3.1 Dithionite-citrate-bicarbonate (DCB) extractable Fe and Al contents of the soils
The DCB extractable Fe oxides (Fed) representing the total free Fe oxides available in the soils were higher than the DCB extractable Al oxides (Ald). The data revealed that Fed were highest in Itagunmodi series followed by Egbeda and Iwo series (Tables 2a-c). The total free Fe oxide (Fed) was lower in the surface soils (528.57-706.79 mg kg− 1) than the subsurface horizons (628.07-1603.25 mg kg− 1) in all the pedons. Specifically, Fed ranged from 530.60-594.10 mg kg− 1 and 723.49-1603.25 mg kg− 1 in the surface and subsurface soils of Itagunmodi series, respectively. Egbeda series had Fed values ranging from 528.57-541.11 mg kg− 1 and 628.07-1540.03 mg kg− 1 in the surface and subsurface soils, respectively. In Iwo series, the values varied from 620.58-706.79 mg kg− 1 in the surface soils and 684.29-985.76 mg kg− 1 in the subsurface soils.
The DCB extractable Al (Ald) varied from 142.33-174.09 mg kg− 1 within the surface soils and 136.22-287.46 mg kg− 1 in the subsurface soil of Itagunmodi series. Egbeda series had Ald values varying from 123.12-192.51 mg kg− 1 in the surface soils and 167.09-373.48 mg kg− 1 in the subsurface soils. In Iwo series, the values varied from 118.21-232.38 mg kg− 1 in the surface soils and 143.23-390.48 mg kg− 1 in the subsurface soils. The values of Ald increased regularly with soil depth in all the pedons except in Itagunmodi soil series I (Tables 2a-c). Comparison of the amount of Al oxides extracted by DCB (Ald) in different soil series indicated that the largest Al oxides were obtained from Iwo followed by Egbeda and Itagunmodi series.
Table 2
a Fe and Al oxides extracted by different extractants in Itagunmodi series
Horizon | Depth | Dithionate extractable | | Oxalate extractable | | Pyrophosphate extractable |
Fed | Ald | Feo | Alo | Fep | Alp |
| (cm) | (mg kg− 1) |
Itagunmodi soil series I | | | | | | |
A | 0–20 | 530.60 | 174.09 | | 292.46 | 60.69 | | 108.10 | 16.66 |
BA | 20–48 | 723.49 | 171.59 | | 301.09 | 38.12 | | 103.66 | 17.53 |
Bt21 | 48–84 | 912.62 | 171.74 | | 307.61 | 37.70 | | 100.87 | 15.20 |
B22 | 84–126 | 1115.11 | 164.85 | | 313.92 | 25.09 | | 94.47 | 12.36 |
B3 | 126–200 | 1407.48 | 136.22 | | 400.54 | 22.59 | | 89.68 | 10.19 |
Itagunmodi soil series II | | | | | | |
A | 0–28 | 594.10 | 161.67 | | 338.33 | 64.70 | | 178.40 | 28.59 |
AB | 28–53 | 789.94 | 177.65 | | 380.59 | 46.30 | | 167.44 | 27.99 |
Bt21 | 53–80 | 981.30 | 180.31 | | 384.33 | 48.02 | | 149.42 | 26.24 |
Bt22 | 80–142 | 1080.39 | 270.84 | | 401.47 | 69.03 | | 146.47 | 22.72 |
Bt23 | 142–200 | 1259.94 | 287.46 | | 415.75 | 58.70 | | 129.27 | 21.12 |
Itagunmodi soil series III | | | | | | | |
A | 0–17 | 576.85 | 142.33 | | 317.52 | 40.52 | | 196.23 | 27.69 |
AB | 17–47 | 841.15 | 195.80 | | 329.53 | 47.70 | | 158.73 | 28.04 |
B21 | 47–86 | 956.43 | 171.73 | | 367.71 | 57.77 | | 120.72 | 30.38 |
B22 | 86–155 | 1551.36 | 295.80 | | 448.92 | 110.45 | | 102.03 | 37.07 |
B23 | 155–220 | 1603.25 | 260.50 | | 482.70 | 38.80 | | 110.96 | 10.32 |
Table 2
b Fe and Al oxides extracted by different extractants in Egbeda series
Horizon | Depth | Dithionate extractable | | Oxalate extractable | | Pyrophosphate extractable |
Fed | Ald | Feo | Alo | Fep | Alp |
| (cm) | (mg kg− 1) |
Egbeda soil series I | | | | | | | | |
