Soil Science Society of America Journal 64:892-897 (2000)
© 2000 Soil Science Society of America
DIVISION S-2-SOIL CHEMISTRY
Phosphate-Induced Clay Dispersion as Related to Aggregate Size and Composition in Hapludoxs
Jose M. Limaa,
Sharon J. Andersonb and
Nilton Curia
a Soil Science Dep., Federal University of Lavras, Minas Gerais, Brazil, 37200-000
b Earth Systems Sci. and Policy, CSUMB, 100 Campus Ctr., Seaside, CA 93955-8001 USA
jmlima{at}ufla.br
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ABSTRACT
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Although phosphate sorption is a well-understood phenomenon in soils, less is known of its effect on electrophoretic mobility (EM), isoelectric point (IEP), and clay particle dispersion in Oxisols. High phosphate sorption and high stability of aggregates are characteristic of Oxisols. Phosphate sorbed as inner-sphere complexes brings negative charge to the surface of particles, affecting EM, IEP, and clay particle dispersion. The objectives of this research were to determine the effect of residual sorbed P (after one sorption–desorption cycle in 0.015 M NaCl) on EM, IEP, and clay particle dispersion in aggregates of Oxisols with different organic matter contents and hematite/goethite ratios. Aggregates of 1 to 2 and 0.1 to 0.2 mm were fractionated from samples of A and B horizons of two Oxisols, both with 165 g kg-1 Fe2O3, that differ in their organic matter and hematite and goethite contents. Phosphate sorption decreased EM and IEP of B horizon aggregates. It also decreased the amount of dispersed clay, as the IEP decreased to values closer to the pH of the soil suspension, decreasing net positive charge. Then, P sorption increased dispersed clay as the IEP became lower than the pH of suspension. The effect was slightly higher on aggregates with higher hematite/goethite ratio. The changes on those parameters were mostly noted for B horizon samples, where phosphate sorption had a major effect on charge balance because of their lower organic matter content. Small aggregates had less dispersed clay than large aggregates.
Abbreviations: DR, Dark-Red Latosol • EM, electrophoretic mobility • IEP, isoelectric point • UL, Una Latosol
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INTRODUCTION
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OXISOLS with medium to high Fe content (greater than 
110 g kg-1) in the clay fraction tend to contain small, very stable aggregates, especially in the B horizon (Resende et al., 1988; Fontes, 1992; Oliveira et al., 1992). There are many studies showing the interaction of Fe and Al oxides with clay minerals, specifically kaolinite (Goldberg and Glaubig, 1987; Goldberg, 1989; Santos et al., 1989; Fontes, 1992). Although many of these studies were done with artificial oxides in the laboratory, they strongly suggest that the interactions of Fe and Al oxides with each other and other soil minerals are of fundamental importance in soil aggregation.
In soils, small aggregates (microaggregates) <0.2 mm in diameter are composed of clay, Fe and Al oxides, and organic matter, whereas larger aggregates are more likely to be composed of microaggregates that are bound together mainly by organic matter and polyvalent metals (Giovanini and Sequi, 1976; Tisdall and Oades, 1982). There are some controversies regarding the role of Fe oxides in soil aggregation. Schwertmann and Kämpf (1985) showed that goethite and hematite in Oxisols tend to form microaggregates among themselves, instead of interacting with kaolinite. On the other hand, Santos et al. (1989) showed that microaggregates of Fe oxides associated with organic matter and kaolinite formed larger aggregates in Oxisols. Fontes (1992) also showed some evidence for interaction of Fe oxides and kaolinite and gibbsite in Oxisols. The interaction of Fe oxides with other minerals in soils also includes x-ray amorphous Fe oxides, as these oxides were observed to be very effective in aggregating soil particles (Arduino et al., 1989; Barberis et al., 1991) because of the higher surface area and charge per unit mass. Either interacting among themselves or with other soil particles, most of the interactions involving Fe oxides are pH dependent. The presence of net surface charge, either positive or negative, produces repulsive forces between like-charged particles and consequently reduces aggregate stability and increases clay dispersion (Gillman, 1974; Sumner, 1992). Aggregate stability decreases and clay dispersibility increases as net surface charge increases. The surface-charge properties of Oxisols are determined by the relative proportions of Fe and Al oxides, kaolinite, and organic matter, all of which cause surface charge in Oxisols to depend strongly on pH. Iron and Al oxides display net positive charge below pH 8 to 9 (Sposito, 1989a, 1989b; Stumm, 1992), although positively and negatively charged surface functional groups can coexist above and below the IEP. Kaolinite has net negative charge above pH 4.5 (Ferris and Jepson, 1975; Sposito, 1989a, 1989b; Stumm, 1992), and organic matter is negatively charged at all soil pH values.
