 .03 0.60 — 100 Acrorthox-B2 1.2 6.07 0.60 0.73 0.13 122 Acrorthox-B2 2.4 6.27 0.79 0.86 0.26 143 Tropudalf-Ap 0.0 5.70 -0 .08 3.15 - 100 Tropudalf-Ap 1.2 5.77 0.16 3.44 0.29 109 Tropudalf-Ap 2.4 5.75 0.57 3.64 0.49 116 Tropudalf-B2 0.0 6.10 -0.04 3.10 — 100 Tropudalf-B2 1.2 6.10 0.30 3.20 0.10 103 Tropudalf-B2 2.4 6.10 0.51 3.28 0.18 106

SULFATE SORPTION 115

TABLE 6 Potassium desorbed at each level of adsorbed sulfate at pH values approximately constant (averages of two

observations)

Soil horizon Solution SOT mM/1 Final pH Adsorbed SOT,

me/100 g Desorbed K+,

me/100 g AK\

me/100 g Relative

desorption

Acrohumox-Ap 0.0 4.00 -0.10 0.88 — 100 Acrohumox-Ap 1.2 4.00 0.33 1.02 0.14 116 Acrohumox-Ap 2.4 4.02 0.41 1.05 0.17 119 Acrohumox-B2 0.0 4.07 -0 .13 0.69 — 100 Acrohumox-B2 1.2 4.04 1.02 0.72 0.03 104 Acrohumox-B2 2.4 4.03 1.49 0.88 0.19 127 Acrorthox-Ap 0.0 4.02 -0.04 0.59 — 100 Acrorthox-Ap 1.2 4.00 0.71 0.77 0.18 130 Acrorthox-Ap 2.4 4.08 0.92 1.00 0.41 169 Acrorthox-B2 0.0 4.01 -0.06 0.07 — 100 Acrorthox-B2 1.2 4.00 1.11 0.13 0.06 186 Acrorthox-B2 2.4 4.07 2.28 0.24 0.17 342 Tropudalf-Ap 0.0 4.21 -0 .08 2.20 — 100 Tropudalf-Ap 1.2 4.20 0.50 2.35 0.15 107 Tropudalf-Ap 2.4 4.30 0.63 2.67 0.47 121 Tropudalf-B2 0.0 4.06 -0.14 2.01 — 100 Tropudalf-B2 1.2 3.99 1.14 2.09 0.08 104 Tropudalf-B2 2.4 4.05 1.94 2.39 0.38 119

(Hingston et al. 1967,1968). Mekaru and Uehara (1972) have shown a strong effect of phosphorus on increasing the net negative charge of these kinds of soils. It appears from these results that sorbed S04

= may behave in the same way by lowering the pH corresponding to the zero point of charge. The increases observed could be of practical interest in soils of the tropics charac-terized by high iron and aluminum oxides con-tent and very low cation exchange capacity at low pH values.

The results of the experiments presented here show clearly that S04

= sorption by the soils studied increases as the S04

= concentration in the reacting solution increases. In addition, S04~ sorption is substantially higher in the B2 hori-zons than in surface horizons, so that much of the S04

= potentially available to plants is stored in the subsoil. Since, in the oxide-rich soil sys-tems at least, specific S0 4" sorption occurs, a mechanism for retention of some S04

= within the soil profile exists. It is likewise clear, how-ever, that S04

= sorption decreases as soil pH increases. The resulting decrease in S04

= sorp-tion with increasing soil pH, and even S04

=

desorption in some instances, has important con-sequences on S04

= supply for plants. Many of these soils have been shown to be deficient in plant-available S04

= . Acid soils that have been limed or those that have a naturally high pH,

such as the Tropudalf studied here, may require regular S04

= applications as a consequence of their lowered S04~ sorption capacity. In addi-tion, liming of acid soils will enhance the move-ment of S04

= out of the limed zone of the soil. In assessing the available S04

= supply of these kinds of soils, the quantities of sorbed S04

= in the subsoil must be considered in addition to that contained in the surface horizons.

REFERENCES

Adams, Fred, and Z. Rawajfih. 1977. Basaluminite and alunite: A possible cause of sulfate retention by acid soils. Soil Sci. Soc. Am. J. 41: 686-692.

American Public Health Association, Inc. 1965. Stan-dard methods for the examination of water, sew-age, and industrial wastes, 12th ed., New York.

American Public Health Association, Inc. 1971. Stan-dard methods for the examination of water, sew-age, and industrial wastes, 13th ed., New York.

Atkinson, R. J., A. M. Posner, and J. P. Quirk. 1967. Adsorption of potential-determining ions at the ferric oxide-aqueous electrolyte interphase. J. Phys. Chem. 71: 550-558.

Aylmore, L. A. G., M. Karim, and J. P. Quirk. 1967. Adsorption and desorption of sulfate ions by soil constituents. Soil Sci. 103: 10-15.

Chao, T. T., M. E. Harward, and S. C. Fang. 1962. Movements of S35 tagged sulfate through soil col-umns. Soil Sci. Soc. Am. Proc. 26: 27-32.

Fox, R. L., S. M. Hansan, and R. C. Jones. 1971. Phosphate and sulfate sorption by Latosols. Int.

116 COUTO, LATHWELL, AND BOULDIN

Symp. Soil Fert. Eval. Proc., New Delhi 1: 857-864.

Hague, I., and D. Walmsley. 1973. Adsorption and desorption of sulfate in some soils of the West Indies. Geoderma 9: 269-278.

Harward, M. E., and H. M. Reisenauer. 1966. Reac-tions and movements of inorganic soil sulfur. Soil Sci. 101: 326-335.

Hingston, F. J., R. J. Atkinson, A. M. Posner, and J. P. Quirk. 1967. The specific adsorption of anions. Nature 215: 1459-1461.

Hingston, F. J., R. J. Atkinson, A. M. Posner, and J. P. Quirk. 1968. Specific adsorption of anions of goethite. Trans. Intern Congr. Soil Sci. 9th Congr.

Adelaide 1: 669-678. Kamprath, E. J., W. L. Nelson, and J. W. Fitts. 1956.

The effect of pH, sulfate, and phosphate concen-tration on the adsorption of sulfate by soils. Soil Sci. Soc. Am. Proc. 20: 463-466.

Koter, M., H. Panak, and T. Wojnowska. 1973. Sorp-tion and desorption of sulfate ions by various soil sorbents. Soils Fert. 36: 95.

Mekaru, T., and G. Uehara. 1972. Anion adsorption in ferruginous tropical soils. Soil Sci. Soc. Am. Proc. 36: 296-300.

Van Raij, B., and M. Peech. 1972. Electrochemical properties of some Oxisols and Alfisols of the tropics. Soil Sci. Soc. Am. Proc. 36: 587-593.