Soil Science Society of America Journal 67:589-595 (2003)
© 2003 Soil Science Society of America
DIVISION S-4—SOIL FERTILITY & PLANT NUTRITION
Extractable Iron and Aluminum Effects on Phosphate Sorption in a Savanna Alfisol
John O. Agbenin*
Dep. of Soil Sci., Faculty of Agriculture, Ahmadu Bello Univ., PMB 1044, Zaria, Kaduna State, Nigeria
* Corresponding author (joagbenin{at}yahoo.com)
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ABSTRACT
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Phosphate management problems in weathered tropical soils are mostly associated with the abundance of free Fe and Al oxides. The objectives of this study were to determine the effect of dithionite- and oxalate-extractable Fe and Al on P sorption in a savanna Alfisol. Twelve pedons from four cultivated fields and three pedons from an uncultivated natural site were examined for profile distribution of dithionite- and oxalate-extractable Fe and Al. Sixty samples, consisting of surface and subsurface samples, were chosen from the pedons to determine P sorption. Dithionite-extractable Fe and Al (Fed and Ald) and oxalate-extractable Al (Alo) increased with soil depth except for soils from one field, while oxalate-extractable Fe (Feo) decreased from the surface up to 20 cm depth, and thereafter remained constant with depth. The soils had low P sorption capacity. Phosphate sorbed ranged from 103 mg kg-1 in the surface soils to 460 mg kg-1 in the subsurface soils representing between 6 and 29% of applied P. Phosphate sorbed was linearly related to Fed (r2 = 0.71), Ald (r2 = 0.69) and Alo (r2 = 0.52), and was unrelated to Feo. Stepwise regression indicated that 73% of the variability of P sorbed by the soils was explained by Fed and Ald. The coefficients of the multiple regression model indicated that a unit change in Ald concentration changed P sorbed by 74 mg kg-1 as compared with 21 mg kg-1 by Fed. Since Ald is Al3+ substituted isomorphically for Fe3+ in crystalline Fe oxides in soils, the degree of this substitution appeared to have a profound effect on P sorption and fertility of savanna Alfisols.
Abbreviations: Ald, dithionite-extractable Al • Fed, dithionite-extractable Fe • Alo, oxalate-extractable Al • Feo, oxalate-extractable Fe
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INTRODUCTION
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A PEDOLOGICAL FEATURE of savanna Alfisols is the abundance of free Fe oxides which may be mobilized and deposited in soil profiles as Fe mottles, concretions, and hardpans (Jones and Wild, 1975). The nature, amounts, and distribution of dithionite- and oxalate-extractable Fe and Al oxides in a soil significantly affect its properties such as charge characteristics and ion adsorption, particularly phosphate sorption (Torrent, 1987; Torrent et al., 1990). The degree of crystallinity of Fe oxides in a soil has been assessed by the ratio of Fe extractable by acid ammonium oxalate, a selective extractant for poorly crystalline or amorphous Fe and Al (Schwertmann, 1964), to Fe extractable by dithionite citrate bicarbonate, a selective extractant for removing both crystalline and noncrystalline Fe oxides and Al substituted in crystalline Fe oxides in soils (Mehra and Jackson, 1960; Holmgren, 1967; Torrent, 1987). Low Feo/Fed ratio indicates a high degree of crystallinity of Fe oxides (Blume and Schwertmann, 1969). For Nigerian savanna soils, dominated by Alfisols or Tropical Ferruginuous soils, crystalline Fe is the predominant Fe form (Juo et al., 1974; Jones and Wild, 1975).
