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Sky Journal of Soil Science and Environmental Management Vol. 3(8), pp. 083 - 090, October, 2014
Available online http://www.skyjournals.org/SJSSEM
ISSN 2315-8794© 2014 Sky Journals
Full Length Research Paper
Relative erodibilities of some soils from Anambra basin
A. C. C. Ezeabasili1*, B. U. Okoro2 and E. J. Emengini3
1
School of Built Environment, University of Salford, Manchester, England, UK.
Department of Civil Engineering, Nnamdi Azikiwe University Awka, Nigeria.
3
Department of Surveying and Geoinformatics, Nnamdi Azikiwe University, Awka, Nigeria.
2
Accepted 9 September, 2014
Some properties of soils relevant to their erodibilities was studied in twelve locations where severe gully
erosion were observed. These properties were used in the calculation of some indices of erodibility such as
clay ratio, and dispersion ratio. Dispersion ratios ranged from 0.16 to 0.18 while the remaining index, clay ratio,
ranged from 0.21 to 24.64. Wischmeier’s erodibility equation was also applied to calculate soil erodibilities from
these properties. Low soil erodibility factors K, were obtained generally and ranged from 0.05 to 0.15. The
indices are major factors in predicting soil erosion and in land use planning and are being introduced in the
area.
Key words: Soil erosion, soil erodibility, erodibility indices, Anambra.
INTRODUCTION
Soil erodibility is the susceptibility of soil to erosion and it
depends on various soil properties such as textures, soil
aggregation, shear strength, infiltration capacity,
permeability, organic content, chemical content, soil
profile, surface stoniness, detaching/ transportation force,
etc. Soil erodibility can be determined by using simple,
measurable, independent variables based on soil
characteristics of high correlation to erodibility
(Wischmeier et al., 1976). Deivid et al. (2003) noted that
soils erosion amounting to reduction of soil quality and
fertility among other problems are linked to agents of
denudation. Water erosion is a complicated process
involving impacts from precipitation, detachment and
subsequent transportation of the detached soil. Water
erosion generally depends on erodibility of soil surface
and deposition, and the kinetic energy of water flow over
land surface. This implies that soils prone to detachment
and overland transportation can be eroded with ease.
Important soil properties and their corresponding qualities
with respect to erodibility have been outlined by Edward
(1961) as follows; texture: permeability, nature of clay:
*Corresponding author. E-mail: [email protected].
infiltration, consistency: water holding capacity, structure
of soil: detachability, coarse fragment (organic matter
content): ease with which particles are moved, thickness
of significant layers (degree of consolidation/ cementation
of soil particles impacts on erodibility status of soil, loose
particle aggregate will be prone to detachment and
transportation by agents of denudation): ease with which
excess water can leave the soil. These factors influence
soil susceptibility to erosion.
Study conducted by Salako et al. (1999) showed that
soil varied with season for the coarse – textured soils of
south-western Nigeria. The maximum wet density (MWD)
was higher in the dry season (January) than the wet
season (July and September) due to closer association of
the soil particles in the dry season and their dispersion in
the wet seasons had similar MWD to both seasons.
Salako (2003) in his work observed that researches
conducted in Nigeria (Aina, 1980; Obi and Salako, 1989),
agreed that rainfall erosivity in the tropics, can be linked
to intensity, amount and sizes of drop.
Bryan (1969) reviewed the use of indices of soil
erodibility stating their limitations. In 1968, he showed
that more indices fail to predict erodibility of soil with
marked differences from the ones for which the index
was developed. This conclusion among others was that
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Sky. J. Soil. Sci. Environ. Manage.
