Assessment of potentially reactive pools of aluminum in Andisols using a five-step sequential extraction procedure

Figures & data

Table 1. Experimental conditions of the single extraction procedure

Extracting reagent	Soil : solution	Shaken time	Liquid separation	References
1 M KCl (Alk)	5 g : 30 mL	Shaken for 30 min (extraction was repeated four times)	Centrifugation at 3000 rpm for 15 min and filtration with advantec paper #6	Lin and Coleman 1960
0.5 M CuCl2 (pH 2.8) (AlCu)	5 g : 50 mL	Shaken for 5 min, equilibrated for 12 h and shaken for 30 min	Filtration with advantec paper #6	Juo and Kamprath 1979
0.1 M Na–pyrophosphate at pH 10 (Alp)	1 g : 100 mL	Shaken for 16 h	Centrifugation at 2500 rpm for 15 min and filtration with advantec paper #6	Bascomb 1968
0.2 M ammonium oxalate at pH 3 (Alo)	1 g : 50 mL	Shaken for 4 h in dark	Centrifugation at 2500 rpm for 15 min and filtration with advantec paper #6	Blackmore 1981
0.5 M NaOH (Aln)	1 g : 100 mL	Shaken for 16 h	Centrifugation at 2500 rpm for 15 min and filtration with advantec #6 paper	Hashimoto and Jackson 1960

Figure 1. Schematic diagram of the sequential extraction procedure employed. Abbreviations: CuCl₂, copper chloride; KCl, potassium chloride; Na₄P₂O₇, sodium pyrophosphate; NaOH, sodium hydroxide; NH₄-Oxalate, ammonium oxalate. s-Al_k, exchangeable aluminum (Al) sequentially extracted; s-Al_Cu, Al weakly bounded to organic matter sequentially extracted; s-Alp, Al strongly bounded to organic matter sequentially extracted; s-Al_o, Al in allophane or immoglite sequentially extracted; s-Aln, crystalline Al sequentially extracted. fr., fraction.

Table 2. Location, pH, carbon content, phosphate retention and some general properties of the studied soils

Soil	Location	pH-H2O	pH-KCl	pH-NaF	Al2O3 (%)	SiO2 (%)	C (%)	Pads (mg kg^−1)	Clay (%)
A1	Tsukuba Ibaraki	6.6	4.6	11.1	33	55	4.41	196	–
A2	Katsuma Tochigi	5.9	5.4	11.2	33	57	1.68	239	–
A3	Nisine Iwate	6.6	6.0	9.5	25	57	4.87	152	–
A4	Kuroiso Nasu	6.9	5.7	10.8	26	61	4.81	159	–
A5	Takizawa Iwate	5.8	5.0	10.9	22	44	7.57	211	–
A6	Kawatabi Miyagi	5.4	4.8	11.4	22	68	0.68	154	7.2%
N1	Kawatabi Miyagi	6.0	4.5	11.4	20	67	10.42	183	–
N2	Kawatabi Miyagi	4.6	4.1	11.4	19	71	10.52	147	–
N3	Tonomi Hyogo	4.4	3.9	11.1	21	69	12.75	187	–
N4	Towono Iwate	6.1	5.4	10.2	22	53	6.57	114	–
Soil pH was measured in water (pH-H2O), 1 M potassium chloride (pH-KCl), 1 M sodium fluoride (NaF). Aluminum oxide (Al2O3), silicon dioxide (SiO2), carbon (C), phosphorus adsorption (Pads), “—” non-analyzed.

Table 3. Aluminum (Al) concentration (in g kg⁻¹) of each fraction extracted by single extraction methods, Al_p/Al_o ratio and classification of the soils studied

Soil	Alk	AlCu	Alp	Alo	Aln	AlCu–Alk	Alp–AlCu	Alo–Alp	Aln–Alo	Alp/Alo	Classification
A1	0.013	1.8	6.9	52.9	41.1	1.7	5.1	46.0	−11.8	0.1	Allophanic
A2	0.009	4.2	4.5	64.8	49.9	4.2	0.3	60.3	−14.9	0.1	Allophanic
A3	0.015	0.4	3.2	14.5	11.0	0.4	2.8	11.3	−3.6	0.2	Allophanic
A4	0.004	1.0	5.0	25.0	14.6	1.0	4.0	20.0	−10.4	0.2	Allophanic
A5	0.006	1.8	9.9	35.9	21.2	1.8	8.1	26.0	−14.7	0.3	Allophanic
A6	0.053	1.7	3.9	17.2	15.6	1.7	2.2	13.3	−1.6	0.2	Allophanic
N1	0.168	3.6	14.9	18.2	14.7	3.5	11.2	3.3	−3.5	0.8	Non-allophanic
N2	0.455	4.1	16.9	18.2	15.1	3.6	12.8	1.3	−3.0	0.9	Non-allophanic
N3	0.554	4.0	18.0	18.1	21.4	3.5	13.9	0.1	3.3	1.0	Non-allophanic
N4	0.004	1.6	6.6	12.8	7.5	1.5	5.1	6.1	−5.3	0.5	Non-allophanic
Abbreviations: Alk, exchangeable Al; AlCu, potentially reactive non-exchangeable Al; AlP, Al in organic complexes. Alo, total non-crystalline Al; Aln, total free Al; AlCu–Alk, Al weakly bounded to organic matter; Alp–AlCu, Al strongly bounded to organic matter; Alo–Alp, Al in allophane and imogolite; Aln-Alo, crystalline Al.