A | 0–24 | 537.89 | 192.51 | | 237.89 | 75.57 | | 24.52 | 10.39 |
AB | 24–43 | 647.75 | 213.18 | | 248.27 | 80.28 | | 34.78 | 12.66 |
Bt21 | 43–89 | 726.25 | 301.49 | | 251.30 | 98.44 | | 79.45 | 15.34 |
Bt22 | 89–130 | 1142.49 | 324.25 | | 381.16 | 156.19 | | 84.13 | 40.46 |
Bt23 | 130–200 | 1540.03 | 373.48 | | 498.55 | 188.99 | | 81.80 | 19.18 |
Egbeda soil series II | | | | | | | | |
A | 0–15 | 541.11 | 130.94 | | 276.37 | 51.47 | | 43.09 | 30.06 |
AB | 15–36 | 628.07 | 167.09 | | 301.91 | 55.07 | | 73.95 | 10.30 |
Bt21 | 36–75 | 707.99 | 171.25 | | 320.21 | 57.62 | | 75.93 | 13.17 |
Bt22 | 75–108 | 802.22 | 219.24 | | 332.54 | 72.90 | | 33.82 | 17.10 |
BC1 | 108–165 | 805.66 | 222.75 | | 374.78 | 73.64 | | 49.17 | 15.25 |
BC2 | 165–200 | 812.08 | 226.93 | | 379.62 | 116.31 | | 46.32 | 7.34 |
Egbeda soil series III | | | | | | | | |
A | 0–20 | 528.57 | 123.12 | | 245.45 | 60.74 | | 24.63 | 33.82 |
AB | 20–37 | 650.68 | 202.28 | | 279.93 | 61.68 | | 23.38 | 19.77 |
Bt21 | 37–78 | 749.78 | 222.76 | | 309.07 | 62.81 | | 29.69 | 9.96 |
Bt22 | 78–131 | 887.36 | 287.19 | | 361.71 | 63.94 | | 50.37 | 9.58 |
Bt23 | 131–200 | 871.88 | 225.64 | | 407.48 | 66.89 | | 32.08 | 10.47 |
Table 2
c Fe and Al oxides extracted by different extractants in Iwo series
Horizon | Depth | Dithionate extractable | | Oxalate extractable | | Pyrophosphate extractable |
Fed | Ald | Feo | Alo | Fepyr | Alpyr |
| (cm) | (mg kg− 1) |
Iwo soil series I | | | | | | | | |
A | 0–27 | 629.23 | 191.41 | | 294.14 | 51.42 | | 165.64 | 22.97 |
AB | 27–52 | 772.43 | 210.35 | | 336.99 | 57.02 | | 157.15 | 21.71 |
B2 | 52–81 | 820.98 | 287.23 | | 349.52 | 76.65 | | 155.59 | 14.88 |
BC1 | 81–120 | 925.23 | 266.67 | | 401.70 | 85.79 | | 134.70 | 13.33 |
BC2 | 120–200 | 830.33 | 216.56 | | 313.13 | 52.34 | | 124.49 | 9.89 |
Iwo soil series II | | | | | | | | |
A | 0–18 | 620.58 | 118.21 | | 305.89 | 35.48 | | 123.30 | 15.15 |
AB | 18–36 | 684.29 | 165.71 | | 320.40 | 48.41 | | 112.83 | 13.60 |
Bt21 | 36–75 | 790.02 | 205.77 | | 346.33 | 62.84 | | 111.67 | 11.79 |
Bt22 | 75–130 | 870.04 | 129.41 | | 401.81 | 73.90 | | 107.38 | 9.43 |
BC | 130–200 | 781.23 | 134.80 | | 227.70 | 49.81 | | 107.11 | 7.68 |
Iwo soil series III | | | | | | | | |
A | 0–20 | 706.79 | 232.38 | | 297.98 | 66.61 | | 186.13 | 49.05 |
AB | 20–43 | 813.22 | 143.23 | | 349.95 | 63.07 | | 204.27 | 21.02 |
Bt21 | 43–80 | 929.99 | 242.53 | | 386.54 | 94.37 | | 243.22 | 35.69 |
Bt22 | 80–144 | 985.76 | 390.48 | | 401.85 | 102.82 | | 157.97 | 20.90 |
BC | 144–200 | 855.23 | 149.42 | | 414.01 | 65.37 | | 272.65 | 41.00 |
3.2 Acid ammonium oxalate extractable Fe and Al contents of the soils
The values of oxalate extractable Fe oxides (Feo) which are indicative of the total amorphous Fe oxide ranged from 292.46-338.33 mg kg− 1 in the surface soils of Itagunmodi series and 301.09–482.70 mg kg− 1 in the subsurface horizons. The Feo contents varied from 237.89-276.37 mg kg− 1 and 248.27-498.55 mg kg− 1 in the surface and subsurface soils of Egbeda series, respectively. Soils of Iwo series had Feo contents varying from 294.14-305.89 mg kg− 1 in the surface soils and 227.70-414.01 mg kg− 1 in the subsurface soils. Constant increase in Feo content with increasing soil depth was observed in all the pedons (Tables 2a-c). The data shows that Fe oxides extracted by oxalate was highest in Itagunmodi series followed by Egbeda and Iwo series.
