- Email: [email protected]
An assessment of EDTA as an extractant of organic-complexed and amorphous forms of Fe and Al in soils
Geoderma, 35 (1985) 109-118 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
109
AN ASSESSMENT OF EDTA AS AN EXTRACTANT OF ORGANICCOMPLEXED AND AMORPHOUS FORMS OF Fe AND Al IN SOILS*
J.A. MCKEAGUE and P.A. SCHUPPLI Land Resource
Research Institute Agriculture
(Received June 6,1984;
Canada, Ottawa, Ont., KlA OC6 (Canada)
accepted after revision December 12,1984)
ABSTRACT McKeague, J.A. and Schuppli, P.A., 1985. An assessment of EDTA as an extractant of organic-complexed and amorphous forms of Fe and Al in soils. Geoderma, 35: 109-118. EDTA solutions proposed as selective extractants of amorphous and organic forms of Fe and Al in soils and synthetic materials were tested. Extraction of Fe and Al from some soil samples continued at a decreasing rate beyond 120 days. For some soil samples and synthetic materials the amounts of Fe and Al extractable by EDTA (90 days) were markedly lower than those extractable by acid ammonium oxalate (2 h). Extraction of samples for 1 h with EDTA was shown to release amounts of Fe and Al far below those considered to be complexed with organic matter. The EDTA extraction procedures tested should not replace either acid ammonium oxalate for estimating amorphous forms of Fe and Al or pyrophosphate for estimating organic forms of Fe and Al in soils.
INTRODUCTION
Selective extraction of different forms of Fe and Al in soils continues to be a subject of interest because physical methods such as X-ray are not adequate for differentiating various amorphous forms (Schwertmann and Taylor, 1977). Extraction procedures proposed for differentiating forms such as crystalline iron oxides, amorphous iron oxides and organic-complexed iron (McKeague et al., 1971) have limitations. Dithionite-citratebicarbonate (Mehra and Jackson, 1960), for example, dissolves some Fe from fine particles of labile silicates and it fails to dissolve completely some crystalline iron oxide particles coarser than approximately 50 pm. Acid ammonium oxalate (referred to as oxalate) dissolves not only amorphous iron oxides (Schwertmann, 1964) but also magnetite (Baril and Bitton, 1969). Sodium pyrophosphate peptizes mineral particles containing Fe and Al in some soils in addition to dissolving organic complexed Fe
*Contribution
No. LRRI 84-32.
110
and Al (Jeanroy and Guillet, 1981; Schuppli et al., 1983). Improved selective extraction procedures are desirable. Among the reagents tested recently, EDTA has been proposed for selective extraction of both amorphous iron oxides (Borggaard, 1976; 1979) and organic-complexed Fe and Al (Farmer et al., 1980; Anderson et al., 1982). EDTA has the advantage over oxalate that the amount of Fe and Al extracted increases slowly with time and reaches a plateau after approximately 3 months, whereas oxalate extracts markedly increased amounts of Fe and Al with time beyond the usual 2 or 4 h shaking period (Borggaard, 1979). EDTA as an extractant of organic complexed Fe and Al might avoid the problem encountered with pyrophosphate extracts having particulate material which may remain in suspension (Schuppli et al., 1983). Borggaard’s (1976; 1979) work with a model substance containing crystalline and amorphous iron oxides and with a soil sample (Borggaard, 1976) indicated that EDTA solutions of a range of concentrations and pH extracted the same amount of Fe in 3 months as that extracted by oxalate in 2 h. EDTA extractable Fe and Al did not increase after 3 months of shaking in most cases, although there were slight increases for a few samples. Later work (Borggaard, 1981) with 17 soil samples, however, showed that for 10 of the samples, Fe extracted in 2 h by oxalate exceeded that extracted by EDTA in 3 to 8 months; for several samples, the difference exceeded 30%. On the other hand, oxalate values were less than 65% of the EDTA-Fe values for a few samples. The 1 h EDTA-extraction for organic-complexed Fe and Al was apparently based only on results showing that more Fe and Al were extracted from organic-rich upper B horizons of some podzols than from lower B horizons rich in amorphous inorganic substances (Farmer et al., 1980; Anderson et al., 1982). Further testing of EDTA as a selective extractant was necessary. The purpose of the work reported here was to determine Fe and Al extracted by EDTA solutions after various periods of equilibration of a range of soil samples, a few synthetic materials and magnetite and thus to assess the usefulness of EDTA as a selective extractant of organic-complexed and amorphous forms of Fe and Al in soils. MATERIALS
AND METHODS
Samples tested Soil samples were selected so as to provide a wide range of organic matter content, oxalate- and pyrophosphate-extractable Fe and Al, clay and other properties (Table I). In addition, a magnetite sample and a few synthetic materials were tested; the latter (Table I) were prepared as outlined by McKeague et al. (1971). They were amorphous to X-rays with the exception of C9 which contained gibbsite.