percentage weight of water stable aggregate greater than
3 mm is reliable as universal indicator for predicting
erodibility. However, he noted that since these
aggregates can also be eroded no standard diameter for
water stable aggregate should be devised. It was noted
that the dispersion ratio is based on the assumption that
only dispersed materials are eroded. No provision was
made for contribution of high velocity raindrops to
dispersal of stable materials. He further suggested a
minimum of 10% clay for any meaningful interpretation
and in laboratory tests found the index to be about
equivalent to the dispersion ratio. Bouyoucos (1935),
considered clay ratio as being a measure of the binding
ability of clay fraction to form soil aggregates (Equation
3). The advantage lies in the fact that it requires only
basic texture data but a setback this index has is that, the
ratio may become meaningless in soils with low clay
content. Another major drawback of the Bouyoucos ratio
is that it does not take into account some of the most
important factors affecting soil erodibility, especially
organic matter. Wischmeier and Mannering (1969) found
that soil loss on field plots is dependent on the inverse of
the clay ratio. Middleton (1930) also considered the
colloid content/moisture equivalent to be an index of
water transmissibility. The infiltration capacity of a soil is
considered in expressing its tendency towards water
erosion since erosion is caused by runoff. The value
obtained when the ratio was divided by dispersion ratio is
called erosion ratio.
Erodibility (K) as defined in the universal soil loss
equation is computed by the ratio of annual soil loss in
tons per acre to El30 computed in imperial units. The soil
loss is monitored on a unit plot that is 22 m long, on a 9%
slope in continuous fallow and is tilled up and down the
slope (Wischmeier and Smith, 1978). Continuous fallows
mean land, which has been tilled and kept free of
vegetation for at least two (2) consecutive years. During
the period of soil loss measurement, the plot is ploughed
and placed in conventional corn seeded condition each
spring and tilted as needed to prevent vegetative growth
and severe surface crusting. When all these conditions
are met then slope length (L), slope gradient (S), crop
factor (C), and conservation practices (P) each equal
unity and
Direct measurement of erodibility (K) as described above
represents the combined effects of all soil properties that
significantly influence the ease with which a particular soil
is eroded by rainfall and runoff, if not protected. Because
of the high cost of field installations and time involved,
direct measurements of erodibility have been made on
only a few bench-mark soils. Wischmeier and Mannering
(1969) proposed an erodibility equation utilizing 15 soil
properties and their interactions (Equation 4). In
subsequent studies the soil properties were reduced to
four, namely texture, organic matter, structure and
permeability. Furthermore, Wischmeier and Smith (1978)
found that very fine sand (VFS) is comparable in
erodibility to silt sized particles. Hence, (VFS) was
transferred to silt fractions to describe a particle size
parameter designated ‘M’). The United State Department
of Agriculture (USDA) erodibility monograph solves this
equation when appropriate data are entered in a proper
sequence. However, Wischmeier et al. (1971), Obi et al.
(1989), Vaneslande et al. (1984), noted that the
monograph is mainly adapted to soils from the temperate
region and has been found to be of limited application in
tropical soils.
MATERIALS AND METHODS
The study area
o
1
o
1
Where;
The study area lies between 5 40 N Lat. 6 35 E and
o
1
o
1
7 05 N Lat. 8 30 E. The vegetation of the area consists
mainly of derived savanna, mostly grasses. Sparse
vegetations with mainly shrubs were found in the
southern areas because of human influence mainly
agricultural. The vegetation and trees originally covering
the soils were either cut down or burnt. In terms of
climate, the area lies in a transition belt between the
humid coastal region and the drier interior area of the
country. A marked dry season (December – March), long
rainy season (April to November) with double maxima
generally in June and September and a high annual
rainfall which averages 2000 – 2500 mm throughout the
state is normally recorded. The intensity and duration of
rainfall here is very significant. As rainfall intensifies, it
takes the form of short violet storms; although rains may
continue for many hours, it is observed that the intensity
for the first one hour or there about is normally very high.
Also early rains which fall in February – March – April –
May have been found to be damaging.
K=
Site selection and sample collection
K=
A = Soil loss in t/acre
El =
(1)
Twelve sites showing moderate to catastrophic erosion
problems were selected for field and laboratory analysis.
Samples were taken from the twelve area and at three
different depths. These areas include; Ngwo, Ekwegbe,
Ezeabasili et al.
Ebe, Udi, Awka, Nsukka, Nnaka, Agulu, ababete, Ideani,
Ihiala and Nnobi. The depths are 0 – 30 cm, 30 – 60 cm
and 60 – 90 cm respectively.