The oxalate extractable Al oxide (Alo) contents of the soils of Itagunmodi series showed a variation of 40.52–64.70 mg kg− 1 and 25.09-110.45 mg kg− 1 in the surface and subsurface soils, respectively. Soils of Egbeda series had Alo contents ranging from 51.47–75.57 mg kg− 1 in the surface soils and 55.07-188.99 mg kg− 1 in the subsurface soils. In Iwo series, Alo contents varying from 35.48–66.61 mg kg− 1 in the surface soils and 48.41-102.82 mg kg− 1 in the subsurface soils. Generally, in all the soil profiles, Alo contents were lower than Fed, Feo and Ald contents, respectively. Comparison of the amount of Al oxides extracted by oxalate (Alo) in different soil series indicated that the largest Al oxides were obtained from Iwo series followed by Egbeda and Itagunmodi series in that order (Tables 2a-c).
3.3 Pyrophosphate extractable Fe and Al contents of the soils
Pyrophosphate extractant dissolves the organically bound form of Fe and Al oxides. This fraction of Fe generally decreased with soil depth with only few exceptions. Specifically, the pyrophosphate extractable Fe oxides (Fep) ranged from 108.10-196.23 mg kg− 1 and 89.68-167.44 mg kg− 1 in the surface and subsurface soils of Itagunmodi series, respectively (Table 2a). The Fep contents varied from 24.52–43.09 mg kg− 1 in the surface and 23.38–84.13 mg kg− 1 in the subsurface soils of Egbeda series (Table 2b). In Iwo soil series, the Fep contents varied from 123.30-186.13 mg kg− 1 and 107.11-272.65 mg kg− 1 in the surface and subsurface soils, respectively (Table 2c). The pyrophosphate extractable Al oxides (Alp) followed the same trend, varying from 7.34–49.05 mg kg− 1 in all soil profiles. This fraction of Al ranged from 16.66–28.59 mg kg− 1 and 10.19–37.07 mg kg− 1 in the surface and subsurface soils of Itagunmodi series, respectively. Soils of Egbeda series had Alp contents ranging from 10.39–33.82 mg kg− 1 in the surface soils and 9.58–40.46 mg kg− 1 in the subsurface soils. Soils of Iwo series had Alp contents varying from 15.15–49.05 mg kg− 1 in the surface soils and 7.68-41.00 mg kg− 1 in the subsurface soils (Table 2c). Comparison of the amounts of Fe and Al oxides extracted by pyrophosphate (Fep and Alp) in different soil series indicated that the largest and lowest extractions were from Iwo and Egbeda series, respectively.
3.4 Active Fe and Al oxides and Clay/Fed ratio for the soils
The active Fe oxides computed as the ratio of the oxalate extractable Fe oxide to the dithionite extractable Fe oxide (Feo/Fed) and active Al oxides computed as the ratio of the oxalate extractable Al oxide to the dithionite extractable Al oxide (Alo/Ald) gives a measure of the reactivity of sesquioxides which indicates the pedogenic (age) development status of the soil. Active Fe oxides (Feo/Fed) ranged from 0.55–0.57 and 0.28–0.48 in the surface and subsurface soils of Itagunmodi, respectively (Table 3a). The Feo/Fed ratio ranged from 0.44–0.51 in the surface and 0.32–0.48 in the subsurface soils of Egbeda series (Table 3b). In Iwo soil series, the Feo/Fed ratio varied from 0.42–0.49 and 0.29–0.48 in the surface and subsurface soils, respectively. The Feo/Fed ratios decreased regularly with soil depth in all the pedons. Active Al oxides (Alo/Ald) ranged from 0.28–0.40 and 0.15–0.37 in the surface and subsurface soils of Itagunmodi, respectively. The Alo/Ald ratio ranged from 0.39–0.49 in the surface and 0.22–0.51 in the subsurface soils of Egbeda series. In Iwo soil series, the Alo/Ald ratio varied from 0.27–0.30 and 0.27–0.57 in the surface and subsurface soils, respectively (Table 3c). The pattern of distribution is similar to the active Fe.
The Clay/Fed and Clay/Ald ratios serve to indicate co-migration of Fe and Al oxides with clay. A higher ratio also indicates younger soil while a lower ratio indicates an older soil. The clay/Fed ratio of the soils of Itagunmodi series ranged from 0.45–0.66 in the surface and 0.19–0.75 in the subsurface soils. The clay/Fed ratio ranged from 0.41–0.55 in the surface and 0.31–0.70 in the subsurface soils of Egbeda series. In Iwo soil series, the clay/Fed ratio varied from 0.45–0.66 and 0.19–0.75 in the surface and subsurface soils, respectively (Table 3c). The Clay/Ald ratio ranged from 1.38–2.67 and 1.01–3.82 in the surface and subsurface soils of Itagunmodi series, respectively. Soils of Egbeda series had Clay/Ald ratios ranging from 10.39–33.82 mg kg− 1 in the surface soils and 9.58–40.46 mg kg− 1 in the subsurface soils. Soils of Iwo series had Alp contents varying from 15.15–49.05 mg kg− 1 in the surface soils and 7.68-41.00 mg kg− 1 in the subsurface soils.