Net surface charge is affected not only by mineralogy, organic matter concentration, and pH, but also by the sorption of ions that form inner-sphere complexes with surface functional groups (Hingston et al., 1972; van Raij and Peech, 1972; Wann and Uehara, 1978; Stumm, 1992). Specific (inner-sphere) sorption of anions decreases the net positive surface charge and thus shifts the IEP (pH at which the EM is zero) to lower pH (Hingston et al., 1974; Sawhney, 1974; Wann and Uehara, 1978; Sposito 1989a, 1989b; Stumm, 1992). In soils with net positive surface charge, that is, soils with pH < IEP, phosphate sorption should cause an increase in aggregate stability and a decrease in clay dispersion by making the net charge of the soil less positive. Chorover et al. (1997) showed that increasing phosphate adsorption on hematite decreases EM to zero; then a charge reversal occurs with further increase in P adsorption. The dispersibility of soil clay should decrease as the difference between IEP and pH decreases. However, if phosphate sorption is sufficient to give the soil net negative charge (i.e., shifting the IEP such that pH > IEP), then additional phosphate sorption should cause an increase in clay dispersion.
Surface charge, P sorption, and aggregation are clearly all interdependent, and all are controlled by the composition of the soil material. Linquist et al. (1997) showed that P sorption is higher for smaller soil aggregates. In a previous study, Lima and Anderson (1997) showed that the composition of 1- to 2-mm Oxisol aggregates differs from that of 0.1- to 0.2-mm Oxisol aggregates, especially in the A horizon. Larger aggregates from the A horizon contain significantly more clay, goethite, and oxalate- and DCB-extractable Fe and Al than do the smaller aggregates; they therefore should sorb more P and be more stable than the smaller aggregates. The effect of P sorption on aggregate stability and on the surface-charge properties and concentration of suspended clay in different aggregate sizes is more difficult to predict because of the complex relationship between composition, surface charge, and P sorption, and because the physical characteristics of the aggregates may also be important.
The objective of this research was to determine whether the concentration and surface charge properties of suspended clay, both before and after phosphate sorption, differs between large and small aggregates from A and B horizons of two Hapludoxs. To distinguish between the chemical effect of compositional differences and the physical effect of aggregation and aggregate size per se, the concentration and IEP of suspended clay were measured both for aggregates and for samples that previously had been dispersed by sonication.
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Materials and methods
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Samples were collected from A and B horizons of two Oxisols that have identical amounts of total Fe oxides (165 g kg-1) but differ in their organic matter contents and hematite/goethite and gibbsite/kaolinite ratios. The two soils, an Una Latosol (hereafter referred as UL) and a Dark-Red Latosol (hereafter DR) (Camargo et al., 1987), were sampled at A and B horizons (0–20 and 60–80 cm, respectively, for A and B horizons of UL and 0–20 and 100–120 cm for DR). The soil sites were 30 m apart in the Campos das Vertentes Physiographic Zone, Minas Gerais State, Brazil (21°20' S, 44°30' W). Both soils are classified as very fine, allitic, isothermic Typic Hapludox according to U.S. soil taxonomy. After air drying, each whole dry sample was placed on the top sieve of a stack of sieves having 2-, 1-, 0.25-, and 0.125-mm openings in order to separate the 1- to 2- and 0.1- to 0.2-mm aggregates. Then, the 1- to 2- and 0.1- to 0.2-mm aggregates were placed on 1- and 0.125-mm sieves, respectively, and the sieves were dipped into a 0.01 M CaCl2 solution at a rate of 35 cycles min-1 for 15 min. The CaCl2 solution was then replaced by deionized water and the dipping cycles were continued for an additional time of 15 min. The CaCl2 solution was used in order to give the samples the same cation saturation condition, and the water was used to remove the excess of CaCl2 solution. The water-stable aggregates in each size fraction were then dried at 40°C in a ventilated oven for 24 h. Relevant chemical, physical, and mineralogical properties of the aggregates are reported in Table 1
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Table 1 Chemical, physical, and mineralogical properties of two aggregate size fractions of A and B horizons of two Oxisols from Brazil (triplicate analyses)