111 TABLE I Identification and some properties of the soil samples and synthetic materials studied*’ Sample No.
Subgroup*’
Hori- pH zon
csscz CSSC19 80-2747 7213 cssc4 CSSCBO 66282 66283 7044 7215 80-2748 66124 SSD190 7555 6546 80-2845 80-2746 cssc5 CSSC16 CSSC23 CSSCll 6617
SM.FHP O.FHP O.FHP O.FHP LU.HFP O.FHP O.HFP O.HFP O.HFP O.FHP O.FHP P.HP O.HP OT.HP P.HP O.HFP O.FHP LU.HFP O.GBL O.LG O.DB BR.GBL
Bhf Bhf Bhf Bhf Bf Bf Bf Bfj Bf Bf Bfj Bh Bh Bh Bh AP Ae Bt Bt Btg Bm Ah
4.7 3.5 4.3 3.8 5.4 4.8 4.1 5.1 4.4 4.1 4.9 3.9 4.1 3.5 4.1 4.7 3.1 5.8 7.1 3.8 7.2 6.3
C (X) 5.2 12.0 7.1 9.6 2.3 3.1 3.9 2.3 4.7 3.8 1.5 3.7 1.9 2.1 3.3 1.9 1.8 0.1 1.2 0.2 0.8 4.1
ct C3 C5 C8 C9
fulvic acid-Fe complex fulvic acid-AI complex hydrous ferric oxide hydrous aluminum oxide
20.9 35.1 0.0 0.0
Clay (%)
Fet*3 Fed (%) (%)
Fe, (%)
AI, (%)
AI, (%)
Fe (%P
;PJ
9 10 20 5 7 7 19 20 11 4 17 1 5 20 16 22 21 27 12 16
3.6 6.1 4.6 5.2 5.2 0.3 2.7 4.8 3.7 3.5 1.7 2.4
0.6 2.7 3.1 3.0 1.1 0.9 2.1 0.7 1.7 0.9 0.9 0.1 0.0 0.0 0.2 1.2 0.4 0.4 0.4 0.4 0.1 0.4
7.7 6.6 8.2 8.4 2.8 8.0 8.6 7.0 7.6 5.3 6.2
1.3 1.5 2.7 1.0 1.4 1.6 2.4 3.0 1.9 1.8 1.8 1.5 0.3 0.2 0.8 0.4 0.2 0.2 0.2 0.2 0.1 0.2
0.4 2.6 1.3 3.0 0.2 0.6 1.0 0.1 0.9 0.5 0.2 0.1 0.0 0.0 0.0 0.7 0.2 0.0 0.1 0.1 0.1 -
0.9 1.6 1.7 1.0 0.5 1.1 0.3 0.4 1.2 0.9 0.4 0.8 0.3 0.2 0.7 0.3 0.1 0.0 0.1 0.1 0.1 -
Al,
Al,
1.3 3.3 3.6 2.0 1.5 4.3 1.8 2.5 1.4 1.7 0.2 0.1 0.0 0.2 1.8 0.7 1.1 1.2 1.4 0.6 0.9
Fet
Fe,
Fer,
29.8
30.0
18.0
61.5
15.2
0.2
Al,
6.8
6.6
28.9
3.5
0.2
*I Methods used are from the manual on soil sampling and methods of analysis (McKeague, 1978) and are listed under materials and methods. A dash (-) indicates that data are not available. *’ Subgroup abbreviations are from Canada Soil Survey Committee (1978). SM.FHP Sombric Ferro-Humic Podzol OT-HP C&stein Humic Podzol GGBL Orthic Gray Brown Luvisol O.HFP Orthic Ferro-Humic Podzol LU.HFP Luvisolic Humo-Ferric Podzol BR.GBL Brunisolic Gray Brown Luvisol O.HFP Orthic Humo-Ferric Podzol O.LG Grthic Luvic Gleysol O.HP Orthic Humic Podzol O.DB Orthic Dark Brown P.HP Placic Humic Podzol *3The suffixes d, o and p refer to dithionite-citrate-bicarbonate-, acid-ammoniumoxalate-, and sodium-pyrophosphate-extractable, respectively; t indicates total.
112
Method of analysis EDTA (Borggaard, 1979; 1981). The extracting solution was 0.1 M EDTA, 0.9 M ammonium acetate adjusted to pH 7.5 with ammonium hydroxide; for brevity, it is referred to as EDTA. The soil : solution ratio was 1 : 50 and the soil samples were ground finer than 0.5 mm. The soil-EDTA suspensions were shaken on an end-over-end shaker at 21°C for various periods of time and centrifuged at 20,000 g for 10 min. A portion of the centrifugate was analyzed by atomic absorption spectrophotometry to determine Fe and Al. EDTA (Farmer et al., 1980). The extracting solution was 0.05 M EDTA neutralized to pH 7 with ammonium hydroxide, the soil : solution ratio was 1 : 5 and the soil samples had passed a 0.5 mm sieve; shaking times were 1, 4 and 16 h. The suspensions were centrifuged at 20,000 g for 10 min and the centrifugates analyzed for Fe and Al as described previously. Other methods. Samples were extracted overnight with 0.43 M acetic acid (ratio 1 : 40) and Fe and Al were determined (Farmer et al., 1980). Other methods from a manual (McKeague, 1978) are listed and the numbers are given: carbon, combustion, 3.611; pH, 0.01 M CaC&, 3.11; clay, pretreatments with hydrogen peroxide and dithionite-citrate-bicarbonate, pipette, 2.111; extractable Fe and Al by dithionite-citrate-bicarbonate, 3.51; acid ammonium oxalate (both 4 h, 3.52, and 2 h extractions); and sodium pyrophosphate, 3.53; total Fe and Al, acid dissolution and atomic absorption (similar to 3.71); X-ray, 5.2. RESULTS
Extraction of Fe and Al from most samples by EDTA increased at a decreasing rate with time for periods usually of 60 to 150 days depending on the sample (Tables II and III). Plots of Fe and Al extracted against log time were more or less linear for some samples from 1 to 30 or 60 days {Fig. 1). Beyond approximately 60 days, the slopes either remained similar (Al, 7213), changed to lower positive gradients (Fe, 80-2747), or approached 0 gradients (Fe, CSSC 19). For some soil samples with low amounts of Fe and Al extractable by EDTA the levels reached a plateau after a relatively short time (Fe in 7555 and SSD 190, 1 day, Table II, Al in 80-2746, 8 days, Table III). Iron extracted from C3, a Fe-fulvic acid complex, reached a plateau after 30 days, but extraction of Fe from C8, a synthetic iron oxide that was amorphous to X-rays increased beyond 30 days (Table II). Extraction of Al from C9, a synthetic aluminum oxide containing gibbsite, increased markedly beyond 90 days of equilibration (Table III). Dissolution of magnetite by EDTA was negligible, Fe in solution after 11 days shaking was only 0.02%. Compared with values for a 2 h oxalate extraction, EDTA extraction for 90 days removed 84.5% as much Fe and 86.5% as much Al for the
113
22 soil samples combined (Tables II and III). For the synthetic materials tested, EDTA (90 days) compared with oxalate (2 h) dissolved widely different amounts of Fe and Al (Tables II and III). For C8 and C9 EDTA values exceeded oxalate values markedly. Amounts of Fe and Al extracted from many samples by EDTA in 1 h were only a small fraction of those extracted by pyrophosphate (Table IV). This is clearly evident for samples of strongly developed podzolic B horizons such as CSSC 19 and 7213 with organic carbon contents in TABLE II EDTA-extractable Fe after various periods extractable Fe after 2 h equilibration*’ Sample No.