3.5 Forms of Fe and Al oxides in the soils
The DCB procedure extracts both the non-crystalline and crystalline Fe and Al oxides; the extracts may also contain trace amounts of water soluble, exchangeable, and organically bound Fe and Al oxides, and a small amount of Fe and Al bearing silicates. The amorphous oxides are extracted with ammonium oxalate solution. Thus, the difference between the values obtained by the two methods represent the amount of Fe and Al oxides present in definite crystalline form. Pyrophosphate extractant dissolves the organically bound form of Fe and Al oxides. In the same vein, the difference between the values obtained by extraction using ammonium oxalate solution and pyrophosphate solution represents the amount of Fe and Al oxides present in inorganic amorphous forms (Tables 3a-c).
3.5.1 Crystalline forms of Fe and Al oxides in the soils
Crystalline forms of Fe oxides \(({\text{F}\text{e}}_{\text{d}}-{\text{F}\text{e}}_{\text{o}})\) in the soils of Itagunmodi series varied from 238.13-255.77 mg kg− 1 and 409.35-1120.55 mg kg− 1 in the surface and subsurface soils, respectively (Table 3a). For Egbeda soils, it was 264.74–300.00 mg kg− 1 in the surface soils and 326.16-1158.88 mg kg− 1 in the subsurface soils. In Iwo series, crystalline Fe oxides \(({\text{F}\text{e}}_{\text{d}}-{\text{F}\text{e}}_{\text{o}})\) ranged from 314.69-408.81 mg kg− 1 and 337.97-583.91 mg kg− 1 in the surface and subsurface soils, respectively. Crystalline forms of Fe oxides were highest in the surface soils of Iwo series, followed by Egbeda and Itagunmodi series in that order, while at the subsurface horizons, soils of Itagunmodi series recorded the highest amount of crystalline Fe oxides, followed by Egbeda and Iwo series, respectively. Itagunmodi series had crystalline forms of Al oxides \(({\text{A}\text{l}}_{\text{d}}-{\text{A}\text{l}}_{\text{o}})\) varying from 96.97–113.40 mg kg− 1 in the surface soils and 113.63-228.76 mg kg− 1 in the subsurface soils (Table 3a). For Egbeda soils, it was 62.38-116.93 mg kg− 1 in the surface soils and 112.02-223.25 mg kg− 1 in the subsurface soils. Iwo soil series had crystalline forms of Al oxides \(({\text{A}\text{l}}_{\text{d}}-{\text{A}\text{l}}_{\text{o}})\) ranging from 82.72-165.77 mg kg− 1 and 55.51-287.65 mg kg− 1 in the surface and subsurface soils, respectively.
Table 3
a Crystalline and amorphous forms of Fe and Al oxides in Itagunmodi soil series
Horizon | Depth | Crystalline | | Amorphous | | Index of degree of activation | | Measure of co-migration of clay |
Fed - Feo | Ald - Alo | Feo - Fep | Alo - Alp | Feo/Fed | Alo/Ald | | Clay/Fed | Clay/Ald |
| (cm) | (mg kg− 1) | | | | | |
Itagunmodi soil series I | | | | | | | | |
A | 0–20 | 238.13 | 113.40 | | 184.36 | 44.03 | | 0.55 | 0.35 | | 0.45 | 1.38 |
BA | 20–48 | 422.40 | 133.46 | | 197.43 | 20.60 | | 0.42 | 0.22 | | 0.75 | 3.15 |
Bt21 | 48–84 | 605.01 | 134.04 | | 206.73 | 22.50 | | 0.34 | 0.22 | | 0.61 | 3.26 |
B22 | 84–126 | 801.19 | 139.76 | | 219.45 | 12.73 | | 0.28 | 0.15 | | 0.41 | 2.79 |
B3 | 126–200 | 1006.94 | 113.63 | | 310.86 | 12.40 | | 0.28 | 0.17 | | 0.37 | 3.82 |
Itagunmodi soil series II | | | | | | | | |
A | 0–28 | 255.77 | 96.97 | | 159.93 | 36.11 | | 0.57 | 0.40 | | 0.61 | 2.60 |
AB | 28–53 | 409.35 | 131.34 | | 213.15 | 18.31 | | 0.48 | 0.26 | | 0.68 | 3.04 |
Bt21 | 53–80 | 596.97 | 132.29 | | 234.91 | 21.78 | | 0.39 | 0.27 | | 0.63 | 3.44 |
Bt22 | 80–142 | 678.92 | 201.81 | | 255.00 | 46.31 | | 0.37 | 0.25 | | 0.57 | 2.29 |