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Three grams of each aggregate size fraction were placed into 250-mL centrifuge bottles and prewetted with a few drops of deionized water. After wetting, 100 mL of deionized water were added to each tube. Half of the samples were sonicated at 100 W for 3 min to destroy the aggregates. Next, 50 mL of 0.03 M NaCl were added to both the aggregated and dispersed samples and the suspensions were adjusted to pH 5 with NaOH or HCl. The suspensions were left to equilibrate overnight without shaking. The pH was then readjusted to 5 as necessary. Then, 50-mL volumes of solutions containing either 0, 15, 40, 80, or 150 mg L-1 P in 0.03 M NaCl were added to duplicate samples to give total initial P concentrations of 0, 0.12, 0.32, 0.64, and 1.2 mM (the final ionic strength was 0.015 M). The NaCl rather than CaCl2 was used as the background solution to avoid Ca–P precipitates. The suspensions were shaken for 48 h in an orbital shaker at 200 rpm at 20 ± 2°C in a constant temperature chamber. A 48-h reaction time was chosen because initial experiments had shown that the amount of phosphate sorbed after 48 h was 90% of the total sorbed after 15 d, for initial concentrations of 1.2 mM. After shaking the suspensions, pH was measured and the tubes were centrifuged for 5 min at 550 g. The supernatant solutions were decanted and saved for P measurements, and each centrifuge bottle was weighed to determine the volume of entrained solution. Supernatant phosphate concentrations were measured using a Lachat flow-injection analyzer (Zellweger Analytics, Lachat Instruments Div., Mequon, WI) with the molybdenum blue-ascorbic reduction method of Murphy and Riley (1962). Sorbed P was calculated as the difference between initial and final solution P concentrations and was expressed in millimoles per kilogram:
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where Pinit and Pfinal are the initial and final solution-phase P concentrations (mmol dm-3), and V is the solution volume (dm3).
For desorption measurements, the samples and entrained solution were shaken with 100 mL of 0.015 M NaCl for 24 h at 20 ± 2 °C, then centrifuged as before. The supernatant solutions were decanted and saved for P analyses. The bottles with soil and entrained solution were weighed to determine the mass of entrained solution. The amount of phosphate desorbed was calculated using the equation:
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where Pd is the P concentration in the desorption supernatant, Vd is the volume of NaCl solution added during desorption step, Ventr is the volume of solution entrained after the sorption step, and Pfinal is defined as above. The amount of phosphate remaining after desorption (residual P, mmol kg-1) was calculated as the difference between sorbed and desorbed P.
To measure the effect of residual P on dispersible clay, the soil and entrained solution that remained in the bottles after desorption were transferred to 50-mL tubes, and the total volume (including entrained solution) was brought to 50 mL with deionized water. The volume of entrained solution was 4.12 ± 0.68 mL, which accounted for an ionic strength equal to 0.00114 ± 0.00017 M in the background of the suspension. The tubes were then shaken overnight in an end-over-end shaker at 90 cycles min-1. The tubes were removed from the shaker and shaken by hand for 30 s to set time zero for settling clay-sized particles according to Stoke's law. After 3 h of settling, 10-mL aliquots were withdrawn from the tubes at 4.5-cm depth and dried in weighed aluminum dishes for 24 h to determine the mass of dispersed clay-sized (<2-mm) material. The amount of dispersed clay was expressed as a fraction of the total clay. The method used for measuring dispersed clay is similar to that suggested by Burt et al. (1993). After the 10-mL aliquot was taken for dispersed clay measurements, the pH of each suspension was measured to determine the difference between suspension pH and IEP (described below). Total clay was measured as described in Lima and Anderson (1997).
For EM measurements, duplicate 0.1-mL aliquots of the dispersed clay suspensions, with clay concentrations ranging from 80 to 150 mg dm-3, were transferred to 50-mL centrifuge tubes containing 40 mL of 0.015 M NaCl solution at pH 3, 5, 7, and 9. The pH of each suspension was measured, and 30 mL of each suspension were placed into a Zeta Meter cell (Zeta Meter System 3.0, Zeta Meter, Staunton, VA) for EM measurements. The voltage applied in the Zeta Meter was adjusted to give a measuring time of 5 s for each particle. A total of 15 to 30 particles were counted in forward or reverse direction for each sample. Isoelectric point was determined graphically from plots of EM vs. pH.