Laboratory analysis of soil samples
Laboratory analysis of soil samples were conducted in
the Agricultural Laboratory of the Nnamdi Azikiwe
University, Awka, Anambra State, Nigeria. Standard
laboratory procedures were followed throughout the
research period.
Indices of Erodibility: Erodibility indices were obtained
using equation (2) – (4).
i.
Dispersion ratio =
(2)
Where D, depict the dispersed silt + clay after 1kg of
oven dry soil in a litre of distilled water is shaken 20
times; T, is total silt + clay determined by the standard
sedimentation method in non dispersed state. This index
has been shown to be accurate only for soils high in silt
and clay and hence does not reflect accurately the
erodibility of soils with a high sand content. This ratio also
indicates a sharp boundary between erodible and non
erodible soils, since dispersion ratio values greater than
10 indicates erodible soils and values less than 10
indicate non-erodible soils.
ii.
Clay ratio =
iii. M = (% Silt + VFS) (100 - % clay)
(3)
(4)
When the silt fraction does not exceed 70%, it was
described by the following equation;
K=M
The prediction accuracy, however, was improved by
including information on organic (matter, soil structure
and permeability as expressed below:
Where;
K = erodibility
M = (% Silt — VFS) (100- % Clay)
A = % organic matter content
B = Soil structured code
C = permeability class.
Statistical analysis of erodibility indices
Descriptive analysis was carried out on Silt/Clay ratio,
clay ratio, dispersion ratio and the erodibility factor to
ascertain variation in sampled soils.
85
RESULTS AND DISCUSSIONS
The results of mechanical analysis in all soils studied
show a high dominance of sand fraction in the top layers
of all the soils tested. In almost all cases percentage
sand decreased with depth. This was not the case with
clay which in almost all the sites, increased with depth.
This might be attributed to clay eluviations from the
surface horizon.
On the other hand, silt content was almost evenly
distributed in two lower layers. Higher sand content in the
top layer is associated with the preferential removal of
clay and silt by erosion. Also transportability of sand
fractions is lower compared to liner soil fractions. Keniper
and Noonan (1970) concluded from the laboratory soil
erosion study that sandy soils with contents greater than
80%, if subjected to raindrop action have high infiltration
rates and low runoff. Most soils studied fall under this
description.
Silt/clay ratio
The variations of the silt/clay ratio are shown (Figures 1 2) and the degree of weathering as reflected by this
variation shows that in most cases, the values decreased
in depth with little variation. This may be as a result of
clay eluviations from the top soil. The maximum value of
silt/clay ratio was observed as 4.5 in Ihiala top soil
(Figure 3) and the minimum was 0.10 at a depth of 30 –
60 cm in Nsukka (Figure 3).
The decrease in silt/clay ratio with depth and the
variation in all the twelve sites in question is in close
agreement with an earlier study by Lal (2000) that
silt/clay ratio for a majority of tropical soils decline with
depth and that the ratio may range widely from soil to soil
even within the same toposequence. Changes in the
silt/clay ration with depth were the least in the soil of
Awka indicating that the soil was perhaps at a more
advanced stage of weathering than others. However, it
was noticed that silt/clay ratio were high for soils of Ihiala,
Ebe, Nnobi at the uppermost layers and for Nanka
between 30 – 60 cm (Figures 1 and 3). This might be
attributed to transportation of silt and clay particles down
the profile by percolating water. Percentage clay was also
found to be an appreciably lower than that of silt. This
index had apparently failed to allow any important
deductions on its relationship to erosion.
Dispersion ratio
Udi has the highest value of the dispersion ration with
81% at the topmost layer while Nsukka has the least with
16% (Figures 5 and 6). For erodible soils, the ratio is
normally found to be above 15% (Middleton, 1930). All
the soils under study (Figures 4 - 6) were therefore
depicted as erodible by this index. Average values for
86
Sky. J. Soil. Sci. Environ. Manage.