Bt23 | 142–200 | 844.19 | 228.76 | | 286.48 | 37.57 | | 0.33 | 0.20 | | 0.52 | 2.30 |
Itagunmodi soil series III | | | | | | | | |
A | 0–17 | 247.32 | 101.81 | | 121.28 | 12.82 | | 0.55 | 0.28 | | 0.66 | 2.67 |
AB | 17–47 | 523.63 | 148.10 | | 170.80 | 19.66 | | 0.39 | 0.24 | | 0.50 | 2.15 |
B21 | 47–86 | 588.72 | 113.96 | | 246.99 | 27.39 | | 0.38 | 0.34 | | 0.46 | 2.56 |
B22 | 86–155 | 1102.45 | 185.34 | | 346.88 | 73.39 | | 0.29 | 0.37 | | 0.19 | 1.01 |
B23 | 155–220 | 1120.55 | 221.70 | | 371.74 | 28.48 | | 0.30 | 0.15 | | 0.22 | 1.38 |
Table 3
b Crystalline and amorphous forms of Fe and Al oxides in Egbeda soil series
Horizon | Depth | Crystalline | | Amorphous | | Index of degree of activation | | Measure of co-migration of clay |
| | Fed - Feo | Ald - Alo | | Feo - Fep | Alo - Alp | | Feo/Fed | Alo/Ald | | Clay/Fed | Clay/Ald |
| (cm) | (mg kg− 1) | | | | | | |
Egbeda soil series I | | | | | | | |
A | 0–24 | 300.00 | 116.93 | | 213.37 | 65.18 | | 0.44 | 0.39 | | 0.41 | 1.14 |
AB | 24–43 | 399.48 | 132.91 | | 213.49 | 67.62 | | 0.38 | 0.38 | | 0.31 | 0.94 |
Bt21 | 43–89 | 474.95 | 203.05 | | 171.85 | 83.10 | | 0.35 | 0.33 | | 0.66 | 1.59 |
Bt22 | 89–130 | 1158.88 | 168.05 | | 297.03 | 115.73 | | 0.33 | 0.48 | | 0.35 | 1.23 |
Bt23 | 130–200 | 643.95 | 184.48 | | 416.75 | 169.81 | | 0.32 | 0.51 | | 0.30 | 1.23 |
Egbeda soil series II | | | | | | | | | | |
A | 0–15 | 264.74 | 79.47 | | 233.29 | 44.14 | | 0.51 | 0.39 | | 0.55 | 2.29 |
AB | 15–36 | 326.16 | 112.02 | | 227.96 | 44.77 | | 0.48 | 0.33 | | 0.51 | 1.92 |
Bt21 | 36–75 | 387.79 | 113.63 | | 244.28 | 44.45 | | 0.45 | 0.34 | | 0.65 | 2.69 |
Bt22 | 75–108 | 469.68 | 146.34 | | 298.72 | 55.80 | | 0.41 | 0.33 | | 0.70 | 2.55 |
BC1 | 108–165 | 430.88 | 149.11 | | 325.61 | 58.39 | | 0.47 | 0.33 | | 0.47 | 1.71 |
BC2 | 165–200 | 432.46 | 110.62 | | 333.30 | 86.25 | | 0.47 | 0.51 | | 0.37 | 1.32 |
Egbeda soil series III | | | | | | | | | | |
A | 0–20 | 283.12 | 62.38 | | 220.82 | 60.74 | | 0.46 | 0.49 | | 0.42 | 1.79 |
AB | 20–37 | 370.74 | 140.61 | | 256.56 | 41.90 | | 0.43 | 0.30 | | 0.34 | 1.09 |
Bt21 | 37–78 | 749.78 | 159.96 | | 279.39 | 52.84 | | 0.41 | 0.28 | | 0.64 | 2.15 |
Bt22 | 78–131 | 578.29 | 223.25 | | 311.34 | 54.36 | | 0.41 | 0.22 | | 0.61 | 1.88 |
Bt23 | 131–200 | 510.17 | 158.75 | | 375.40 | 56.42 | | 0.47 | 0.30 | | 0.60 | 2.30 |
Table 3
c Crystalline and amorphous forms of Fe and Al oxides in Iwo soil series
Horizon | Depth | Crystalline | | Amorphous | | Index of degree of activation | | Measure of co-migration of clay |
Fed - Feo | Ald - Alo | Feo - Fep | Alo - Alp | Feo/Fed | Alo/Ald | Clay/Fed | Clay/Ald |
| (cm) | (mg kg− 1) | | | | | |
Iwo soil series I | | | | | | | | | | | |
A | 0–27 | 335.08 | 139.98 | | 128.51 | 28.45 | | 0.47 | 0.27 | | 0.45 | 1.38 |
AB | 27–52 | 435.44 | 153.34 | | 179.84 | 35.31 | | 0.44 | 0.27 | | 0.75 | 3.15 |
B2 | 52–81 | 471.47 | 210.58 | | 193.93 | 61.77 | | 0.43 | 0.27 | | 0.61 | 3.26 |
BC1 | 81–120 | 523.53 | 180.88 | | 267.00 | 72.46 | | 0.43 | 0.32 | | 0.41 | 2.79 |
BC2 | 120–200 | 517.20 | 164.22 | | 188.64 | 42.45 | | 0.38 | 0.24 | | 0.37 | 3.82 |
Iwo soil series II | | | | | | | | | | | |
A | 0–18 | 314.69 | 82.72 | | 182.59 | 20.34 | | 0.49 | 0.30 | | 0.61 | 2.60 |
AB | 18–36 | 469.62 | 117.29 | | 207.57 | 34.82 | | 0.47 | 0.29 | | 0.68 | 3.04 |
Bt21 | 36–75 | 337.97 | 142.93 | | 234.66 | 51.05 | | 0.44 | 0.31 | | 0.63 | 3.44 |
