3.1. Morphological Properties of the Soils
The soil color was dark reddish brown (5YR 3/4, dry) to (5YR 3/2, moist) at surface soil that have same hue and value with slight variation; reddish brown (2.5YR 4/3) to dark reddish brown (2.5YR 3/3) dry and dark reddish brown (2.5YR 3/4) to (2.5YR 2.5/3) moist in sub surface areas of profile 1. This was observed at foot slope areas of fallow land of the site (Table 1 and Appendix Table 3).
At back slope of cultivated land the color of the soil under profile 2 was reddish brown (5YR 4/4, dry) to dark reddish brown (5YR 3/2, moist) at surface soil (Table 1 and Appendix Table 4). In addition, dark reddish brown (2.5YR 3/4) to dark red (2.5YR 3/6) dry and dark reddish brown (2.5YR 2.5/4) to (2.5YR 2.5/3) moist in sub surface areas of the profile.
On the other hand, soil color patterns in profile 3 summit area of grazing land was brown (7.5YR 4/3, dry) to dark brown (7.5YR 3/3, moist) in the surface soil; dark reddish brown (5YR 3/3) to reddish brown (2.5YR 5/3) dry and dark reddish brown (2.5YR 3/4) to (2.5YR 2.5/4) moist in sub surface. The color pattern showed variation with depth and slope of the soil in both moist and dry conditions that could be probably due to difference in OM content of the soils and soil type of the study site. The dark brown soil color at the surface horizon of the fallow and grazing land pedons (foot slope and summit area) could be attributed to the relatively higher accumulation and decomposition of OM content of the study site. In line with this, many authors reported that the surface horizons have darker color than the corresponding subsurface horizons because of relatively higher soil OM contents (Teshome et al. 2016; Agbugba 2018).
The surface soil structure of foot slope (profile 1) was weak to moderate grade, fine and medium size and subangular structure (Table 1 and Appendix Table 3) While the soils at the back slope and summit area (profile 2 and 3), respectively were weak grade, fine/thin and fine medium size, subangular structure (Table 1and Appendix Tables 4 and 5). The existing slight variability in structure characteristics could be related to position of the profile in the landscape and horizons in the profile, and contents of OM.
In the subsoil horizons soil structure was moderate, medium, subangular blocky to moderate to strong, medium and angular blocky; grade, size and type at pedon 1; the subsoil horizons soil structure was moderate, medium, subangular blocky at pedon 2 and weak to moderate, medium, subangular to moderate, medium subangular blocky at pedon 3 (Appendix Table 3). The variations in structure among horizons might be due to vertical variability in the development of soil structure, and hence in the development of the representative soil profile. The development of blocky structure types could be related to the low level of OM, reduction in abundance of plant roots and higher clay in subsurface. The weak strength of the structure in these horizons might be due to relative high OM and low clay content of the soils. This is in agreement with the findings of Demelash (2010) who indicated that the weak strength of the structure in the horizons is suitable for agriculture.