Figure 1. Silt/ Clay ratio for samples from Ngwo, Ebe, Nnobi and Ekwegbe.
Figure 2. Silt/ Clay ratio for samples from Agulu, Udi, Ideani, Abatete.
Figure 3. Silt/ Clay ratio for samples from Ihiala, Nanka, Awka, Nsukka.
Ezeabasili et al.
Figure 4. Dispersion ratio for samples from Ngwo, Ebe, Nnobi and Ekwegbe.
Figure 5. Dispersion ratio for samples from Agulu, Udi, Ideani, Abatete.
Figure 6. Dispersion ratio for samples from Ihiala, Nanka, Awka, Nsukka.
87
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Sky. J. Soil. Sci. Environ. Manage.
Figure 7. Clay ratio for samples from Ngwo, Ebe, Nnobi and Ekwegbe.
Figure 8. Clay ratio for samples from Agulu, Udi, Ideani, Abatete.
different sites show that vulnerability of some of the soils
to erosion is in following descending order: Udi, Ngwo,
Awka, Nanka, Ideani and Agulu.
Clay ratio
The clay ratio proposed by Bouyoucos (1935) is a
measure of the amount binding due to clay. As shown in
figures, clay ratio decreases with increase in depth. This
may be interpreted to mean that the binding influence of
clay and hence the resistance of these soils to erosion
increased with increase in depth. Higher clay ratio here
indicates lower binding influence due to clay and
therefore greater susceptibility to erosion. Based on this,
the relative resistance of these soils to erosion was found
to be in the following descending order using the result of
the first 0 – 30 cm depth; Nsukka, Ekwgbe, Awka,
Abalete Agulu Nanka, Ideani, Nnobi, Ebe, Ihiala, Udi and
Ngwo (Figures 7 - 9).
However, Bryan (1969) cautioned that the minimum of
10% clay is required for any meaningful interpretation.
Therefore, Ngwo, Ebe, Nnobi Udi, Ideani, Ihiala that
possess less than 10% clay may not pass the test as set
by ratio. However, as the depth increases percentage
clay increases and the clay ratio becomes more reliable.
Wischmeier and Mannering (1969) used the binding
influence of clay as reflected by inverse of clay ratio for
soil toss in field plots.
Predicted erodibility factor (K)
Values of K predicted by Wischmeier equation are
presented in Table 1; generally, K values obtained were
low. The result reported here are in close agreement will
those of Amon (1984) and Obi et al. (1989) on soils of
South-Eastern. Generally, in the soils under study higher
K values were recorded on soils with higher slit content.
Furthermore, coefficient of variation (CV%) in silt/ clay
ratio, clay ratio, dispersion ratio and the erodibility factor
of the sampled soils are depicted in Table 1. Awka soil
had the least variation in silt/clay ratio. It had a coefficient
of variation of 5.556%, this agrees with previously
observed fact that soil in Awka may be in an advanced
stage of weathering than others. Soil with moderate
Ezeabasili et al.
Figure 9. Clay ratio for samples from Ihiala, Nanka, Awka, Nsukka.