Free Fe oxides in soils are rarely pure (Schwertmann and Herbillon, 1992). The central Fe3+ ions are easily replaced by Al3+ present in the same pedoenvironment where free Fe oxides are formed especially in weathered soils, such as these, where the degree of substitution of Al3+ for Fe3+ may reach the maximum possible extent (Schwertmann and Herbillon, 1992). Dithionite Al is thought to be substituted into crystalline Fe oxides such as goethite and hematite (Holmgren, 1967). The effect of such substitutions is the structural distortion of crystalline Fe oxides with implication for anion retention and surface area (Schwertmann and Herbillon, 1992). Ainsworth and Sumner (1985) reported higher P sorption per unit surface area for Al-substituted goethites than for pure goethites, though such effects were not observed by Torrent (1987) in Mediterranean soils.
Despite the high intensity of weathering and the abundance of free Fe oxides in savanna soils, their P sorption capacity is considered low (Mokwunye, 1975; Agbenin, 1996). Tiessen et al. (1991) reported that ferruginous Fe nodules are the major P sinks in several Alfisols from semiarid regions of Ghana and northeast Brazil. Systematic investigation has not been performed to determine the components of savanna soils which affect P sorption. Phosphate sorption can be significantly affected by the amounts, forms, and distribution of oxalate- and dithionite-extractable Fe and Al in soils (Torrent, 1987; Torrent et al., 1990). There is little information on the effect of oxalate- and dithionite-extractable Fe and Al on P sorption in savanna soils. Such data are required to design improved methods of P management practices in the soils. The objectives of this study were to determine the status and effect of oxalate- and dithionite-extractable Fe and Al on P sorption in a typical savanna Alfisol.
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MATERIALS AND METHODS
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Sampling Site
The soil samples for this study were chosen from 15 pedons of a cultivated field and an uncultivated natural site at Samaru: 11°11'E long and 7°38'E long in the northern Guinea savanna of Nigeria. The geology of the area is dominated by Precambrian Basement Complex rocks consisting mainly of granite and gneiss. Geomorphologically, the area is a gently undulating plain which is a part of the Kaduna-Zaria Plains (Montimore, 1970). The site is a nearly level plain with <2% slope. The field is well drained and the soils are classified as Kaolinitic, Kanhaplic Haplustalf or Tropical Ferruginuous soils, the dominant soils in the region (Jones and Wild, 1975).
Of the 15 pedons sampled for this study, 12 pedons were from the cultivated site, and the three other pedons were from an uncultivated natural site (labeled F-1) close to the cultivated site up to a depth of 100 cm. The 12 pedons from the cultivated site were taken from four fields, indexed F-2 to F-5, under continuous cultivation since 1949 at the Institute for Agricultural Research Farm, Ahmadu Bello University, Zaria, Nigeria. Details of management of the cultivated fields before sampling are given in Table 1. The soil samples were air-dried and screened through a 2-mm sieve.
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Table 1. Site history, range, and means (in parentheses) of particle size distribution, organic carbon (OC), and cation exchange capacity (CEC) of soils of the field sites samples for this study.
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Soil Analysis and Phosphate Sorption
The particle size distribution in the soils was determined by the pipette method (Gee and Bauder, 1986). Soil pH was determined in 0.01 M CaCl2 at a soil:solution ratio of 1:2. Organic C was determined using a C and N Analyzer (CHN Rapid, Fruhe Heraeus, Hanau, Germany). Cation exchange capacity was determined by the summation of exchangeable cations and exchange acidity. Exchangeable cations were displaced by neutral 1 M NH4OAc. Potassium and Na in the extracts were determined by Flame Emission Spectroscopy, while Ca and Mg were determined by Atomic Absorption Spectroscopy. The exchange acidity was displaced by 1 M KCl and titrated with 0.1 M NaOH to the first permanent endpoint using phenolphthalein indicator (Thomas, 1982). The amorphous Fe and Al (Feo and Alo) were determined by extraction with ammonium oxalate in the dark (Jackson et al., 1986), while the dithionite citrate bicarbonate-extractable Fe and Al (Fed and Ald) were determined by the method of Mehra and Jackson (1960).