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Results and discussion
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Phosphate Sorption and Desorption
Phosphate sorption, and the effect of residual P on IEP and dispersibility of clay were not significantly different for aggregated vs. dispersed samples. Therefore, only the results for aggregated samples are shown and discussed below.
Sorbed, desorbed, and residual P concentrations for initial P concentrations ranging from 0.12 to 1.2 mM are shown in Fig. 1 . For the A horizons, small aggregates sorbed less P than large aggregates, which can be accounted for by their lower clay and Fe and Al oxide contents (Table 1) that represent fewer available sites for P sorption. The difference between P sorption on small and large aggregates from the A horizons increases as initial P concentration increases. It seems that saturation of the available sites in small aggregates of A horizons was reached at the 0.64 mM initial P concentration. Any P ions that were sorbed above this saturation were easily desorbed, which is shown by the higher desorption/sorption ratio at 1.2 mM initial P (comparing the bars in Fig. 1). Residual P does not increase as initial P concentration goes from 0.64 to 1.2 mM. Both 1- to 2- and 0.1- to 0.2-mm aggregates in B horizons sorbed similar amounts of P. Also desorption was similar between these aggregate sizes.
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Fig. 1 Effect of aggregate size on residual sorbed P (bottom bars), desorbed P (upper bars), and total sorbed P (combined bar height) at four initial P concentrations for A and B horizons of Una (UL) and Dark-Red (DR) Latosols. Average and standard deviation for triplicate samples
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Aggregates from the B horizons sorbed more, and desorbed less P, than did aggregates from the A horizons. Residual P, which represents irreversibly sorbed P, is greater in B than in A horizons. Higher organic matter contents in the A horizon aggregates (Table 1) caused lower P sorption by making the A horizon material more negatively charged and possibly blocking some phosphate sorption sites.
Effect of Residual Phosphorus on Electrophoretic Mobility and Isoelectric Point
The effects of residual phosphate and aggregate size on the EM and IEP of suspended clay are shown in Fig. 2 and 3
, respectively. The size of the aggregates had no effect on both parameter values, despite the differences in residual P of large and small aggregates in the A horizons.
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Fig. 2 Effect of pH, aggregate size, and initial P concentration on electrophoretic mobility of dispersed clay from A and B horizons of Una (UL) and Dark-Red (DR) Latosols
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Fig. 3 Effect of residual P and aggregate size on isoelectric point (IEP) of dispersed clay from A and B horizons of Una (UL) and Dark-Red (DR) Latosols. Average and standard deviation for duplicate samples. Standard deviations smaller than the symbol sizes are not shown
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Residual P had little effect on the EM and IEP of suspended clay from the A horizon material, though IEP of clay from large aggregates decreased slightly. The IEP values at zero residual P in the A horizons are much lower than in B horizons (3.8 vs. 6.6; Fig. 3). Mainly the higher organic matter and, to some extent, the lower content of Fe oxides (Table 1) are responsible for the lower IEP in the A horizons. Most of the P sorption in the A horizon may occur by displacing organic compounds (Afif et al., 1995), which causes IEP values to be approximately constant as residual P increases, though the small decrease in IEP suggests that some P had been sorbed at positive sites. The quantity of residual P had a much higher effect on EM and IEP of B horizon material. The IEP values dropped less and desorption of P was slightly lower in the B horizon of UL soil. The higher amount of goethite, which is more reactive with phosphate than hematite (Bigham et al., 1978; Sposito, 1989a), probably caused more stable complexes to be formed in the UL-B horizon than in the DR-B horizon. According to Hingston et al. (1974), the more stable complexes produce less negative charge. Thus, the P complexes formed in DR soil were likely to produce more negative charge and, therefore, to decrease the IEP values of DR-B horizon, because of its higher hematite and lower goethite contents, compared with the UL-B horizon (Table 1). The IEP values at zero residual P (6.5 for B horizons and 3.7 for A horizons) show that B horizon aggregates were positively charged and A horizon aggregates were negatively charged at the initial condition (prior to P sorption) when pH values were adjusted to 5.