CSSCB cssc19 80-2747 7213 cssc4 CSSCBO 66282 66283 7044 7215 80-2748 66124 SSD190 7555 6546 80-2846 80-2745 csscs CSSC16 CSSC23 CSSCll 6617 Sum %
EDTA-extractable Fe (%) Days of extraction: 1 4 8 30
of
equilibration
compared
with oxalate-
Oxalate Fe 60
90
120
150
;%h)
0.05 1.60 0.56 1.04 0.06 0.25 0.15 0.02 0.13 0.08 0.06 0.04 0.01 0.01 0.01 0.07 0.14 0.03 0.03 0.04 0.01 0.07
0.12 1.96 0.84 1.43 0.18 0.42 0.31 0.07 0.37 0.20 0.20 0.04 0.01 0.01 0.01 0.09 0.37 0.05 0.08 0.09 0.02 0.08
0.21 2.22 1.09 1.74 0.35 0.55 0.48 0.12 0.61 0.30 0.32 0.04 0.01 0.01 0.01 0.10 0.47 0.07 0.11 0.14 0.03 0.11
0.30 2.36 1.29 2.26 0.52 0.69 0.70 0.22 0.84 0.50 0.52 0.08 0.01 0.01 0.01 0.10 0.61 0.11 0.16 0.23 0.04 0.14
0.40 2.48 1.70 2.41 0.72 0.90 0.99 0.41 1.26 0.64 0.73 0.08 0.01 0.01 0.01 0.13 0.72 0.18 0.24 0.33 0.07 0.20
0.45 2.54 1.76 2.50 0.72 0.93 1.03 0.42 1.29 0.75 0.74 0.09 0.01 0.01 0.02 0.15 0.87 0.20 0.26 0.34 0.08 0.21
0.47 2.54 1.83 2.54 0.74 0.92 1.07 0.45 1.34 0.76 0.73 0.09 0.01 0.01 0.02 0.16 0.91 0.21 0.28 0.37 0.08 0.22
0.49 2.54 1.86 2.62 0.78 0.93 1.10 0.48 1.37 0.76 0.73 0.09 0.01 0.01 0.02 0.15 0.91 0.21 0.29 0.37 0.08 0.22
0.55 2.59 2.48 3.05 0.93 0.90 1.44 0.52 1.66 0.84 0.83 0.10 0.01 0.01 0.02 0.21 0.85 0.22 0.27 0.35 0.08 0.28
4.44 24.5
6.95 38.2
9.09 50.0
11.70 64.05
14.62 80.5
15.37 87.5
15.75 86.7
16.02 89.3
18.19
3.24 10.1
7.2 21.2
22.6 22.9
22.4 23.0
22.9 23.2
Synthetic material c3 C8
1.15 2.67
30.0 11.5
*’ The sum of Fe extracted from the 22 soil samples by EDTA (Borggaard, 1979) after various times of equilibration is expressed as a percentage of the sum of Fe extracted by oxalate from the same samples. Values are single determinations.
114
the range of 10 to 12% (Tables IV and I). For some samples of low carbon content such a CSSC5 and CSSC23, extractable Fe and Al increased regularly from 1 to 4 to 16 h to 1 day equilibration; the data for 4 and 16 h are not reported. Acetic acid (Farmer et al., 1980) extracted less than 0.1% Fe from any sample, extractable Al ranged from less than 0.1% for several samples to 0.47% for a Bhf horizon, 80-2747. The amounts extracted are not reported in detail. TABLE
III
EDTA-extractable Al after various extractable Ai after 2 h equilibration*’ Sample No.
EDTA-extractable Days of extraction: 1 4 8
periods
of
equilibration
compared
Oxalate
Al (%) 30
60
90
120
150
0.54 1.02 1.46 0.61 0.50 0.88 0.48 0.32 1.06 0.66 0.56 0.47 0.18 0.18 0.44 0.07 0.24 0.04 0.05 0.09 0.02 0.06
0.63 1.12 1.74 0.68 0.67 0.98 0.65 0.50 1.15 0.69 0.74 0.68 0.19 0.19 0.47 0.08 0.26 0.04 0.05 0.10 0.02 0.07
0.74 1.18 1.96 0.70 0.87 1.12 0.88 0.78 1.35 0.90 1.02 0.87 0.23 0.21 0.59 0.08 0.29 0.06 0.07 0.11 0.02 0.08
0.85 1.19 2.08 0.72 1.03 1.22 1.22 1.37 1.55 1.05 1.42 0.88 0.24 0.21 0.60 0.09 0.34 0.10 0.10 0.15 0.04 0.10
0.87 1.20 2.13 0.70 1.13 1.38 1.26 1.64 1.60 1.17 1.49 0.87 0.23 0.21 0.60 0.09 0.35 0.11 0.10 0.17 0.04 0.10
0.91 1.21 2.20 0.72 1.16 1.40 1.25 1.73 1.61 1.24 1.49 0.86 0.22 0.22 0.60 0.08 0.35 0.11 0.11 0.17 0.05 0.10
0.92 1.24 2.28 0.76 1.22 1.48 1.25 1.75 1.65 1.24 1.46 0.94 0.22 0.22 0.60 0.08 0.36 0.11 0.11 0.17 0.05 0.10
1.06 1.31 2.66 0.87 1.28 1.56 1.64 1.90 1.94 1.22 1.62 1.08 0.20 0.18 0.59 0.15 0.35 0.11 0.13 0.14 0.05 0.15
Sum %
7.46 37.0
9.93 49.2
11.70 58.0
14.11 70.0
16.55 82.0
17.44 86.5
17.79 88.2
18.21 90.4
20.19
Synthetic
materials 5.2 0.06
5.3 0.06
5.3 1.76
5.6 4.31
5.7 5.22
8.05
9.57
c5 c9
5.2 0.06
oxalate-
Al
;G)
0.40 0.91 1.08 0.57 0.27 0.65 0.30 0.15 0.66 0.56 0.29 0.46 0.16 0.18 0.42 0.06 0.18 0.02 0.02 0.06 0.01 0.05
csscs cssc19 80-2747 7213 cssc4 CSSCBO 66282 66283 7044 7215 80-2748 66124 SSD190 7555 6546 86-2746 80-2845 cssc5 CSSC16 CSSC23 CSSCll 6617
with
6.6 1.3
*I The sum of Al extracted from the 22 soil sample by EDTA (Borggaard, 1979) after various times of equilibration is expressed as a percentage of the sum of Al extracted by oxalate from the same samples. Values are single determinations.
115
0.6 -
t’
-
7213,AI,
0.6-
0.4-
0.2 -
1
4
30
6
60
90
120
150
DAYS
Fig. 1. Changes in amounts of Fe and Al extracted by EDTA (Borggaard, 1979) with time of shaking (in days). DISCUSSION
The conclusion (Borggaard, 1979) that EDTA-extractable Fe in soils reaches a limit after 3 months shaking of a soil suspension is not borne out by this study. For some samples Fe and Al do reach a plateau in 3 months, but for others concentrations increase at a decreasing rate for at least 5 months. Similarly, the claim that EDTA extracts approximately the same amount of Fe in 3 months as is extracted by oxalate in 2 h (Borggaard, 1979) is shown to be incorrect both by data reported here and by his data (Borggaard, 1981). The suitability of EDTA as a reference method for estimating amorphous iron oxide (Borggaard, 1981) is not supported. Research on the adequacy of acid ammonium oxalate for differentiating amorphous from crystalline iron oxides (Schwertmann, 1964) is much more extensive than that on EDTA. Neither extractant, however, was
116
TABLE
IV
Comparison Fe and AI*’
of
Sample No.