The difference between Ald and Alo was assumed to be the Al3+ substituted for Fe3+ in the crystalline Fe oxides hereby designated as Ald–o, although Holmgren (1967) indicated that Ald is Al substituted for Fe3+ in crystalline Fe oxides. The degree of substitution, calculated as mole % Al substitution, was determined by the molar concentration of Ald–o divided by the molar concentration of Fed + Ald–o. This is similar to the procedure adopted by Torrent (1987) except that citrate-bicarbonate buffer was used to remove amorphous Al from the soil in that case, and the difference between Ald and the citrate-bicarbonate buffer extractable Al was assumed to be Al3+ substituted for Fe3+ in crystalline Fe oxides. Thus, it is possible that the mole % Al substitution, as calculated in this study, might be slightly underestimated for the soils. However, this is unlikely to have had a significant impact on our results because Norrish and Taylor (1961) concluded that Fed and Ald originate from the same Fe oxides in soils, mostly goethite, the dominant crystalline Fe oxide in savanna Alfisols.
To characterize the P sorption capacity of the soils, 60 soil samples consisting of surface and subsurface samples were used, chosen from the 15 pedons to give a wide range of oxalate and dithionite Fe and Al. The P sorption capacity of the soils was determined by a single point P sorption method of Bache and Williams (1971). The amount of P sorbed from a P application of 50 mmol kg-1 soil (1.6 g kg-1 soil) was called the P sorption index of the soils defined as x/log c, where x is P sorbed and c is the equilibrium P concentration in solution. The higher the index, the higher the P sorption capacity of soils. Furthermore, the use of this index provides a specific and realistic estimate of the phosphate buffer capacity of soils (Bache and Williams, 1971).
Two grams of <2-mm sieved soil samples were weighed into centrifuge tubes in duplicates and equilibrated with the P solution concentration at a soil:solution ratio of 1:20. Two drops of toluene were added to the centrifuge tubes. The pH of the soil suspension was determined. The supporting electrolyte was 0.01 M KCl. The tubes were shaken for 24 h, and the suspensions were filtered through Whatman No. 42 Filter paper (Whatman Chemical Separation, Inc., Clifton, NJ) into 100-mL volumetric flasks. The P in the filtrates was determined colorimetrically by the method of Murphy and Riley (1962). Phosphate sorbed was the difference between P added and P in the supernatant solution. The relations between P sorbed and oxalate- and dithionite-extractable Al and Fe were evaluated by linear and multiple regression analyses.
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RESULTS AND DISCUSSION
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Oxalate- and Dithionite-Extractable Iron and Aluminum
The soils studied in this report had no evidence of Fe concretions within the 150-cm depth. The concentrations of Fed increased from the topsoil to the subsoil but remained nearly constant with depth below 20 cm while Feo decreased from the topsoil to the subsoil in F-1, F-2, and F-3, but remained fairly constant with depth in F-4 and F-5 (Fig. 1)
. The Fed concentration ranging from 3.6 to 13.3 g kg-1 soil were higher than values reported for most Nigerian soils by Ogunsola et al. (1989) but were, however, close to the range of Fed concentrations in some coastal sandy soils of southern Nigeria (Lekwa and Whiteside, 1986). The concentrations of Feo ranged from 176 to 628 mg kg-1, which were close to the values reported for several soils across Nigeria (Ogunsola et al., 1989). The degree of crystallinity of Fe oxides can be assessed by Feo/Fed. The generally low Feo/Fed ratio (Fig. 2)
indicated the preponderance of crystalline Fe forms, mainly goethite, as widely reported in savanna soils (Jones and Wild, 1975).
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Fig. 1. Distribution of Fed (A) and Feo (B) in representative pedons of a cultivated and uncultivated savanna Alfisol.
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Fig. 2. The depth function of the ratio of Feo to Fed as an index of the crystallinity of Fe oxides in representative pedons of a cultivated and uncultivated savanna Alfisol.