Effect of Residual Phosphorus on Suspended Clay
The ratio of suspended clay/total clay (clay ratio) related to residual P is shown in Fig. 4
. Residual P had no effect on the clay ratio in A horizon aggregates. The clay ratio varied as a function of residual P in the B horizon, and the effect was smaller in small than in large aggregates of the B horizon. The clay ratio decreased as the residual P increased to 10 mmol kg-1, then the clay ratio increased for values of residual P above 10 mmol kg-1. For both soils and horizons small aggregates had lower clay ratio, except for 10 mmol kg-1 residual P, where both B horizon aggregate sizes had the ratio close to zero.
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Fig. 4 Effect of residual sorbed P and aggregate size on dispersed total clay for A and B horizons of Una (UL) and Dark-Red (DR) Latosols. Average and standard deviation for duplicate samples
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The effect of residual P on the clay ratio is better illustrated by the difference between IEP and pH (Fig. 5)
, which is an index of the net charge on the clay surface. Smaller aggregates had a lower clay ratio than larger aggregates in both A and B horizons.
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Fig. 5 Effect of isoelectric point (IEP) - pH and aggregate size on dispersed total clay for A and B horizons of Una (UL) and Dark-Red (DR) Latosols. Average and standard deviation for duplicate samples
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The values of IEP in the A horizons were always 2 to 3 pH units below the experimental suspension pH. The material from B horizons of both soils showed a positive side (IEP higher than pH) and a negative side (IEP lower than pH). This caused the clay ratio to decrease as P sorption increased up to levels where IEP - pH was close to zero; then, phosphate sorption increased the ratio, increasing clay dispersion (Fig. 5). Smaller aggregates were more stable than larger aggregates, as shown by their lower clay ratio (Fig. 4 and 5).
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Conclusions
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The amount of residual P (difference between sorption and desorption) was lower for small aggregates. In both aggregate sizes, an increase in residual P caused a greater decrease in IEP for B horizon than for A horizon material. Dispersion of clay was affected by residual P only in B horizon aggregates. Small aggregates dispersed less (were more stable) than large aggregates. The greater the difference between IEP and pH, in absolute value, the greater was the amount of dispersed clay.USDA Soil Survey Laboratory Staff 1992
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NOTES
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Project developed at the Dep. of Crop and Soil Sci., Michigan State Univ., and sponsored by CNPq - Brazil.
Received for publication November 23, 1998.
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REFERENCES
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- Afif E., Barron V., Torrent J. Organic matter delays but does not prevent phosphate sorption by cerrado soils from Brazil. Soil Sci. 1995;159:207-211.
- Arduino E., Barberis E., Boero V. Iron oxides and particle aggregation in B horizon of some Italian soils. Geoderma 1989;45:319-329.
- Barberis E., Marsan F.A., Boero V., Arduino E. Aggregation of soil particles by iron oxides in various size fractions of soil horizons. J. Soil Sci. 1991;42:535-542.
- Bigham J.M., Golden D.C., Buol S.W., Weed S.B., Bowen L.H. Iron oxide mineralogy of well-drained Ultisols and Oxisols: II. Influence on color, surface area, and phosphate retention. Soil Sci. Soc. Am. J. 1978;42:825-830.[Abstract/Free Full Text]
- Burt R., Reinsch T.G., Miller W.P. A micro-pipette method for water dispersible clay. Commun. Soil Sci. Plant Anal. 1993;24:2531-2544.
- Camargo M.N., Klamt E., Kauffman J.H. Soil classification as used in Brazilian soil surveys. Wageningen, the Netherlands: ISRIC, 1987.
- Chorover J., Zhang J., Amistadi M.K., Buffle J. Comparison of hematite coagulation by charge screening and phosphate adsorption: Differences in aggregate structure. Clays Clay Miner. 1997;45:690-708.[Abstract]
- Ferris A.P., Jepson W.P. The exchange capacity of kaolinite and the preparation of homoionic clays. J. Colloid Interface Sci. 1975;51:245-259.
- Fontes M.P.F. Iron oxide-clay mineral association in Brazilian Oxisols: A magnetic separation study. Clays Clay Miner. 1992;40:175-179.[Abstract]
- Gillman G.P. The influence of net charge on water dispersible clay and sorbed sulfate. Aust. J. Soil Res. 1974;12:173-176.
