Mechanisms for high potassium selectivity of soils dominated by halloysite from northern California, USA

Figures & data

Table 1. Selected physical and chemical properties of soil samples.

	Particle size distribution				pH	pH				Exchangeable bases				Base
Soil sample	Coarse sand	Fine sand	Silt	Clay	(H2O)	(KCl)	T-C	T-N	CEC	Ca	Mg	K	Na	Saturation
	– – – % – – – –						– - g kg^−1 –		– – – – cmolc kg^−1 – – – –					%
LeTrab 2Bt3	4	26	37	33	5.9	4.4	3.0	0.2	24.7	3.8	0.8	4.0	0.2	36
Red Tank 2Bt3	2	12	35	51	6.2	5.1	2.0	0.2	23.7	4.8	0.6	4.7	0.1	43
Aiken Bt3	3	34	24	39	5.9	4.5	8.0	0.4	18.2	3.6	2.0	0.7	0.0	35
Aiken Bt4	3	24	22	51	5.8	4.2	5.0	0.4	12.6	1.9	2.0	0.8	0.0	37
Soil sample	FeCD	Alo	Feo	Sio	Alp	Fep
	– – – g kg^−1 – – – –
LeTrab 2Bt3	93	5.0	4.6	1.2	1.9	1.3
Red Tank 2Bt3	45	6.5	5.3	2.6	3.9	2.3
Aiken Bt3	22	4.6	3.0	1.0	1.9	0.8
Aiken Bt4	23	3.3	3.3	0.9	1.7	0.5

T-C: Total carbon; T-N: Total nitrogen; CEC: Cation exchange capacity; Fe_CD: Citrate-dithionite extractable Fe (Holmgren 1967).

Al_o, Fe_o and Si_o: Acid oxalate-extractable Al, Fe and Si (Blakemore et al. 1981).

Al_p and Fe_p: Sodium pyrophosphate- extractable Al and Fe (Blakemore et al. 1981).

Figure 1. Transmission electron micrographs of the citrate-dithionite-treated clay fractions of LeTrab 2Bt3 and Red Tank 2Bt3 samples. Reference scales are 0.5 µm.

Figure 2. X-ray diffractograms of CD-air dried and CD-reduced pressure dried clay fractions of LeTrab 2Bt3 and Red Tank 2Bt3 samples.

Table 2. Permanent and variable negative charges of clay fractions after the treatments.

	Permanent charge		Variable charge		Total charge
Sample treatment	LeTrab	Red Tank	LeTrab	Red Tank	LeTrab	Red Tank
	cmolc kg^−1
Air-dried	7.7	9.5	13.6 (5.4)*	10.4 (5.4)	21.3	19.9
Reduced pressure-dried	6.5	6.9	8.3 (5.4)	7.7 (5.4)	14.8	14.6
CD-air dried	12.6	17.1	11.6 (7.6)	12.0 (7.2)	24.2	29.1
CD-reduced pressure-dried	8.4	10.4	8.0 (7.5)	10.7 (7.2)	16.4	21.1

The values are reported based on the actual treated weight.

*Value in parentheses shows the pH value associated with the measurement.

Table 3. Average and standard deviation values of adsorbed Ca²⁺+K⁺ in K⁺-Ca²⁺ exchange equilibrium study for clay fractions of LeTrab and Red Tank soils.

		Exch. Ca^2++K^+
Sample	Treatment	cmolc kg^−1	SD
LeTrab	Air-dried	11.7	0.41
	H2O2-air dried	12.4	0.26
	Reduced pressure-dried	7.8	0.13
	CD-air-dried	17.7	0.07
	CD-reduced pressure-dried	16.4	0.14
Red Tank	Air-dried	13.3	0.20
	H2O2-air dried	13.2	0.09
	Reduced pressure-dried	8.4	0.04
	CD-air-dried	15.1	0.13
	CD-reduced pressure-dried	12.8	0.16

The values are reported based on the actual treated weight.

Figure 3. Equivalent fractions of adsorbed K⁺ (E_K) vs. equivalent fraction of solution K⁺ (Ē_K) for clay fractions with the treatments. Non-preference line (dashed line) calculated following Sposito et al. (1983).

Figure 4. The Vanselow selectivity coefficient (ln K_V) as a function of the equivalent adsorbed fraction of K⁺ (E_K) for clay fractions with the treatments.

Figure 5. Transmission electron micrographs of the citrate-dithionite-treated clay fractions of Aiken Bt3 and Aiken Bt4 soils.

Figure 6. X-ray diffractograms of CD-air dried clay fractions of Aiken Bt3 and Aiken Bt4 soils.

Table 4. Average and standard deviation values of adsorbed Ca²⁺+K⁺ in K⁺-Ca²⁺ exchange equilibrium study for fine earth fractions (<2 mm) of LeTrab, Red Tank and Aiken soils.

	Exch. Ca^2++K^+
Soil Sample	cmolc kg^−1	SD
LeTrab 2Bt3	11.4	0.27
Red Tank 2Bt3	10.6	0.12
Aiken Bt3	7.5	0.05
Aiken Bt4	7.0	0.09

Figure 7. Equivalent fractions of adsorbed K⁺ (EK) vs. equivalent fraction of solution K⁺ (Ē_K) (left) and the Vanselow selectivity coefficient (ln K_V) as a function of the equivalent adsorbed fractions of K⁺ (E_K)(right) for fine earth fractions (<2 mm). Non-preference line (dashed line) calculated following Sposito et al. (1983).

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