Table 1 Some morphological characteristics of the study site soils
Horizon
|
depth (cm)
|
Color
|
structure
|
Boundary2
|
consistence
|
Dry
|
Moist
|
Grade
|
Size
|
Type
|
Dry
|
Moist
|
Wet
|
Pedon 1: Foot slope of fallow land at Kule site
|
Ap
|
0–20
|
5YR 3/4
|
5YR 3/2
|
WMM
|
FM
|
SA
|
C
|
HA
|
FR
|
SST/SPL
|
AB
|
20–45
|
2.5YR 4/3
|
2.5YR 3/4
|
MO
|
ME
|
SA
|
DS
|
SHA
|
FR
|
ST/PL
|
Bt1
|
45–100
|
2.5YR 2.5/4
|
2.5YR 2.5/4
|
MO
|
ME
|
SB
|
DS
|
SO
|
VFR
|
ST/PL
|
Bt2
|
100–200
|
2.5YR 3/3
|
2.5YR 2.5/3
|
MS
|
ME
|
AW
|
D
|
SO
|
VFR
|
ST/PL
|
Pedon 2: Back slope of cultivated land at Goro site
|
Ap
|
0–22
|
5YR 4/4
|
5YR 3/2
|
WE
|
FI
|
SA
|
D
|
HA
|
FI
|
SST/SPL
|
Bt1
|
22–25
|
2.5YR 3/4
|
2.5YR 2.5/4
|
MO
|
ME
|
SB
|
DW
|
SHA
|
FR
|
ST/PL
|
Bt2
|
25–130
|
2.5YR 3/6
|
2.5YR 3/6
|
MO
|
ME
|
SB
|
DW
|
SO
|
VFR
|
ST/PL
|
Bt3
|
130–200
|
2.5YR 3/4
|
2.5YR 2.5/3
|
MO
|
ME
|
SB
|
CS
|
SO
|
VFR
|
ST/PL
|
Pedon 3: Summit area of grazing land at Chate site
|
Ah
|
0–18
|
7.5YR 4/3
|
7.5YR 3/3
|
WE
|
FM
|
SA
|
G
|
SHA
|
FI
|
ST/PL
|
A
|
18–45
|
5YR 3/3
|
2.5YR 3/4
|
WM
|
ME
|
SA
|
G
|
SO
|
VFR
|
ST/PL
|
Bt1
|
45–98
|
2.5YR 4/4
|
2.5YR 3/4
|
MO
|
ME
|
SB
|
C
|
SO
|
VFR
|
ST/PL
|
Bt2
|
98–150
|
2.5YR 5/3
|
2.5YR 2.5/4
|
MO
|
ME
|
SB
|
C
|
SO
|
VFR
|
VST/VPL
|
*WM = Weak to moderate; M = Medium; MS = Moderate to strong; W = Wavy; WE = Weak; MO = Moderate; FM = Fine and medium; FI = Fine/thin; ME = Medium; SA=; Subangular; SB = Subangular blocky; AW = Angular blocky; C = Clear; S = Smooth; Diffuse; G = Gradual; HA = Hard; SHA = Slightly hard; SO = Soft; FI = Firm; VFR = Very friable; SST = Slightly sticky; ST = Sticky; SPL = Slightly plastic; SST = Slightly sticky; PL = Plastic; VST = Very sticky; VPL = Very plasticity |
In all profiles the soil consistence of the surface horizons varied from hard to slightly plastic in all profiles. With depth of most of the profiles, consistence of each horizon increased by one or more grades from its respective overlying horizon. As a result, soft, very friable, very sticky and very plastic consistence characteristics were common in the lower horizons (Table 1 and Appendix Tables 3 to 5). This might be due to high contents of OM, relatively low contents of clay and effects of continuous plowing of the topsoil horizons. As reported by Wakene and Heluf (2003) and Demelash (2010) the presences of high OM in the surface horizon change the nature of its consistence. In addition, this is in agreement with (Teshome et al. 2016) who conducted that the friable consistencies of the soils were workable at appropriate moisture content.
Most profile on foot slope of fallow land gentle slopping lands has clear boundaries on the surface that changed to diffuse smooth to diffuse boundary in their subsoil horizons. These situations may be the presence of similar characteristics of subsoil horizons within a profile. The existence of similar subsoil horizons within a profile could be related to high and uniform contents of clay in the horizons. The existing differences in boundary characteristics between topsoil and subsoil horizons of these profiles could reflect effects of ploughing, relatively high contents of OM and low contents of clay of the topsoil horizons. On the other hand, back slopes (profile 2) showed diffuse boundaries in the upper few horizons and diffuse wavy grading to clear smooth boundary in the underlying horizons. Therefore, the profile indicated the presence of different soil moisture parts between its evidences genetic horizons was drained (Table 1 and Appendix Table 3 to 5). In line with this, Mohammad and Solomon (2010) reported that the profile showed the presence of different soil moisture regimes between its recognized genetic horizons. The gradual boundary in the lower horizons on profile 3 indicated absence of distinct morphological differences between horizons of the soils (Table 1 and Appendix Table 5). This may reconfirm early stage of soil development of the profile in particular. This is supported by the findings of Seid et al. (2018) who showed the gradual boundaries in the lower horizons reflect absence of distinct morphological differences between the subsequent subsoil horizons.