Table 1. Descriptive analysis of Silt, Clay, Dispersion Ratio and Erodibility factor (K) for eroded soils of Anambra state
Ngwo
Ebe
Nnobi
Ekwegbe
Agulu
Udi
Ideani
Abatete
Ihiala
Nanka
Awka
Nsukka
1.170
0.551
0.303
47.055
1.030
0.494
0.244
47.987
0.573
0.153
0.023
26.643
0.533
0.136
0.018
25.457
0.793
0.180
0.032
22.701
0.560
0.218
0.048
38.919
0.550
0.320
0.103
58.267
1.873
2.276
5.181
121.504
0.660
0.488
0.238
73.964
0.360
0.020
0.000
5.556
0.213
0.127
0.016
59.354
8.770
2.005
4.021
22.864
7.677
2.248
5.054
29.285
4.823
0.404
0.163
8.379
4.720
2.382
5.673
50.460
14.830
8.570
73.443
57.787
3.857
5.432
29.510
140.855
4.417
2.584
6.677
58.506
8.073
8.797
77.387
108.963
7.497
2.522
6.359
33.638
6.073
0.574
0.329
9.451
5.457
1.590
2.527
29.130
0.337
0.006
0.000
1.715
0.460
0.087
0.008
18.827
0.203
0.006
0.000
2.839
0.283
0.064
0.004
22.691
3.543
5.085
25.853
143.498
0.347
0.198
0.039
56.991
0.207
0.040
0.002
19.555
0.267
0.176
0.031
66.061
0.300
0.070
0.005
23.333
0.343
0.006
0.000
1.682
0.217
0.098
0.010
45.300
0.113
0.035
0.001
0.110
0.036
0.001
0.090
0.026
0.001
0.080
0.000
0.000
0.083
0.006
0.000
0.360
0.450
0.203
0.097
0.006
0.000
0.137
0.047
0.002
0.080
0.026
0.001
0.080
0.010
0.000
0.073
0.012
0.000
Silt/ clay ratio
Mean
0.440
Standard Deviation
0.190
Sample Variance
0.036
coefficient of variation
43.182
Clay ratio
Mean
16.377
Standard Deviation
7.468
Sample Variance
55.770
coefficient of variation
45.601
DISPERSION RATIO
Mean
0.533
Standard Deviation
0.064
Sample Variance
0.004
coefficient of variation
12.055
ERODIBILITY FACTOR (K)
Mean
0.073
Standard Deviation
0.021
Sample Variance
0.000
89
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Sky. J. Soil. Sci. Environ. Manage.
Minimum
Maximum
coefficient of variation
0.050
0.090
28.386
0.080
0.150
30.987
0.080
0.150
32.778
variation are soils of Ekwegbe, Agulu, Udi, Ideani,
Ngwo, Ebe and Nnobi; soil of Abatete, Nanka and
Nsukka had high silt/ clay ratio high, while Ihiala
soil had the highest silt/ clay ratio.
CV% of clay ratio ranges from 8.379 to 140.855
with lowest value in soil of Ekwegbe and highest
in soil of Ideani. The difference in variation
amongst sampled soils further reflects the
susceptibility and relative resistance to erosion.
Variation in dispersion ratio of sampled soils
shows that vulnerability to dispersion of soil will be
of the form, least vulnerable (Ebe, Ekwegbe,
Awka, Ngwo, Nnobi), vulnerable (Agulu, Abatete,
Nanka) and most vulnerable (Ideani, Ihiala,
Nsukka and Udi). Sampled soils with highest
values shows that they will be easily dispersed
compared to others with low variation. Differences
in variation in depth may be due to the varied
cementation and aggregate consolidation; this
does impact on the dispersion ratio. Erodibility
factor (k) for sampled soils shows that Agulu soil
has uniform variability, other soils have erodibility
factor varying slightly to moderately while only
Ideani soil has a high variation in erodibility factor.
Conclusion
The soils of Anambra basin are predominantly
sandy. They are characterized by low silt, low
clay, low organic matter and very high
permeability. Values of soil erodibility factor (K)
estimated by Wischmeier equation were generally
low. Although erodibility values were low, the
Table 1 cont.
0.070
0.120
29.397
0.080
0.080
0.000
0.080
0.090
6.928
0.090
0.880
125.123
0.090
0.100
5.973
potential soil loss, the product of erosivity and
erodibility is high, annual seasonal rainfall reaches
about 200 mm in most parts and with very high
intensities.
One is therefore left with the
conclusion that erosivity more than erodibility
contributes to the severity of erosion in the area.
High dispersion and clay ratio were recorded and
were very useful as indices. They were close to
research results of others who worked on similar
areas. Generally, they decreased with depth. The
high sand content and the high dispersion ratios
inferred that most of the soils are highly
detachable. However, with remarkably good
properties exhibited by a majority of these soils,
particularly the high infiltration rate, it can be
concluded that adequate vegetative cover and
higher organic matter are the main characteristics
the soil should possess to resist erosion among
others properties earlier mentioned.
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