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Discussion of Alo and Ald in soils is often difficult to undertake because the identity of Al removed by oxalate and dithionite is not fully known. It is believed that Ald is reactive amorphous Al occluded in crystalline Fe oxides (Norrish and Taylor, 1961; Holmgren, 1967) probably involved in isomorphous substitution with Fe3+ (Fitzpatrick and Schwertmann, 1982; Torrent, 1987), thus causing structural distortion of the crystalline Fe oxides with implications for surface reactions and anion retention. The Ald increased with soil depth (Fig. 3A)
following the depth distribution of Fed except for F-4 field where its distribution is fairly uniform. The Alo appeared to increase from the topsoil to the subsoil (Fig. 3B). Oxalate-extractable Al is thought to be amorphous and organically bound Al (McKeague and Day, 1966), but the increasing Alo with depth suggested that it was possibly not associated with organic matter. The depth increase of Alo might suggest co-migration with clay probably present as surface coatings supported by the correlation with clay (r = 0.57; P < 0.01).
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Fig. 3. Distribution of Ald (A) and Alo (B) in representative pedons of a cultivated and uncultivated savanna Alfisol.
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Phosphate Sorption
The concentrations of oxalate- and dithionite-extractable Fe and Al in 30 of the soils used for the P sorption study are shown in Table 2. The concentrations of oxalate- and dithionite-extractable Al and Fe in the other 30 soils are within the range represented in Table 2. This is also applicable to the amount of P sorbed by the soils. Phosphate sorption index was two to three times greater in the subsurface than in surface soils except for Soils 18 to 20 (Table 2). Total P sorbed by the soils ranged from 103 mg kg-1 in the topsoil to 460 mg kg-1 in the subsoil, representing between 6 and 29% of applied P. This is additional evidence that savanna Alfisols have low capacity to sorb P, as reported by Mokwunye (1975) and Agbenin (1996).
The differences in P sorption index among the surface soils might be related to variation in clay contents. Surprisingly, however, Soils 18 to 20, subsurface samples from F-4 field, with similar silicate clay contents as other subsurface samples, sorbed significantly less P than other subsurface soils (Table 2), suggesting that total silicate clays might not exert a significant influence on the P sorption in savanna Alfisols.
A regression of P sorbed on Fed (Fig. 4A)
indicated that 71% of the variance in P sorbed or P sorption index was explained by Fed. Similar regression of P sorbed on Ald (Fig. 4B) explained 69% of the observed variance. There was no significant linear relation between Feo and P sorbed (Fig. 5A)
, suggesting that Feo was not an effective P sink in these soils corroborating observations in some semiarid soils from northeast Brazil (Agbenin and Tiessen, 1995). However, Alo explained up to 52% variance of P sorbed by the soils (Fig. 5B). The results of this study did not support the observations of Wang et al. (1991) that P seems to interact more with noncrystalline Fe than with crystalline Fe, but are consistent with those of Torrent (1987), who showed that Fed was a more effective P sink than Feo in some Mediterranean soils. Stepwise regression indicated that Ald and Fed together explained 73% of the variance of P sorbed as expressed in the equation:
Similar relations between P sorbed and Fed and Ald were reported for soils from semiarid northeastern Brazil by Agbenin and Tiessen (1994), soils from Malawi studied by Karim and Adams (1984), and some Puerto Rican soils by Jones (1981). The dependence of P sorption on Fed and Ald seems to explain the low amounts of P sorbed by Soils 18 to 20, even though they were subsurface soils with similar silicate clay contents as the other subsurface soils which generally sorbed more P than surface soils (Table 2). This, thus, provides some evidence that Fed and Ald are the phosphate scavengers in savanna Alfisols.