3.2. Soil Physical Properties
3.2.1. Particle size distribution
The particle size distribution showed that the soils were generally clay in all pedon profiles except pedon 2 horizon 3; pedon 3 horizon 1 and 4 which were sandy clay, clay loam and sandy clay loam, respectively (Table 2). The textures of soils were clay throughout with clay content varying from 38–46% in the surface to 30 to 52% in the subsurface horizons. In profile 3, the particle size analysis indicated that, the proportions of clay, silt and sand showed an irregular variation with depth. In addition, the soil on summit in profile 3 was young soils of exposed bedrock or saprophyte soil horizons and depth limit was 150 cm. This might be due to the reflection of the basaltic and trachybasalt parent material on which the soils were formed that weathers to fine textured soils. This is in agreement with the findings of Alemayehu (2015) who pointed out that the geological formation of the area is Gibe and Arjo lower basalts.
The silt/clay ratio of the plough layers was found to be higher than the sub-surface soils (Table 2). The higher silt/clay ratio in the surface horizon and decrement depth wise showed that the presence of clay migration from the upper to the lower horizon which may be attributed to illuviation and pedoturbation processes and an indication of sub soil horizons were more weathered than surface horizons. This is in agreement with the findings of Alemayehu (2015) and Seid et al. (2018) who reported that the decrease in silt/clay ratio with depth.
Table 2 Particle size distribution, bulk density and total porosity of the soil profiles
Horizon
|
Depth (cm)
|
Particle Size Distribution (%)
|
TC
|
Si/C ratio
|
BD (g/cm3 )
|
TP (%)
|
|
|
Sand
|
Clay
|
Silt
|
Pedon 1: Foot slope of fallow land at Kule site
|
|
AP
|
0–20
|
42.00
|
46.00
|
12.00
|
C
|
0.26
|
1.01
|
61.91
|
AB
|
20–45
|
40.00
|
48.00
|
12.00
|
C
|
0.25
|
1.03
|
61.01
|
Bt1
|
45–100
|
44.00
|
50.00
|
6.00
|
C
|
0.12
|
1.08
|
59.40
|
Bt2
|
100–200+
|
44.00
|
50.00
|
6.00
|
C
|
0.12
|
1.10
|
58.53
|
Pedon 2: Back slope of cultivated land at Goro site
|
|
Ap
|
0–22
|
42.00
|
42.00
|
16.00
|
C
|
0.38
|
1.09
|
58.87
|
Bt1
|
22–25
|
43.00
|
51.00
|
6.00
|
C
|
0.12
|
1.10
|
58.51
|
Bt2
|
25–130
|
50.00
|
44.00
|
6.00
|
SC
|
0.14
|
1.25
|
52.83
|
Bt3
|
130–200+
|
42.00
|
52.00
|
6.00
|
C
|
0.12
|
1.28
|
51.70
|
Pedon 3: Summit area of grazing land at Chate site
|
|
Ah
|
0–18
|
42.00
|
38.00
|
20.00
|
CL
|
0.53
|
1.09
|
58.87
|
A
|
18–45
|
44.00
|
48.00
|
8.00
|
C
|
0.17
|
1.15
|
56.79
|
Bt1
|
45–98
|
42.00
|
50.00
|
8.00
|
C
|
0.16
|
1.17
|
55.88
|
Bt2
|
98–150
|
54.00
|
30.00
|
16.00
|
SCL
|
0.53
|
1.20
|
54.72
|
*BD – Bulk density; C = Clay; S = Sand; CL = Clay Loam; SCL = Sandy clay loam; Si/C = Silt clay ratio; TC –Textural class; TP – Total porosity |
3.2.2. Bulk Density and Porosity
The BD of soils showed variation among the different slopes (Table 2). Among the surface horizons, the lower BD (1.01 g cm− 3) was recorded under fallow land of FS while the higher (1.09 g cm− 3) was recorded under the BS of cultivated land and summit area of grazing land. The higher BD recorded at the surface horizon of the cultivated land field may be attributed to the intensive cultivation practices and the relatively low OM content. The highest BD of subsurface was recorded at pedon 2 of back slope cultivated land (1.28 g cm− 3) while the lowest was recorded at pedon 1 of foot slope (1.03 g cm− 3). This may be due to erosion and compaction of subsurface soils. The low BD found at bottom layers indicated that the soils were not compacted and have more porosity. This was apparently due to decreased in OM contents with depth, less aggregation, less root penetration and more compaction of the subsurface soils due to the weight of the overlying layers of soils as reported by (Abera and Kefyalew 2017).