Fe (%)
csscz cssc19 80-2747 7213 cssc4 cssc20 66282 66283 7044 7215 80-2748 66124 SSD190 7555 6546 80-2746 30-2845 cssc5 CSSC16 CSSC23 CSSCll 6617 c3 c5 C8 c9
Fe and AI extracted
in 1 h by
EDTA
with
Ai (%)
EDTA*’
Pyro
EDTA*’
Pyro
0.01
0.40 2.60 1.72 3.77 0.25 0.65 0.78 0.05 0.91 0.54 0.16 0.11 0.01 0.02 0.03 0.21 0.73 0.03 0.09 0.05 0.05 0.25
0.07 0.23 0.23 0.14 0.04 0.07 0.08 0.02 0.07 0.09 0.04 0.20 0.15 0.16 0.06 0.01 0.03 0.01 0.01 0.04 0.01 0.03
0.90 1.60 1.78 0.94 0.50 1.10 0.66 0.36 1.17 0.87 0.46 0.94 0.24 0.22 0.67 0.09 0.32 0.03 0.06 0.10 0.06 0.12
5.1
6.5
0.01
0.20
0.42 0.12 0.25 0.01 0.02 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.02 0.19
pyrophosphate-extractable
18.0
0.53
*’ Values less than 0.01% are reported as 0.01% as precision ularly for AI. *ZThe EDTA procedure was that of Farmer et al. (1980).
is poor
at that level, partic-
effective in dissolving an X-ray amorphous sample, C8 (Table II). Oxalate has the marked advantage that the extraction time required is only a few hours, but it has the disadvantage that it attacks magnetite (Baril and Bitton, 1969). Hydroxylamine hydrochloride-hydrochloric acid (NH,OH.HCl) avoids the latter problem (Ross et al., 1985). It requires only overnight equilibration and extracts a similar amount of iron from soils to that extracted by oxalate. Further testing would be required, however, before NH,OH.HCl could be accepted as a better selective extractant than oxalate of amorphous iron oxides. Estimates of organic forms of Fe and Al by a l-hour EDTA extraction
117
(Anderson et al., 1982) are shown to be far below those by pyrophosphate for most samples. The fact that very little Fe and Al is extracted by this EDTA procedure from some well-developed podzolic B horizons containing more than 3% organic C (CSSC2 and 20) indicates that it does not provide a reasonable estimate of organic forms of Fe and Al. Furthermore, even a l-day EDTA extraction was ineffective in dissolving Fe from a synthetic Fe-fulvic acid complex, C3 (Table II). Though pyrophosphate overestimates organic-complexed Fe and Al in some samples (Jeanroy and Guillet, 1981; Schuppli et al., 1983), it provides a generally more valid estimate (McKeague et al., 1971) than the EDTA procedure. Improved selective extraction procedures for forms of Fe, Al and other elements in soil will probably continue to be sought unless more sensitive and definitive physical methods are developed. Before new procedures are proposed, however, they should be subjected to thorough testing to ensure that they are better than procedures currently in use. The testing should involve the study of well characterized mineral samples and model synthetic materials including a wide range of forms, in addition to numerous soil samples having a wide range of properties. The latter procedure would have led Borggaard to somewhat different conclusions regarding EDTA as an extractant of amorphous Fe (Borggaard, 1976; 1979). A later study (Borggaard, 1981) showed that Fe extractable by EDTA (3-8 months) as compared with Fe extractable by oxalate (2 h) was considerably lower for some samples and somewhat higher for others. Testing of EDTA (1 h) as an extractant of organic-complexed Fe and Al would probably have led Farmer et al. (1980) and Anderson et al. (1982) to the conclusion apparently reached by Farmer et al. (1983). In the abstract of the latter paper, pyrophosphate-extraction is suggested as a satisfactory way of characterizing organic forms of Fe and Al in podzol B horizons. CONCLUSIONS
On the basis of results presented here and those already published, it seems that there is no sound basis for using EDTA-extraction either as a reference procedure (3-month extraction) for estimating amorphous iron oxide, or as a procedure for estimating organic forms of Fe and Al. Acid ammonium oxalate remains the best-tested method for estimating amorphous iron oxide. A 2 h extraction is probably best for that purpose (Schwertmann, 1964), but a 4 h extraction may be preferable for estimating amorphous forms of Al (Farmer et al., 1983). Pyrophosphate-extraction, with precautions to remove suspended material before analysis (Schuppli et al., 1983) remains the best method to date for estimating organic forms of Fe and Al. Both reagents have limitations as selective extractants but other reagents should be tested thoroughly before they are suggested as improvements.
118 ACKNOWLEDGEMENT
X-ray analysis knowledged.
of samples
C8 and C9 by G.C. Scott
are gratefully
ac-
REFERENCES Anderson, H.A., Berrow, M.L., Farmer, V.C., Hepburn, A., Russell, J.D. and Walker, A.D., 1982. A reassessment of podzol formation processes. J. Soil Sci., 33: 125 -136. Baril, R. and Bitton, G., 1969. Teneurs elevees de fer libre et identification taxonomique de certains sols du Quebec contenant de la magnetite. Can. J. Soil Sci., 49: 1-9. Borggaard, O.K., 1976. Selective extraction of amorphous iron oxide by EDTA from a mixture of amorphous iron oxide, goethite and hematite. J. Soil Sci., 27: 478486. Borggaard, O.K., 1979. Selective extraction of amorphous iron oxides by EDTA from a Danish sandy loam. J. Soil Sci., 30: 727-734. Borggaard, O.K., 1981. Selective extraction of amorphous iron oxides by EDTA from soils from Denmark and Tanzania. J. Soil Sci., 32: 427-432. Farmer, V.C., Russell, J.D. and Berrow, M.L., 1980. Imogolite and proto-imogolite allophane in spodic horizons: evidence for a mobile aluminum silicate complex in podzol formation. J. Soil Sci., 31: 673-684. Farmer, V.C., Russell, J.D. and Smith, B.F.L., 1983. Extraction of inorganic forms of translocated Al, Fe and Si from a podzol Bs horizon. J. Soil Sci., 34: 571-576. Jeanroy, E. and Guillet, B., 1981. The occurrence of suspended ferruginous particles in pyrophosphate extracts of some soil horizons. Geoderma, 26: 95-105. McKeague, J.A. (Editor), 1978. Manual on Soil Sampling and Methods of Analysis. Can. Sot. Soil Sci., Ottawa, 212 pp. McKeague, J.A., Brydon, J.E. and Miles, N.M., 1971. Differentiation of forms of extractable iron and aluminum in soils. Soil Sci. Sot. Am. Proc., 35: 33-38. Mehra, O.P. and Jackson, M.L., 1960. Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. 7th Natl. Conf. Clays Clay Min., pp. 317-327. Ross, G.J., Wang, C. and Schuppli, P.A., 1985. A comparison of acid hydroxylamine and acid ammonium oxalate solution as extractants of free Fe and Al from soils. Soil Sci. Sot. Am. J. In Press. Schuppli, P.A., Ross, G.J. and McKeague, J.A., 1983. The effective removal of suspended materials from pyrophosphate extracts of soils from tropical and temperate regions. Soil Sci. Sot. Am. J., 47: 1026-1032. Schwertmann, U., 1964. The differentiation of iron oxide in soils by a photochemical extraction with acid ammonium oxalate. Z. Pflanzenernahr. Dung. Bodenkd., 105: 194-201. Schwertmann, U. and Taylor, R.M., 1977. Iron oxides. In: J.B. Dixon and S.B. Weed (Editors), Minerals in Soil Environments. Soil Sci. Sot. Am., Madison, Wise., pp. 145-180.