The relative importance of Fed and Ald as P sinks in these soils can be deduced from the magnitude of the stepwise regression coefficients. A unit change in Ald changes P sorbed by 74 mg kg-1 as against 21 mg kg-1 by Fed. Because Ald is essentially reactive amorphous Al occluded in Fe oxides or Al substituted for Fe3+ in crystalline Fe oxides (Holmgren, 1967), such substitutions are likely to lead to structural distortions and porosity of the crystalline Fe oxides to which P can readily diffuse. In several Fe nodules isolated from semiarid or savanna Alfisols from Ghana, x-ray microprobe scans revealed a high concentration of Al while micromorphological investigations of the same Fe nodules revealed significant porosity (Tiessen et al., 1991). Mole % calculation of substituted Al3+ for Fe3+ in Fe oxides in this savanna Alfisol showed a range of 11 mol % in the surface soil to 25 mol % in the subsurface (Table 2), which is within prediction in highly weathered tropical soils. Schwertmann and Herbillon (1992) estimated that up to 33 mol % Al3+ substitution for Fe3+ in goethites could occur for goethites in association with gibbsite, and in highly weathered soils.
It is, however, interesting to observe in this study that Al substitution in crystalline Fe oxides expressed as mole % showed significant linear relations with sorbed P in the soils [P sorbed = -229 + 25 (Al substitution mole %); r2 = 0.60, P < 0.001] but fitting an exponential function marginally improved R2 to 62% (Fig. 6)
. This corroborates the observation of Schwertmann and Herbillon (1992) that the degree of substitution of Fe3+ by Al3+ in crystalline Fe oxides might be a factor in P fertility and sorption capacity of weathered tropical soils. Similarly, Ainsworth and Sumner (1985) reported higher P sorption per unit surface area of structurally Al-substituted goethites than pure goethites. Reports that noncrystalline Fe and Al are good predictors of P sorption (Broomfield, 1965; Borggaard et al., 1990; Yuan and Lavkulich, 1994) were not supported by this study. In less weathered soils, predictive variables for P sorption models are mostly Alo and Feo (van der Zee and Riemsdijk, 1988; Borggaard et al., 1990; Yuan and Lavkulich, 1994; Beauchemin and Simard, 2000). The overriding influence of Alo and Feo on P sorption in less weathered soils has led to the proposition of the ratio of oxalate-extractable P (Pox) to Alo + Feo as the P saturation index of soils (van der Zee and Riemsdijk, 1988; Beauchemin and Simard, 2000; Pautler and Sims, 2000), suggesting that concentrations of Alo and Feo are good estimates of P sorption sites in those soils. My results suggest that models of P sorption developed for soils outside this region are clearly inappropriate for savanna soils.
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Fig. 6. The relationship between Al substitution in Fe oxides expressed as mole % and P sorption in a savanna Alfisol.
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CONCLUSIONS
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The data presented in this study lead to the following conclusions: The soils exhibited low P sorption capacity, sorbing only between 6 and 29% of applied P, further supporting the notion that savanna soils have low P sorption capacity. Phosphate sorption index increased drastically from the surface to the subsurface following the increases in Ald, Fed, and Alo.
The active P sorption components of savanna soils appeared to be Ald and Fed. The ability of Fed to sorb P in the soils appeared to depend largely on the degree of Al substitution for the central Fe3+ in Fe oxides. When estimating P requirement of savanna Alfisols, one should consider the concentrations of P, Fed, and Ald in the soil.
The two- to three-fold increases in P sorbed by the subsurface samples compared with surface samples would imply that should the topsoil be eroded, the P requirements of savanna Alfisol would increase drastically. The implication of the low P sorption capacity of the surface soils is that frequent application of small doses of fertilizer-P might be much more appropriate to meet crop requirement than a single large dose application, in accord with current fertilization practices in the region.
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ACKNOWLEDGMENTS
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The author is indebted to Alexander Von Humboldt Foundation for Research Fellowship and to the Institute of Soil Science, Justus-Liebig University, Giessen, for laboratory facilities. I acknowledge the contributions of the technical staff of the institute to this work.
Received for publication February 28, 2001.
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TOP
ABSTRACT
INTRODUCTION