The total porosity was relatively higher (61.91%) in the foot slope soil (Profile 1) and the lowest total porosity depth limit was recorded in back slope soil (51.70%) (Table 2). The higher values of total porosity corresponded to the higher amount of OM contents and lower BD values. Similar results reported by Abera and Kefyalew (2017) who said that the higher porosity seems lower BD for soils. In line with this, other authors reported that the total porosity of profiles varied inversely following the variation in BD values and correlation of OM (Seid et al. 2018). Thus, porosity values of the recognized pedons in the surface layers are in the acceptable range for crop production.
3.3. Soil Chemical Properties
3.3.1. Soil pH
The surface soil pH (H2O) ranged from 4.65 in the back slope soil of Profile 2 to 5.2 in the summit area soil of Profile 3 (Table 3). The soil pH value of foot slope soil of profile 1 showed an irregular increase with depth of the representative profile. The increased in soil pH values could be due to decrement of leaching. The relatively higher soil pH observed in the grazing land of summit area (profile 3) could be partly due to the presence of relatively higher total exchange bases and CEC than in the soil on the cultivated field of back slope (profile 2). This finding is in agreement with the findings of (Abera and Kefyalew 2017), who reported that use of acidifying mineral fertilizers and intensive cultivation that enhanced leaching of basic cations. Similar result was reported by Seid et al. (2018) who indicated that the decreased in soil pH values could be due to seasonal soil water saturation.
Table 3 Soil pH, exchangeable acidity, exchangeable aluminium, acid saturation and aluminium saturation
Horizon
|
Depth (cm)
|
pH (H2O)
|
Exch. Acidity
|
Exch. Al3+
|
Exch. H+
|
AS
|
Al S
|
|
|
|
|
Cmol (+)Kg− 1
|
%
|
Pedon 1: Foot slope of fallow land at Kule site
|
Ap
|
0–20
|
4.73
|
1.85
|
1.08
|
0.77
|
16.30
|
9.53
|
AB
|
20–45
|
4.56
|
3.40
|
2.80
|
0.60
|
23.69
|
19.51
|
Bt1
|
45–100
|
4.70
|
2.46
|
2.40
|
0.06
|
12.65
|
12.32
|
Bt2
|
100–200+
|
4.97
|
0.79
|
0.60
|
0.19
|
5.68
|
4.30
|
Pedon 2: Back slope of cultivated land at Goro site
|
Ap
|
0–22
|
4.65
|
1.12
|
0.90
|
0.22
|
9.68
|
7.78
|
Bt1
|
22–25
|
4.51
|
3.44
|
2.90
|
0.64
|
36.34
|
29.77
|
Bt2
|
25–130
|
4.48
|
3.54
|
3.30
|
0.24
|
36.84
|
34.34
|
Bt3
|
130–200+
|
4.47
|
4.42
|
3.80
|
0.62
|
39.43
|
33.90
|
Pedon 3: Summit area of grazing land at Chate site
|
Ah
|
0–18
|
5.20
|
0.26
|
0.00
|
0.26
|
2.73
|
0.00
|
A
|
18–45
|
5.00
|
1.14
|
1.00
|
0.14
|
7.57
|
6.62
|
Bt1
|
45–98
|
4.91
|
3.74
|
2.75
|
0.99
|
27.12
|
19.92
|
Bt2
|
98–150
|
4.85
|
5.30
|
4.58
|
0.72
|
30.18
|
26.08
|
* Exch. = exchangeable; Exch. Al3+ =Exchangeable aluminum; AS = Acid Saturation; AL S = Aluminum Saturation |
3.3.2. Exchangeable acidity, exchangeable aluminum and exchangeable hydrogen
The exchangeable acidity observed on the surface horizons of the profiles were relatively low (0.26 to 1.85 cmolckg−1) in summit area of profile 3 and foot slope of profile 1, respectively (Table 3). The highest exchangeable acidity (0.79 to 5.30 cmolckg−1) was observed at the subsurface soil of foot slope and summit area of the profiles at the study sites (Table 3). The exchangeable acidity increased with depth in profile 2 and 3 except in the foot slope profile1. This is in agreement with the findings of Seid et al. (2018) who pointed out that the decomposition of OM and removal of basic cations may be the cause of acidification.