Bt22 | 75–130 | 468.23 | 55.51 | | 294.43 | 64.47 | | 0.46 | 0.57 | | 0.57 | 2.29 |
BC | 130–200 | 553.53 | 84.99 | | 120.60 | 42.13 | | 0.29 | 0.37 | | 0.52 | 2.30 |
Iwo soil series III | | | | | | | | | | | |
A | 0–20 | 408.81 | 165.77 | | 111.85 | 17.56 | | 0.42 | 0.29 | | 0.66 | 2.67 |
AB | 20–43 | 463.26 | 80.16 | | 145.69 | 42.05 | | 0.43 | 0.44 | | 0.50 | 2.15 |
Bt21 | 43–80 | 543.45 | 148.17 | | 143.33 | 58.67 | | 0.42 | 0.39 | | 0.46 | 2.56 |
Bt22 | 80–144 | 583.91 | 287.65 | | 243.87 | 81.93 | | 0.41 | 0.26 | | 0.19 | 1.01 |
BC | 144–200 | 441.23 | 84.04 | | 141.35 | 24.37 | | 0.48 | 0.44 | | 0.22 | 1.38 |
3.5.2 Amorphous forms of Fe and Al oxides in the soils
Amorphous forms of Fe oxides \(({\text{F}\text{e}}_{\text{o}}-{\text{F}\text{e}}_{\text{p}})\) in the soils of Itagunmodi series varied from 121.28-184.36 mg kg− 1 in the surface soils and 170.80-371.74 mg kg− 1 in the subsoils (Table 3a). It was 213.37-233.29 mg kg− 1 in the surface soils and 171.85-416.75 mg kg− 1 in the subsoils of Egbeda series (Table 3b). Amorphous forms of Fe oxides \(({\text{F}\text{e}}_{\text{o}}-{\text{F}\text{e}}_{\text{p}})\) in soils of Iwo series ranged from 111.85-182.59 mg kg− 1 and 120.60-294.43 mg kg− 1 in the surface and subsurface, respectively (Table 3c). Amorphous forms of Fe oxides in the surface and subsurface horizons were highest in Egbeda series followed by Itagunmodi series and lowest in Iwo series. Itagunmodi series had amorphous forms of Al oxides \(({\text{A}\text{l}}_{\text{o}}-{\text{A}\text{l}}_{\text{p}})\) varying from 12.82–44.03 mg kg− 1 in the surface and 12.40-73.39 mg kg− 1 in the subsurface soils.
3.6 Relationship of the Extractable Forms of Fe and Al oxides with Soil Properties
The relationships between different extractable forms of Fe and Al oxides and some physical and chemical properties of the soils studied are presented in Table 4. In Itagunmodi soil series, OM correlated significantly positively with Fep (0.57*), and negatively with Fed (-0.83**) and Feo (-0.60*). Thus, the higher the OM content of the soils of Itagunmodi series, the higher the pyrophosphate extractable forms of Fe oxides and the lower the amount of dithionite and oxalate extractable forms of Fe oxides present in the soils and vice-versa. Available phosphorus showed significant positive correlation with Fep (0.57*) and Alp (0.57*). An increase in the available phosphorus content of the soils will result in an increase in the amount of pyrophosphate extractable forms of Fe and Al oxides and vice-versa. The pH of the soils was found to have a significant negative correlation with Fed (-0.69**), Ald (-0.77**) and Feo (-0.59*). The higher the pH of the soil, the lower the dithionite extractable forms of Fe and Al oxides and the oxalate extractable forms of Fe oxides and vice-versa.
In Egbeda soil series, relationship between clay and Fed (0.49*), Ald (0.50*), and Feo (0.540*) were significant. Thus, the higher the clay content of the soils of Egbeda series, the higher the dithionite extractable forms of Fe and Al oxides and the oxalate extractable forms of Fe oxides and vice-versa. Sand showed a significant negative correlation with Ald (-0.49*), and Feo (-0.61*). The lower the sand content, the higher the dithionite extractable forms of Al oxides and the oxalate extractable forms of Fe oxides. Organic matter showed significant negative correlation with Feo (-0.67*). The higher the OM content of the soils, the lower the oxalate extractable forms of Fe oxides.