109
AN ASSESSMENT OF EDTA AS AN EXTRACTANT OF ORGANICCOMPLEXED AND AMORPHOUS FORMS OF Fe AND Al IN SOILS*
J.A. MCKEAGUE and P.A. SCHUPPLI Land Resource
Research Institute Agriculture
(Received June 6,1984;
Canada, Ottawa, Ont., KlA OC6 (Canada)
accepted after revision December 12,1984)
ABSTRACT McKeague, J.A. and Schuppli, P.A., 1985. An assessment of EDTA as an extractant of organic-complexed and amorphous forms of Fe and Al in soils. Geoderma, 35: 109-118. EDTA solutions proposed as selective extractants of amorphous and organic forms of Fe and Al in soils and synthetic materials were tested. Extraction of Fe and Al from some soil samples continued at a decreasing rate beyond 120 days. For some soil samples and synthetic materials the amounts of Fe and Al extractable by EDTA (90 days) were markedly lower than those extractable by acid ammonium oxalate (2 h). Extraction of samples for 1 h with EDTA was shown to release amounts of Fe and Al far below those considered to be complexed with organic matter. The EDTA extraction procedures tested should not replace either acid ammonium oxalate for estimating amorphous forms of Fe and Al or pyrophosphate for estimating organic forms of Fe and Al in soils.
INTRODUCTION
Selective extraction of different forms of Fe and Al in soils continues to be a subject of interest because physical methods such as X-ray are not adequate for differentiating various amorphous forms (Schwertmann and Taylor, 1977). Extraction procedures proposed for differentiating forms such as crystalline iron oxides, amorphous iron oxides and organic-complexed iron (McKeague et al., 1971) have limitations. Dithionite-citratebicarbonate (Mehra and Jackson, 1960), for example, dissolves some Fe from fine particles of labile silicates and it fails to dissolve completely some crystalline iron oxide particles coarser than approximately 50 pm. Acid ammonium oxalate (referred to as oxalate) dissolves not only amorphous iron oxides (Schwertmann, 1964) but also magnetite (Baril and Bitton, 1969). Sodium pyrophosphate peptizes mineral particles containing Fe and Al in some soils in addition to dissolving organic complexed Fe
*Contribution
No. LRRI 84-32.
110
and Al (Jeanroy and Guillet, 1981; Schuppli et al., 1983). Improved selective extraction procedures are desirable. Among the reagents tested recently, EDTA has been proposed for selective extraction of both amorphous iron oxides (Borggaard, 1976; 1979) and organic-complexed Fe and Al (Farmer et al., 1980; Anderson et al., 1982). EDTA has the advantage over oxalate that the amount of Fe and Al extracted increases slowly with time and reaches a plateau after approximately 3 months, whereas oxalate extracts markedly increased amounts of Fe and Al with time beyond the usual 2 or 4 h shaking period (Borggaard, 1979). EDTA as an extractant of organic complexed Fe and Al might avoid the problem encountered with pyrophosphate extracts having particulate material which may remain in suspension (Schuppli et al., 1983). Borggaard’s (1976; 1979) work with a model substance containing crystalline and amorphous iron oxides and with a soil sample (Borggaard, 1976) indicated that EDTA solutions of a range of concentrations and pH extracted the same amount of Fe in 3 months as that extracted by oxalate in 2 h. EDTA extractable Fe and Al did not increase after 3 months of shaking in most cases, although there were slight increases for a few samples. Later work (Borggaard, 1981) with 17 soil samples, however, showed that for 10 of the samples, Fe extracted in 2 h by oxalate exceeded that extracted by EDTA in 3 to 8 months; for several samples, the difference exceeded 30%. On the other hand, oxalate values were less than 65% of the EDTA-Fe values for a few samples. The 1 h EDTA-extraction for organic-complexed Fe and Al was apparently based only on results showing that more Fe and Al were extracted from organic-rich upper B horizons of some podzols than from lower B horizons rich in amorphous inorganic substances (Farmer et al., 1980; Anderson et al., 1982). Further testing of EDTA as a selective extractant was necessary. The purpose of the work reported here was to determine Fe and Al extracted by EDTA solutions after various periods of equilibration of a range of soil samples, a few synthetic materials and magnetite and thus to assess the usefulness of EDTA as a selective extractant of organic-complexed and amorphous forms of Fe and Al in soils. MATERIALS
AND METHODS
Samples tested Soil samples were selected so as to provide a wide range of organic matter content, oxalate- and pyrophosphate-extractable Fe and Al, clay and other properties (Table I). In addition, a magnetite sample and a few synthetic materials were tested; the latter (Table I) were prepared as outlined by McKeague et al. (1971). They were amorphous to X-rays with the exception of C9 which contained gibbsite.
111 TABLE I Identification and some properties of the soil samples and synthetic materials studied*’ Sample No.
Subgroup*’
Hori- pH zon
csscz CSSC19 80-2747 7213 cssc4 CSSCBO 66282 66283 7044 7215 80-2748 66124 SSD190 7555 6546 80-2845 80-2746 cssc5 CSSC16 CSSC23 CSSCll 6617
SM.FHP O.FHP O.FHP O.FHP LU.HFP O.FHP O.HFP O.HFP O.HFP O.FHP O.FHP P.HP O.HP OT.HP P.HP O.HFP O.FHP LU.HFP O.GBL O.LG O.DB BR.GBL
Bhf Bhf Bhf Bhf Bf Bf Bf Bfj Bf Bf Bfj Bh Bh Bh Bh AP Ae Bt Bt Btg Bm Ah
4.7 3.5 4.3 3.8 5.4 4.8 4.1 5.1 4.4 4.1 4.9 3.9 4.1 3.5 4.1 4.7 3.1 5.8 7.1 3.8 7.2 6.3
C (X) 5.2 12.0 7.1 9.6 2.3 3.1 3.9 2.3 4.7 3.8 1.5 3.7 1.9 2.1 3.3 1.9 1.8 0.1 1.2 0.2 0.8 4.1
ct C3 C5 C8 C9
fulvic acid-Fe complex fulvic acid-AI complex hydrous ferric oxide hydrous aluminum oxide
20.9 35.1 0.0 0.0
Clay (%)
Fet*3 Fed (%) (%)
Fe, (%)
AI, (%)
AI, (%)
Fe (%P
;PJ
9 10 20 5 7 7 19 20 11 4 17 1 5 20 16 22 21 27 12 16
3.6 6.1 4.6 5.2 5.2 0.3 2.7 4.8 3.7 3.5 1.7 2.4
0.6 2.7 3.1 3.0 1.1 0.9 2.1 0.7 1.7 0.9 0.9 0.1 0.0 0.0 0.2 1.2 0.4 0.4 0.4 0.4 0.1 0.4
7.7 6.6 8.2 8.4 2.8 8.0 8.6 7.0 7.6 5.3 6.2
1.3 1.5 2.7 1.0 1.4 1.6 2.4 3.0 1.9 1.8 1.8 1.5 0.3 0.2 0.8 0.4 0.2 0.2 0.2 0.2 0.1 0.2
0.4 2.6 1.3 3.0 0.2 0.6 1.0 0.1 0.9 0.5 0.2 0.1 0.0 0.0 0.0 0.7 0.2 0.0 0.1 0.1 0.1 -
0.9 1.6 1.7 1.0 0.5 1.1 0.3 0.4 1.2 0.9 0.4 0.8 0.3 0.2 0.7 0.3 0.1 0.0 0.1 0.1 0.1 -
Al,
Al,
1.3 3.3 3.6 2.0 1.5 4.3 1.8 2.5 1.4 1.7 0.2 0.1 0.0 0.2 1.8 0.7 1.1 1.2 1.4 0.6 0.9
Fet
Fe,
Fer,
29.8
30.0
18.0
61.5
15.2
0.2
Al,
6.8
6.6
28.9
3.5
0.2
*I Methods used are from the manual on soil sampling and methods of analysis (McKeague, 1978) and are listed under materials and methods. A dash (-) indicates that data are not available. *’ Subgroup abbreviations are from Canada Soil Survey Committee (1978). SM.FHP Sombric Ferro-Humic Podzol OT-HP C&stein Humic Podzol GGBL Orthic Gray Brown Luvisol O.HFP Orthic Ferro-Humic Podzol LU.HFP Luvisolic Humo-Ferric Podzol BR.GBL Brunisolic Gray Brown Luvisol O.HFP Orthic Humo-Ferric Podzol O.LG Grthic Luvic Gleysol O.HP Orthic Humic Podzol O.DB Orthic Dark Brown P.HP Placic Humic Podzol *3The suffixes d, o and p refer to dithionite-citrate-bicarbonate-, acid-ammoniumoxalate-, and sodium-pyrophosphate-extractable, respectively; t indicates total.