At subsurface horizons, highest exchangeable aluminum (34.34%) was obtained in the cultivated land of BS site (Table 3). This might be due to cultivation and continuous use of inorganic fertilizers in the cultivated land coupled with leaching of bases due to high rainfall. This is in agreement with the findings of (Abera and Kefyalew 2017) who concluded that intensive cultivation and continuous use of inorganic fertilizers were the root cause of soil acidity.
3.3.3. Organic matter, total nitrogen, C: N ratio and available P
The values of soil OM in the topsoil layers varied from 3.70% in profile 1 found at the foot slope to 5.23% in profile 3 at the summit area. The OM content of the subsoil layers of the soil profiles along the toposequence ranged from 1.06% in profile 1 (foot slope) to 3.17% in profile 1 and 3 (foot slope and summit areas of the site), respectively (Table 4). This finding is in agreement with the findings of Mohammad and Solomon (2010) who reported more addition of decomposable organic materials in the surface horizons. According to the ratings given by London (1991), it was rated as high in topsoil and very low to medium in subsoil.
Total N was relatively high in the surface horizons and systematically decreased with subsurface. Total N on the surface horizons of the soils in the site were ranged from 0.19 to 0.27% from pedon 2 to 3 under back slope and summit area, respectively (Table 4). This decrement might be due to the decrement of OM. This finding is in agreement with the findings of (Alemayehu, 2015) who indicated that the low N levels related to continued nutrient mining by plant. According to the rating of (Landon, 1991) it was low at top soils.
The carbon-to-nitrogen ratio (C: N) showed an irregular variation with depth in all profiles. The highest C: N ratio (11.74) was observed under the surface horizons of profile 2 of back slope and the lowest C: N ratio (8.6) was recorded at profile 1 of foot slope of the site (Table 4). While the lowest (9.0) and highest (16.60) C: N ratios of the subsurface soil were recorded under pedon 2 of back slope (Table 4). The wide C: N ratios observed in the soils of the study sites indicated low level of mineralization of OM and low level of release of N to the soil systems. This is inline with the findings of Abera and Kefyalew (2017) who differentiate conditions of mineralization in the recognized horizons.
Table 4 Organic matter, organic carbon, total nitrogen, C:N ratio and Av .P of the soils in the study area
Horizon
|
Depth(cm)
|
OC (%)
|
OM (%)
|
TN (%)
|
C:N ratio
|
Av. P (mg kg − 1)
|
Pedon 1: Foot slope of fallow land at Kule site
|
Ap
|
0–20
|
2.15
|
3.70
|
0.25
|
8.60
|
1.59
|
AB
|
20–45
|
1.84
|
3.17
|
0.19
|
9.68
|
1.32
|
Bt1
|
45–100
|
1.69
|
2.91
|
0.15
|
11.27
|
0.26
|
Bt2
|
100–200+
|
0.61
|
1.06
|
0.06
|
10.17
|
0.15
|
Pedon 2: Back slope of cultivated land at Goro site
|
Ap
|
0–22
|
2.23
|
3.84
|
0.19
|
11.74
|
5.88
|
Bt1
|
22–25
|
1.67
|
2.88
|
0.17
|
9.82
|
0.38
|
Bt2
|
25–130
|
1.35
|
2.32
|
0.15
|
9.00
|
0.99
|
Bt3
|
130–200+
|
0.83
|
1.43
|
0.05
|
16.60
|
0.97
|
Pedon 3: Summit area of grazing land at Chate site
|
Ah
|
0–18
|
3.03
|
5.23
|
0.27
|
11.22
|
2.67
|
A
|
18–45
|
1.84
|
3.17
|
0.17
|
10.82
|
2.80
|
Bt1
|
45–98
|
1.00
|
1.73
|
0.09
|
11.11
|
1.70
|
Bt2
|
98–150
|
0.77
|
1.32
|
0.07
|
11.00
|
0.65
|