- Giovanini G., Sequi P. Iron and aluminum as cementing substances of soil aggregates. II: Changes in aggregate stability of soil aggregates following extraction of iron and aluminum by acetylacetone in a non-polar solvent. J. Soil Sci. 1976;27:148-153.
- Goldberg S. Interaction of aluminum and iron oxides and clay minerals and their effect of soil physical properties: a review. Commun. Soil Sci. Plant Anal. 1989;20:1181-1207.
- Goldberg S., Glaubig R.A. Effect of saturating cation, pH, and aluminum and iron oxide on the flocculation of kaolinite and montmorilonite. Clays Clay Miner. 1987;35:220-227.[Abstract]
- Hingston F.J., Posner A.M., Quirk J.P. Anion adsorption by goethite and gibbsite. I: The role of proton in determining adsorption envelopes. J. Soil Science 1972;23:177-192.
- Hingston F.J., Posner A.M., Quirk J.P. Anion adsorption by goethite and gibbsite. II: Desorption of anions from hydrous oxide surface. J. Soil Sci. 1974;25:16-26.
- Kämpf N., Schwertmann U. Quantitative determination of goethite and hematite in kaolinitic soils by x-ray defraction. Clay Miner. 1982;17:359-363.[Abstract]
- Lima J.M., Anderson S.J. Aggregation and aggregate size effects on extractable iron and aluminum in two Hapludoxs. Soil Sci. Soc. Am. J. 1997;61:965-970.[Abstract/Free Full Text]
- Linquist B.A., Singleton P.W., Yost R.S., Cassman K.G. Aggregate size effects on the sorption and release of phosphate in an ultisol. Soil Sci. Soc. Am. J. 1997;61:160-166.[Abstract/Free Full Text]
- Mehra O.P., Jackson M.L. Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. Clays Clay Miner. 1960;7:317-327.
- Murphy J., Riley J.P. A modified single solution method for determination of phosphate in natural waters. Anal. Chim. Acta. 1962;27:31-36.
- Oliveira J.B., Jacomine P.K.T., Camargo M.N. Classes gerais de solos do Brasil: guia auxiliar para reconhecimento. Jaboticabal, Sao Paulo, Brazil: FUNEP, 1992.
- Resende M., Curi N., Santana D.P. Pedologia e fertilidade do solo: Interacoes e aplicacoes. Brasilia-DF, Brazil: MEC-ESAL-POTAFOS, 1988.
- Santos M.C.D., Mermut A.R., Ribeiro M.R. Submicroscopy of clay microaggregates in Oxisol from Pernambuco, Brazil. Soil Sci. Soc. Am. J. 1989;53:1985-1991.
- Sawhney B.L. Charge characteristics of soils as affected by phosphate sorption. Soil Sci. Soc. Am. Proc. 1974;38:159-160.
- Schwertmann U. Differenzierung der eisenoxide des bodens durch extraktion mit ammonium-oxalat-lösung. Zeitschrift füer Pflanzenernährung 1964;105:194-202.
- Schwertmann U., Kämpf N. Properties of goethite and hematite in kaolinitic soils of Southern and Central Brazil. Soil Sci. 1985;139:344-350.
- Sposito G. The chemistry of soils. New York: Oxford Univ. Press, 1989 a.
- Sposito G. Surface reactions in natural and aqueous colloidal systems. Chimia 1989;43:169-176 b.
- Stumm W. Chemistry of the solid–water interface: Processes at the mineral–water and particle–water interface in natural systems. New York: John Wiley and Sons, 1992.
- Sumner M.E. The electrical double layer and clay dispersion. In: Sumner M.E., Stewart B.A., eds. Soil crusting: Chemical and physical processes. Boca Raton, FL: Lewis Publ, 1992:1-31.
- Tisdall J.M., Oades J.M. Organic matter and water-stable aggregates in soils. J. Soil. Sci. 1982;33:141-163.
- USDA Soil Survey Laboratory Staff. 1992. Soil survey laboratory methods manual. Soil Survey Investigation Rep. 42.
- Wann S.S., Uehara G. Surface charge manipulation of constant surface potential soil colloids. I: Relation to sorbed phosphorus. Soil Sci. Soc. Am. J. 1978;42:565-570.[Abstract/Free Full Text]
- van Raij B., Peech M. Electrochemical properties of some Oxisols and Alfisols of the Tropics. Soil Sci. Soc. Am. Proc. 1972;36:587-593.
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ABSTRACT
INTRODUCTION