Table 4. Aluminum (Al) concentration (in g kg⁻¹) of each fraction extracted by the five-step sequential procedure and the sum quantity of extractable Al (Sum-s-Al)

Soil	s-Alk	s-AlCu	s-Alp	s-Alo	s-Aln	Sum-s-Al
A1	0.049	1.7	3.5	36.3	17.6	59.1
A2	0.021	4.3	1.5	54.1	11.5	71.4
A3	0.015	0.4	2.5	10.4	11.1	24.4
A4	0.004	1.1	4.1	14.3	11.6	31.0
A5	0.006	1.8	9.5	20.4	8.3	40.1
A6	0.062	1.7	2.0	10.2	3.6	17.7
N1	0.143	2.8	10.6	3.2	2.8	19.6
N2	0.455	3.5	12.7	2.6	2.6	21.9
N3	0.554	3.5	11.2	3.1	7.2	25.5
N4	0.004	0.7	5.8	5.4	2.0	13.9
Abbreviations: s-Alk, exchangeable Al sequentially extracted; s-AlCu, Al weakly bounded to organic matter sequentially extracted; s-Alp, Al strongly bounded to organic matter sequentially extracted; s-Alo, Al in allophane or immoglite sequentially extracted; s-Aln, crystalline Al sequentially extracted.

Table 5. Relationships of extractable aluminum (Al) between single extraction methods and sequential method

Single methods	Alk	AlCu–Alk	Alp–AlCu	Alo–Alp	Aln–Alo
Sequential procedure	s-Alk	0.995***
	s-AlCu		0.938***
	s-Alp			0.934***
	s-Alo				0.985***
	s-Aln					0.318
P < 0.01, n = 10.
Abbreviations: Alk, exchangeable Al; AlCu-Alk, Al weakly bounded to organic matter; Alp-AlCu, Al strongly bounded to organic matter; Alo-Alp, Al in allophane and imogolite; Aln-Alo, crystalline Al.

Table 6. Comparison of molar ratio aluminum/silicon (Al/Si) of allophane and allophane content calculated by single extraction methods and sequential procedure

Soils	Single extraction methods			Sequential procedure
	Sio (g kg^−1)	(Alo–Alp)/Sio	Allophane (%)	s-Sio (g kg^−1)	(s-Alo)/(s-Sio)	s-Allophane (%)
A1	21.5	2.2	15.0	17.9	2.1	12.6
A2	32.9	1.9	23.0	33.9	1.7	23.7
A3	7.2	1.6	5.1	4.6	2.3	3.2
A4	8.5	2.4	6.0	5.5	2.7	3.9
A5	13.4	2.0	9.3	9.5	2.2	6.6
A6	2.7	5.1	1.9	4.4	2.4	3.1
N1	2.9	1.2	1.5	0.5	6.1	0.4
N2	2.8	0.5	1.4	0.4	6.4	0.3
N3	2.8	0.1	1.4	0.4	7.9	0.3
N4	4.8	1.3	2.4	2.0	2.9	1.4
Abbreviations: Sio, Si in allophane or immoglite; Alo-Alp, Al in allophane and imogolite; s-Sio, Si in allophane or immoglite sequentially extracted; s-Alo, Al in allophane or immoglite sequentially extracted; s-Allophane, sequentially extracted allophane.

Figure 2. X-Ray diffraction diffractograms (Cu radiation) of oriented clay (<2 µm) specimen, air dry, magnesium ion (Mg²⁺) saturated. The scans correspond to the 2θ scale. Ch., chlorite; Vt., vermiculite; Gb., gibbsite; Qz., quartz; Kt., kaolinite; Sp., serpentine; Cb., cristobolites.