The Olsen extractable soil available P ranged in the surface horizons of profile 1 (1.59 mg kg− 1) and profile 2 (5.88 mg kg− 1) described at the foot slope and back slope of the sites under the fallow land and cultivated land of the study sites, respectively (Table 4). The reason of available P content of the top soils was greater than that of the sub-soils in all profiles, which may be due to desorption of P. In addition, results on cultivated lands of profile 2 could attribute to the relatively higher available p contents in the surface layers, which may be application of P containing fertilizer and compost or residue of inorganic fertilizers. In both of these profiles, available P decreased linearly with increasing soil depth persistently in all the landforms of the study area, because of fixation by Al+ 3. Available P declined with increasing depth that could be attributed to decrease in soil OM asserted by their positive significant correlation. This is supported by the former study conducted by Teshome et al (2016) that showed available P decreased with increasing depth. The low available P content of the soils might be fixation by Al and Fe, as it has been suggested for some other southwest highland soils of Ethiopia (Demelash, 2010). In addition, authors (Abdissa et al., 2018; Endalkachew et al., 2018; Seid et al., 2018) reported that P deficiencies in Ethiopian soils were well documented in various researches. According to the rating of Olsen extractable soil available P were low to medium in the surface horizons and low in the subsurface horizons (Olsen, 1934).
3.3.4. Exchangeable bases, cation exchange capacity and base saturation
Exchangeable sodium was trace in all the study site profiles (Table 5). This could be due to effective leaching loss of sodium because of high amount of annual rainfall (1913.8 mm) at the area (Fig. 2 and Appendix Table 1). The result obtained in the present study was in line with the opinion of (Landon, 1991; Abera and Kefyalew, 2017) who indicated that soil sodicity is not expected in soils of high rainfall areas of tropical environments.
The higher exchangeable K (0.29 cmolckg− 1) was observed in the fallow land of foot slope pedon 1 while the lowest K was recorded in back slope and summit area (0.22 cmolckg− 1). The content of exchangeable K was relatively high in the surface horizons and showed a decreasing pattern with depth of all of the profiles (Table 5). Exchangeable potassium showed some variation due to differences in slope position. This might be attributed to higher nutrient cycling in the surface layers of the soils as compared to the subsurface horizons and high intensity of weathering, intensive cultivation and use of acid forming inorganic fertilizers. In line with this, other authors reported that the K decreased with increasing depth (Abera and Kefyalew, 2017). According to the rating of (Landon, 1991) it was rated as medium in surface soil and low in subsurface soil.
The higher exchangeable Ca (4.62 cmolc kg− 1) was observed in back slope of cultivated land (pedon 2), while the lowest Ca was recorded (0.82 cmolc kg− 1) at foot slope of fallow land (pedon 1) (Table 5). The high contents of exchangeable Ca in the surface horizon of profile 2 of back slope may reveal low rates of leaching cations down ward as well as young soil formation stage of the soils. In the subsoil horizons, contents of exchangeable Ca showed an irregular increasing trend in profiles 1 and 3 (Table 5). The high levels of exchangeable Ca in subsoil horizons (Profiles 1 and 3) could be related to its imperfect drainage pattern, which restricts the downward movement of solutes as well as a Ca rich basaltic parent material. In line with this, Nahusenay et al. (2014) and Endalkachew et al. (2018) indicated that accumulation of exchangeable Ca with depth could be due to leaching from the overlying horizons. According to the rating set by (Landon, 1991) all of the soils in the study area were rated as very low to medium in surface and medium to high under subsurface soils.