112
Method of analysis EDTA (Borggaard, 1979; 1981). The extracting solution was 0.1 M EDTA, 0.9 M ammonium acetate adjusted to pH 7.5 with ammonium hydroxide; for brevity, it is referred to as EDTA. The soil : solution ratio was 1 : 50 and the soil samples were ground finer than 0.5 mm. The soil-EDTA suspensions were shaken on an end-over-end shaker at 21°C for various periods of time and centrifuged at 20,000 g for 10 min. A portion of the centrifugate was analyzed by atomic absorption spectrophotometry to determine Fe and Al. EDTA (Farmer et al., 1980). The extracting solution was 0.05 M EDTA neutralized to pH 7 with ammonium hydroxide, the soil : solution ratio was 1 : 5 and the soil samples had passed a 0.5 mm sieve; shaking times were 1, 4 and 16 h. The suspensions were centrifuged at 20,000 g for 10 min and the centrifugates analyzed for Fe and Al as described previously. Other methods. Samples were extracted overnight with 0.43 M acetic acid (ratio 1 : 40) and Fe and Al were determined (Farmer et al., 1980). Other methods from a manual (McKeague, 1978) are listed and the numbers are given: carbon, combustion, 3.611; pH, 0.01 M CaC&, 3.11; clay, pretreatments with hydrogen peroxide and dithionite-citrate-bicarbonate, pipette, 2.111; extractable Fe and Al by dithionite-citrate-bicarbonate, 3.51; acid ammonium oxalate (both 4 h, 3.52, and 2 h extractions); and sodium pyrophosphate, 3.53; total Fe and Al, acid dissolution and atomic absorption (similar to 3.71); X-ray, 5.2. RESULTS
Extraction of Fe and Al from most samples by EDTA increased at a decreasing rate with time for periods usually of 60 to 150 days depending on the sample (Tables II and III). Plots of Fe and Al extracted against log time were more or less linear for some samples from 1 to 30 or 60 days {Fig. 1). Beyond approximately 60 days, the slopes either remained similar (Al, 7213), changed to lower positive gradients (Fe, 80-2747), or approached 0 gradients (Fe, CSSC 19). For some soil samples with low amounts of Fe and Al extractable by EDTA the levels reached a plateau after a relatively short time (Fe in 7555 and SSD 190, 1 day, Table II, Al in 80-2746, 8 days, Table III). Iron extracted from C3, a Fe-fulvic acid complex, reached a plateau after 30 days, but extraction of Fe from C8, a synthetic iron oxide that was amorphous to X-rays increased beyond 30 days (Table II). Extraction of Al from C9, a synthetic aluminum oxide containing gibbsite, increased markedly beyond 90 days of equilibration (Table III). Dissolution of magnetite by EDTA was negligible, Fe in solution after 11 days shaking was only 0.02%. Compared with values for a 2 h oxalate extraction, EDTA extraction for 90 days removed 84.5% as much Fe and 86.5% as much Al for the
113
22 soil samples combined (Tables II and III). For the synthetic materials tested, EDTA (90 days) compared with oxalate (2 h) dissolved widely different amounts of Fe and Al (Tables II and III). For C8 and C9 EDTA values exceeded oxalate values markedly. Amounts of Fe and Al extracted from many samples by EDTA in 1 h were only a small fraction of those extracted by pyrophosphate (Table IV). This is clearly evident for samples of strongly developed podzolic B horizons such as CSSC 19 and 7213 with organic carbon contents in TABLE II EDTA-extractable Fe after various periods extractable Fe after 2 h equilibration*’ Sample No.