In Iwo soil series, clay was positively and significantly correlated with Fed (0.59*), sand showed a significant negative correlation with Fed (-0.72**), Alo (-0.53*), and Feo (-0.61*) while silt was significantly correlated with Fep (0.61*). The pH was negatively and significantly correlated with Fep (-0.55*). Also, Fed showed significant negative correlation with OM (-0.64**) and available phosphorus (-0.51*).
3.7 Interrelationship Among Different Forms of Fe and Al Oxides
In the soils of Itagunmodi series, Fed was positively and significantly correlated with Ald (0.62**), and Feo (0.84**) and negatively correlated with Fep (-0.54*) (Table 5). Thus, the higher the dithionite extractable forms of Fe oxides, the higher the dithionite extractable Al oxides and oxalate extractable Fe oxides and the lower the pyrophosphate extractable forms of
Table 4
Correlation matrix for the extractable forms of Fe and Al oxides with selected soil properties
| Sand | Silt | Clay | Silt/Clay | Silt/Silt + Clay | pH(H2O) | pH(KCl) | OM | Avail. P |
Itagunmodi soil series |
Fed | -0.31 | 0.22 | 0.09 | 0.19 | 0.09 | -0.69** | -0.85** | -0.83** | -0.11 |
Ald | -0.32 | 0.26 | 0.07 | 0.28 | 0.19 | -0.77** | -0.69** | -0.41 | 0.39 |
Feo | -0.47 | 0.41 | 0.09 | 0.32 | 0.24 | -0.59* | -0.70** | -0.60* | 0.24 |
Alo | 0.07 | 0.40 | -0.29 | 0.49 | 0.46 | -0.25 | -0.23 | 0.06 | 0.50 |
Fep | -0.28 | 0.22 | 0.07 | 0.06 | 0.13 | 0.40 | 0.37 | 0.57* | 0.57* |
Alp | -0.13 | 0.38 | -0.13 | 0.32 | 0.34 | 0.08 | -0.04 | 0.20 | 0.57* |
Egbeda soil series |
Fed | -0.46 | -0.08 | 0.49* | -0.39 | -0.41 | -0.21 | -0.27 | -0.48 | -0.04 |
Ald | -0.49* | -0.03 | 0.50* | -0.33 | -0.35 | -0.27 | -0.26 | -0.36 | 0.03 |
Feo | -0.61** | 0.18 | 0.54* | -0.27 | -0.29 | -0.41 | -0.45 | -0.67** | -0.13 |
Alo | -0.15 | 0.01 | 0.14 | -0.09 | -0.12 | -0.20 | -0.34 | -0.23 | -0.16 |
Fep | -0.29 | -0.19 | 0.36 | -0.43 | -0.44 | 0.17 | -0.15 | -0.31 | -0.28 |
Alp | 0.39 | -0.47 | -0.21 | -0.25 | -0.22 | 0.35 | 0.33 | 0.24 | -0.21 |
Iwo soil series |
Fed | -0.72** | 0.44 | 0.59* | -0.20 | -0.21 | -0.34 | -0.41 | -0.64** | -0.51* |
Ald | -0.26 | 0.01 | 0.28 | -0.34 | -0.29 | -0.19 | -0.28 | 0.01 | -0.33 |
Feo | -0.48 | 0.23 | 0.43 | -0.15 | -0.20 | -0.26 | -0.34 | -0.48 | -0.03 |
Alo | -0.53* | 0.28 | 0.46 | -0.22 | -0.22 | -0.38 | -0.51* | -0.26 | -0.28 |
Fep | -0.02 | 0.61* | -0.28 | 0.64** | 0.60* | -0.55* | -0.72** | 0.10 | -0.10 |
Alp | 0.13 | 0.39 | -0.34 | 0.47 | 0.48 | -0.40 | -0.52* | 0.46 | 0.11 |
* Significant at 5% level, ** Significant at 1% level |
Fed = DCB extractable Fe oxides; Ald = DCB extractable Al oxides |
Feo = oxalate extractable Fe oxides; Alo = oxalate extractable Al oxides |
Fep = pyrophosphate extractable Fe oxides; Alp = pyrophosphate extractable Al oxides |
Fe oxides and vice-versa. Ald was positively and significantly correlated with Feo (0.71**), and Alo (0.64**). The higher the dithionite extractable forms of Al oxides, the higher the oxalate extractable forms of Fe and Al oxides and vice-versa. The Alp was significantly correlated with Alo (0.72**) and Fep (0.57*). The pyrophosphate extractable forms of Al oxides are directly proportional to the oxalate extractable forms of Al oxides and the pyrophosphate extractable forms of Fe oxides. The crystalline form of Fe oxides was significantly correlated with the crystalline Al oxides (0.67**) and amorphous form of Fe oxides (0.93**). The higher the crystalline form of Fe oxides, the higher the crystalline Al oxides and amorphous Fe oxides and vice-versa. This relationship was similar to that observed between crystalline Al oxides and amorphous Fe oxides (0.69**).