The highest exchangeable Mg was observed under the surface horizons of the fallow land foot slope pedon 1 (8.39 cmolc kg− 1) and grazing land of summit area pedon 2 (5.61 cmolckg− 1) and the lowest exchangeable Mg recorded in the surface horizon of the cultivated field (Tables 4 and 5). This may be due to the higher OM content on the surface horizons of these soils. This is in agreement with the findings of different investigators (Wakene and Heluf, 2003; Abera and Kefyalew, 2017) who indicated that cultivation enhances leaching of Mg especially in acidic tropical soils. In the subsurface horizons, exchangeable Mg was also highest in the foot slope soil (Profile 1) (5.07 cmolckg− 1) to (8.12 cmolckg− 1) described at the site followed by (6.61 cmolckg− 1) grazing land of summit area profile 3. Hence, the surface and subsurface horizons of the soils under the three slopes contained optimum levels of exchangeable Mg.
Table 5. Exchangeable bases and percent base saturation of the soils in the study area
Horizon
|
Depth (cm)
|
Exchangeable bases
|
CEC Soil
|
CEC Clay
|
ECEC
|
PBS
|
EBS
|
Na
|
K
|
Mg
|
Ca
|
TEB*
|
(%)
|
…………………… (cmolc kg-1) ………………………
|
Pedon 1: Foot slope of fallow land at Kule site
|
|
Ap
|
0-20
|
Trace
|
0.29
|
8.39
|
0.82
|
9.49
|
36.41
|
79.15
|
11.34
|
26.09
|
89.79
|
AB
|
20-45
|
Trace
|
0.18
|
5.07
|
5.70
|
10.95
|
33.77
|
70.35
|
14.35
|
32.43
|
73.74
|
Bt1
|
45-100
|
Trace
|
0.10
|
7.78
|
9.14
|
17.02
|
25.85
|
51.70
|
19.48
|
65.85
|
87.64
|
Bt2
|
100-200+
|
Trace
|
0.10
|
8.12
|
4.93
|
13.15
|
20.13
|
40.26
|
13.94
|
65.34
|
95.64
|
Pedon 2: Back slope of cultivated land at Goro site
|
|
Ap
|
0-22
|
Trace
|
0.22
|
5.61
|
4.62
|
10.45
|
25.70
|
61.19
|
11.57
|
40.67
|
68.21
|
Bt1
|
22-25
|
Trace
|
0.12
|
2.60
|
3.48
|
6.20
|
20.30
|
39.80
|
9.74
|
30.54
|
49.36
|
Bt2
|
25-130
|
Trace
|
0.08
|
2.80
|
3.19
|
6.07
|
18.15
|
41.25
|
9.61
|
33.47
|
47.04
|
Bt3
|
130-200+
|
Trace
|
0.12
|
3.43
|
3.24
|
6.79
|
10.45
|
20.10
|
11.21
|
64.98
|
50.26
|
Pedon 3: Summit area of grazing land at Chate site
|
|
Ah
|
0-18
|
Trace
|
0.22
|
7.84
|
1.35
|
9.41
|
28.71
|
75.55
|
9.67
|
32.79
|
100.00
|
A
|
18-45
|
Trace
|
0.23
|
6.61
|
7.12
|
13.96
|
25.85
|
53.85
|
15.10
|
53.99
|
93.32
|
Bt1
|
45-98
|
Trace
|
0.18
|
5.51
|
4.37
|
10.06
|
30.84
|
61.68
|
13.80
|
32.63
|
78.53
|
Bt2
|
98-150
|
Trace
|
0.16
|
5.30
|
6.80
|
12.26
|
24.31
|
81.03
|
17.56
|
50.42
|
72.80
|
*TEB = Total Exchangeable Base; PBS = Percent Base Saturation; CEC= cation exchange capacity; EBS = Effective Base Saturation, ECEC= effective cation exchange capacity.
The highest values of total exchangeable bases (TEB) of subsurface soil (17.02 cmolckg-1) was recorded under foot slope Pedon 1 of fallow land, whereas the smallest value (6.07 cmolckg-1) was recorded under Pedon 2 of back slope cultivated land (Table 5). The concentrations of the sum of the TEB were higher at lower elevations in the foot slope and summit area. The possible reason for the highest concentration of the TEB at the lower elevation of the study area could be leaching, drainage and run off from the adjacent mountains. This result is inline with (Alemayehu 2015; Teshome et al. 2016) who indicated that the difference was attributed by the combined effect of slope position and land use.