CSSCB cssc19 80-2747 7213 cssc4 CSSCBO 66282 66283 7044 7215 80-2748 66124 SSD190 7555 6546 80-2846 80-2745 csscs CSSC16 CSSC23 CSSCll 6617 Sum %
EDTA-extractable Fe (%) Days of extraction: 1 4 8 30
of
equilibration
compared
with oxalate-
Oxalate Fe 60
90
120
150
;%h)
0.05 1.60 0.56 1.04 0.06 0.25 0.15 0.02 0.13 0.08 0.06 0.04 0.01 0.01 0.01 0.07 0.14 0.03 0.03 0.04 0.01 0.07
0.12 1.96 0.84 1.43 0.18 0.42 0.31 0.07 0.37 0.20 0.20 0.04 0.01 0.01 0.01 0.09 0.37 0.05 0.08 0.09 0.02 0.08
0.21 2.22 1.09 1.74 0.35 0.55 0.48 0.12 0.61 0.30 0.32 0.04 0.01 0.01 0.01 0.10 0.47 0.07 0.11 0.14 0.03 0.11
0.30 2.36 1.29 2.26 0.52 0.69 0.70 0.22 0.84 0.50 0.52 0.08 0.01 0.01 0.01 0.10 0.61 0.11 0.16 0.23 0.04 0.14
0.40 2.48 1.70 2.41 0.72 0.90 0.99 0.41 1.26 0.64 0.73 0.08 0.01 0.01 0.01 0.13 0.72 0.18 0.24 0.33 0.07 0.20
0.45 2.54 1.76 2.50 0.72 0.93 1.03 0.42 1.29 0.75 0.74 0.09 0.01 0.01 0.02 0.15 0.87 0.20 0.26 0.34 0.08 0.21
0.47 2.54 1.83 2.54 0.74 0.92 1.07 0.45 1.34 0.76 0.73 0.09 0.01 0.01 0.02 0.16 0.91 0.21 0.28 0.37 0.08 0.22
0.49 2.54 1.86 2.62 0.78 0.93 1.10 0.48 1.37 0.76 0.73 0.09 0.01 0.01 0.02 0.15 0.91 0.21 0.29 0.37 0.08 0.22
0.55 2.59 2.48 3.05 0.93 0.90 1.44 0.52 1.66 0.84 0.83 0.10 0.01 0.01 0.02 0.21 0.85 0.22 0.27 0.35 0.08 0.28
4.44 24.5
6.95 38.2
9.09 50.0
11.70 64.05
14.62 80.5
15.37 87.5
15.75 86.7
16.02 89.3
18.19
3.24 10.1
7.2 21.2
22.6 22.9
22.4 23.0
22.9 23.2
Synthetic material c3 C8
1.15 2.67
30.0 11.5
*’ The sum of Fe extracted from the 22 soil samples by EDTA (Borggaard, 1979) after various times of equilibration is expressed as a percentage of the sum of Fe extracted by oxalate from the same samples. Values are single determinations.
114
the range of 10 to 12% (Tables IV and I). For some samples of low carbon content such a CSSC5 and CSSC23, extractable Fe and Al increased regularly from 1 to 4 to 16 h to 1 day equilibration; the data for 4 and 16 h are not reported. Acetic acid (Farmer et al., 1980) extracted less than 0.1% Fe from any sample, extractable Al ranged from less than 0.1% for several samples to 0.47% for a Bhf horizon, 80-2747. The amounts extracted are not reported in detail. TABLE
III
EDTA-extractable Al after various extractable Ai after 2 h equilibration*’ Sample No.
EDTA-extractable Days of extraction: 1 4 8
periods
of
equilibration
compared
Oxalate
Al (%) 30
60
90
120
150
0.54 1.02 1.46 0.61 0.50 0.88 0.48 0.32 1.06 0.66 0.56 0.47 0.18 0.18 0.44 0.07 0.24 0.04 0.05 0.09 0.02 0.06
0.63 1.12 1.74 0.68 0.67 0.98 0.65 0.50 1.15 0.69 0.74 0.68 0.19 0.19 0.47 0.08 0.26 0.04 0.05 0.10 0.02 0.07
0.74 1.18 1.96 0.70 0.87 1.12 0.88 0.78 1.35 0.90 1.02 0.87 0.23 0.21 0.59 0.08 0.29 0.06 0.07 0.11 0.02 0.08
0.85 1.19 2.08 0.72 1.03 1.22 1.22 1.37 1.55 1.05 1.42 0.88 0.24 0.21 0.60 0.09 0.34 0.10 0.10 0.15 0.04 0.10
0.87 1.20 2.13 0.70 1.13 1.38 1.26 1.64 1.60 1.17 1.49 0.87 0.23 0.21 0.60 0.09 0.35 0.11 0.10 0.17 0.04 0.10
0.91 1.21 2.20 0.72 1.16 1.40 1.25 1.73 1.61 1.24 1.49 0.86 0.22 0.22 0.60 0.08 0.35 0.11 0.11 0.17 0.05 0.10
0.92 1.24 2.28 0.76 1.22 1.48 1.25 1.75 1.65 1.24 1.46 0.94 0.22 0.22 0.60 0.08 0.36 0.11 0.11 0.17 0.05 0.10
1.06 1.31 2.66 0.87 1.28 1.56 1.64 1.90 1.94 1.22 1.62 1.08 0.20 0.18 0.59 0.15 0.35 0.11 0.13 0.14 0.05 0.15
Sum %
7.46 37.0
9.93 49.2
11.70 58.0
14.11 70.0
16.55 82.0
17.44 86.5
17.79 88.2
18.21 90.4
20.19
Synthetic
materials 5.2 0.06
5.3 0.06
5.3 1.76
5.6 4.31
5.7 5.22
8.05
9.57
c5 c9
5.2 0.06
oxalate-
Al
;G)
0.40 0.91 1.08 0.57 0.27 0.65 0.30 0.15 0.66 0.56 0.29 0.46 0.16 0.18 0.42 0.06 0.18 0.02 0.02 0.06 0.01 0.05
csscs cssc19 80-2747 7213 cssc4 CSSCBO 66282 66283 7044 7215 80-2748 66124 SSD190 7555 6546 86-2746 80-2845 cssc5 CSSC16 CSSC23 CSSCll 6617
with
6.6 1.3
*I The sum of Al extracted from the 22 soil sample by EDTA (Borggaard, 1979) after various times of equilibration is expressed as a percentage of the sum of Al extracted by oxalate from the same samples. Values are single determinations.
115
0.6 -
t’
-
7213,AI,
0.6-
0.4-
0.2 -
1
4
30
6
60
90
120
150
DAYS
Fig. 1. Changes in amounts of Fe and Al extracted by EDTA (Borggaard, 1979) with time of shaking (in days). DISCUSSION
The conclusion (Borggaard, 1979) that EDTA-extractable Fe in soils reaches a limit after 3 months shaking of a soil suspension is not borne out by this study. For some samples Fe and Al do reach a plateau in 3 months, but for others concentrations increase at a decreasing rate for at least 5 months. Similarly, the claim that EDTA extracts approximately the same amount of Fe in 3 months as is extracted by oxalate in 2 h (Borggaard, 1979) is shown to be incorrect both by data reported here and by his data (Borggaard, 1981). The suitability of EDTA as a reference method for estimating amorphous iron oxide (Borggaard, 1981) is not supported. Research on the adequacy of acid ammonium oxalate for differentiating amorphous from crystalline iron oxides (Schwertmann, 1964) is much more extensive than that on EDTA. Neither extractant, however, was
116
TABLE
IV
Comparison Fe and AI*’
of
Sample No.