The higher the crystalline form of Fe oxides, the higher the crystalline and amorphous forms of Al oxides and vice-versa. Fed was positively correlated with Feo (0.71**), Ald (0.56*), and Alo (0.86**) in Iwo soil series (Table 5). The higher the dithionite extractable forms of Fe oxides, the higher the dithionite extractable Al oxides, and oxalate extractable Fe and Al oxides and vice-versa. The Alo was significantly correlated with Ald (0.75**) and Feo (0.71**). Thus, the oxalate extractable forms of Al oxides have a directly proportional relationship with the dithionite extractable forms of Al oxides and oxalate extractable forms of Fe oxides.
The Alp was highly significantly correlated with Fep (0.82**). The higher the pyrophosphate extractable forms of Al oxides, the higher the pyrophosphate extractable forms of Fe oxides. The amorphous form of Al oxides was significantly correlated with the crystalline Fe (0.64**) and Al oxides (0.52*). Thus, the higher the amorphous form of Al oxides, the higher the crystalline Fe and Al oxides and vice-versa.
Table 5
Correlation matrix for different forms of Fe and Al oxides
| Fed | Ald | Feo | Alo | Fep | Alp | Fed-Feo | Ald-Alo | Feo-Fep |
Itagunmodi soil series | | | | | | | |
Ald | 0.62** | | | | | | | | |
Feo | 0.84** | 0.71** | | | | | | | |
Alo | 0.14 | 0.64** | 0.35 | | | | | | |
Fep | -0.54* | -0.14 | -0.12 | 0.10 | | | | | |
Alp | -0.21 | 0.20 | 0.08 | 0.72** | 0.57* | | | | |
Fed-Feo | 0.99** | 0.58* | 0.78** | 0.10 | -0.60* | -0.26 | | | |
Ald-Alo | 0.70** | 0.92** | 0.71** | 0.30 | -0.23 | -0.11 | 0.67** | | |
Feo-Fep | 0.95** | 0.65** | 0.88** | 0.24 | -0.58* | -0.20 | 0.93** | 0.69** | |
Alo-Alp | 0.29 | 0.73** | 0.41 | 0.94** | -0.15 | 0.44 | 0.26 | 0.44 | 0.41 |
Egbeda soil series | | | | | | | |
Ald | 0.87** | | | | | | | | |
Feo | 0.89** | 0.64** | | | | | | | |
Alo | 0.86** | 0.82** | 0.64** | | | | | | |
Fep | 0.55* | 0.55* | 0.41 | 0.56* | | | | | |
Alp | 0.12 | -0.01 | -0.05 | 0.27 | 0.15 | | | | |
Fed-Feo | 0.67** | 0.71** | 0.50* | 0.62** | 0.42 | 0.33 | | | |
Ald-Alo | 0.59* | 0.85** | 0.44 | 0.39 | 0.37 | -0.26 | 0.57* | | |
Feo-Fep | 0.78** | 0.52* | 0.95** | 0.51* | 0.11 | -0.11 | 0.40 | 0.36 | |
Alo-Alp | 0.86** | 0.79** | 0.63** | 0.98** | 0.53* | 0.25 | 0.53* | 0.37 | 0.50 |
Iwo soil series | | | | | | | |
Ald | 0.56* | | | | | | | | |
Feo | 0.71** | 0.36 | | | | | | | |
Alo | 0.86** | 0.75** | 0.71** | | | | | | |
Fep | 0.26 | 0.09 | 0.41 | 0.35 | | | | | |
Alp | 0.00 | 0.13 | 0.19 | 0.25 | 0.82** | | | | |
Fed-Feo | 0.77** | 0.45 | 0.25 | 0.60* | 0.08 | -0.11 | | | |
Ald-Alo | 0.41 | 0.98** | 0.22 | 0.61* | 0.01 | 0.09 | 0.36 | | |
Feo-Fep | 0.43 | 0.25 | 0.56* | 0.35 | -0.52* | -0.57* | 0.16 | 0.20 | |
Alo-Alp | 0.81** | 0.63** | 0.55* | 0.79** | -0.19 | -0.40 | 0.64** | 0.52* | 0.69 |
* Significant at 5% level, ** Significant at 1% level |
Fed = DCB extractable Fe oxides; Ald = DCB extractable Al oxides |
Feo = oxalate extractable Fe oxides; Alo = oxalate extractable Al oxides |
Fep = pyrophosphate extractable Fe oxides; Alp = pyrophosphate extractable Al oxides |
Fed-Feo = crystalline forms of Fe oxides; Ald-Alo = crystalline forms of Al oxides |
Feo-Fep = amorphous forms of Fe oxides; Alo-Alp = amorphous forms of Al oxides |