There was no significant variation in the cation exchange capacity (CEC) of the soils along the toposequence in the surface and subsurface layers of the pedons. In the surface horizons, CEC varied from 25.70 cmolckg-1 in profile 2 to 36.41 cmolckg-1 in profile 1 (Table 5). In the subsurface horizons, the CEC varied from 10.45 cmolckg-1) Profile 2 to 33.77 cmolckg-1 Profile 1. This showed that soil pH and CEC were correspondent to each other. This result may argument with other scholars (Teshome et al. 2018) indicated that the soil pH and CEC have direct relationship. The low CEC of soils have also been implicated with low yield in most agricultural soils. Any intervention such as applying both manure and the required amount of fertilizer with the aim of improving the CEC of the soil is recommended (Degife et al. 2019). According to ratings by (Landon, 1991), the CEC contents of surface soil was high and subsurface varied from high to low in all profiles.
The CEC clay varied from 61.19 to 79.15 cmolckg-1 of profile 2 and 1, and 39.80 to 81.03 cmolckg-1 of profile 2 and 3 surface and subsurface, respectively (Table 5). The decline in total CEC or CEC/clay with depth of profiles reflects the role of OM in influencing the CEC of a soil. This is supported by the significant and strong positive relationship (r =0.47) between CEC clay and OM (Table 10). Profile 1 and 3 exhibited higher CEC than the profile 2 of back slope cultivated field, which might be due to the greater OM content of the profile (Table 4). This is along the findings of Seid et al. (2018) who found mixed clay mineralogy as broad range of CEC-clay values.
The effective cation exchange capacity (ECEC) soils of all the study sites recorded in an irregular increase with depth. The lowest and highest ECEC 9.67 cmolckg-1 to 11.57 cmolckg-1 of soil were recorded in the surface layers of grazing land of summit area and cultivated land of back slope, respectively (Table 5). Under subsurface horizon the lowest and highest ECEC 9.61 cmolckg-1 soil to 19.48 cmolckg-1 soil back slope and foot slope were recorded, respectively (Table 5). The ECEC of the soils showed incremental change parallel to the pH of the soil. This indicated that the pH of profile 1 were increased with depth as well as ECEC also increased. This is in agreement with the findings of Abdissa et al. (2018) who stated that ECEC increased with increasing pH of soils.
The percentage base saturation (PBS) under surface horizons ranged from 26.09 to 40.67% at foot slope of pedon 1 and back slope of pedon 2, respectively and under subsurface horizons 30.54 to 65.85% at back slope of pedon 2 and foot slope of pedon 1, respectively (Table 5). The PBS was generally showed an unsystematic increase with depth of almost two profiles except profile 1 has slight increase. The surface horizons showed lower PBS as compared to their respective subsurface horizons in almost all the profiles of the soils. This is indicated by Mohammad and Solomon (2010) PBS low under acidic soil due to leaching.
3.4. Soil Classification
The soils in the study sites were identified and classified in to three major soil units (Luvic Nitisol, Haplic Acrisol and Haplic Cambisol) (Table 6 and Fig. 4).
Table 6. Summarized soil classification of RSG of soil types of the study area according to WRB 2014
Table 6 Summarized soil classification of RSG of soil types of the study area according to WRB, 2014
Pedon
|
Diagnostic horizon
|
According to WRB field classification (WRB,2014)
|
Classification code
|
Soil type (RSG) according to (WRB, 2014)
|
Surface
|
Subsurface
|
WRB/FAO (UNESCO)
|
USDA classification
|
1
|
Umbric
|
Nitic
|
Luvic Nitisol (Eutric, Humic)
|
lv, NT (eu, hu)
|
Nitisol
|
Alfisol
|
2
|
Umbric
|
Argic
|
Haplic Acrisol (Hyperdystric,Clayic)
|
ha, AC ( jd, ce)
|
Acrisol
|
Ultisol
|
3
|
Umbric
|
Cambic
|
Haplic Cambisol (Eutric, Clayic)
|
ha, CM (eu, ce)
|
Cambisol
|
Inceptisol
|