Fe (%)
csscz cssc19 80-2747 7213 cssc4 cssc20 66282 66283 7044 7215 80-2748 66124 SSD190 7555 6546 80-2746 30-2845 cssc5 CSSC16 CSSC23 CSSCll 6617 c3 c5 C8 c9
Fe and AI extracted
in 1 h by
EDTA
with
Ai (%)
EDTA*’
Pyro
EDTA*’
Pyro
0.01
0.40 2.60 1.72 3.77 0.25 0.65 0.78 0.05 0.91 0.54 0.16 0.11 0.01 0.02 0.03 0.21 0.73 0.03 0.09 0.05 0.05 0.25
0.07 0.23 0.23 0.14 0.04 0.07 0.08 0.02 0.07 0.09 0.04 0.20 0.15 0.16 0.06 0.01 0.03 0.01 0.01 0.04 0.01 0.03
0.90 1.60 1.78 0.94 0.50 1.10 0.66 0.36 1.17 0.87 0.46 0.94 0.24 0.22 0.67 0.09 0.32 0.03 0.06 0.10 0.06 0.12
5.1
6.5
0.01
0.20
0.42 0.12 0.25 0.01 0.02 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.02 0.19
pyrophosphate-extractable
18.0
0.53
*’ Values less than 0.01% are reported as 0.01% as precision ularly for AI. *ZThe EDTA procedure was that of Farmer et al. (1980).
is poor
at that level, partic-
effective in dissolving an X-ray amorphous sample, C8 (Table II). Oxalate has the marked advantage that the extraction time required is only a few hours, but it has the disadvantage that it attacks magnetite (Baril and Bitton, 1969). Hydroxylamine hydrochloride-hydrochloric acid (NH,OH.HCl) avoids the latter problem (Ross et al., 1985). It requires only overnight equilibration and extracts a similar amount of iron from soils to that extracted by oxalate. Further testing would be required, however, before NH,OH.HCl could be accepted as a better selective extractant than oxalate of amorphous iron oxides. Estimates of organic forms of Fe and Al by a l-hour EDTA extraction
117
(Anderson et al., 1982) are shown to be far below those by pyrophosphate for most samples. The fact that very little Fe and Al is extracted by this EDTA procedure from some well-developed podzolic B horizons containing more than 3% organic C (CSSC2 and 20) indicates that it does not provide a reasonable estimate of organic forms of Fe and Al. Furthermore, even a l-day EDTA extraction was ineffective in dissolving Fe from a synthetic Fe-fulvic acid complex, C3 (Table II). Though pyrophosphate overestimates organic-complexed Fe and Al in some samples (Jeanroy and Guillet, 1981; Schuppli et al., 1983), it provides a generally more valid estimate (McKeague et al., 1971) than the EDTA procedure. Improved selective extraction procedures for forms of Fe, Al and other elements in soil will probably continue to be sought unless more sensitive and definitive physical methods are developed. Before new procedures are proposed, however, they should be subjected to thorough testing to ensure that they are better than procedures currently in use. The testing should involve the study of well characterized mineral samples and model synthetic materials including a wide range of forms, in addition to numerous soil samples having a wide range of properties. The latter procedure would have led Borggaard to somewhat different conclusions regarding EDTA as an extractant of amorphous Fe (Borggaard, 1976; 1979). A later study (Borggaard, 1981) showed that Fe extractable by EDTA (3-8 months) as compared with Fe extractable by oxalate (2 h) was considerably lower for some samples and somewhat higher for others. Testing of EDTA (1 h) as an extractant of organic-complexed Fe and Al would probably have led Farmer et al. (1980) and Anderson et al. (1982) to the conclusion apparently reached by Farmer et al. (1983). In the abstract of the latter paper, pyrophosphate-extraction is suggested as a satisfactory way of characterizing organic forms of Fe and Al in podzol B horizons. CONCLUSIONS
On the basis of results presented here and those already published, it seems that there is no sound basis for using EDTA-extraction either as a reference procedure (3-month extraction) for estimating amorphous iron oxide, or as a procedure for estimating organic forms of Fe and Al. Acid ammonium oxalate remains the best-tested method for estimating amorphous iron oxide. A 2 h extraction is probably best for that purpose (Schwertmann, 1964), but a 4 h extraction may be preferable for estimating amorphous forms of Al (Farmer et al., 1983). Pyrophosphate-extraction, with precautions to remove suspended material before analysis (Schuppli et al., 1983) remains the best method to date for estimating organic forms of Fe and Al. Both reagents have limitations as selective extractants but other reagents should be tested thoroughly before they are suggested as improvements.
118 ACKNOWLEDGEMENT
X-ray analysis knowledged.
of samples
C8 and C9 by G.C. Scott
are gratefully
ac-
REFERENCES Anderson, H.A., Berrow, M.L., Farmer, V.C., Hepburn, A., Russell, J.D. and Walker, A.D., 1982. A reassessment of podzol formation processes. J. Soil Sci., 33: 125 -136. Baril, R. and Bitton, G., 1969. Teneurs elevees de fer libre et identification taxonomique de certains sols du Quebec contenant de la magnetite. Can. J. Soil Sci., 49: 1-9. Borggaard, O.K., 1976. Selective extraction of amorphous iron oxide by EDTA from a mixture of amorphous iron oxide, goethite and hematite. J. Soil Sci., 27: 478486. Borggaard, O.K., 1979. Selective extraction of amorphous iron oxides by EDTA from a Danish sandy loam. J. Soil Sci., 30: 727-734. Borggaard, O.K., 1981. Selective extraction of amorphous iron oxides by EDTA from soils from Denmark and Tanzania. J. Soil Sci., 32: 427-432. Farmer, V.C., Russell, J.D. and Berrow, M.L., 1980. Imogolite and proto-imogolite allophane in spodic horizons: evidence for a mobile aluminum silicate complex in podzol formation. J. Soil Sci., 31: 673-684. Farmer, V.C., Russell, J.D. and Smith, B.F.L., 1983. Extraction of inorganic forms of translocated Al, Fe and Si from a podzol Bs horizon. J. Soil Sci., 34: 571-576. Jeanroy, E. and Guillet, B., 1981. The occurrence of suspended ferruginous particles in pyrophosphate extracts of some soil horizons. Geoderma, 26: 95-105. McKeague, J.A. (Editor), 1978. Manual on Soil Sampling and Methods of Analysis. Can. Sot. Soil Sci., Ottawa, 212 pp. McKeague, J.A., Brydon, J.E. and Miles, N.M., 1971. Differentiation of forms of extractable iron and aluminum in soils. Soil Sci. Sot. Am. Proc., 35: 33-38. Mehra, O.P. and Jackson, M.L., 1960. Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate. 7th Natl. Conf. Clays Clay Min., pp. 317-327. Ross, G.J., Wang, C. and Schuppli, P.A., 1985. A comparison of acid hydroxylamine and acid ammonium oxalate solution as extractants of free Fe and Al from soils. Soil Sci. Sot. Am. J. In Press. Schuppli, P.A., Ross, G.J. and McKeague, J.A., 1983. The effective removal of suspended materials from pyrophosphate extracts of soils from tropical and temperate regions. Soil Sci. Sot. Am. J., 47: 1026-1032. Schwertmann, U., 1964. The differentiation of iron oxide in soils by a photochemical extraction with acid ammonium oxalate. Z. Pflanzenernahr. Dung. Bodenkd., 105: 194-201. Schwertmann, U. and Taylor, R.M., 1977. Iron oxides. In: J.B. Dixon and S.B. Weed (Editors), Minerals in Soil Environments. Soil Sci. Sot. Am., Madison, Wise., pp. 145-180.