River bank erosion control measure effec

Geoforum, Vol. 25, No. 1, pp. 105-113, 1994
Copynght @ 1994 Elsevier Science Ltd
Printed m Great B&m.
All rights reserved
CCM-7185/94 $6.00+0.00

River Bank Erosion Control Measure
Effects on Soil Physical and Chemical
Properties in the Niger Delta Area of
Nigeria

ADENIYI GBADEGESIN,*
ADEFEMI OLOKESUSIt and VICTOR
ADEYEYE,? Ibadan, Nigeria zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONM
paper evaluates the effectiveness of the use of synthetic fibres for river
bank erosion control as it affects soil physical and chemical properties in the Niger
Delta area of Nigeria. The method of investigation involves a comparison of the
mean values of the soil physical and chemical properties in three different zones of
the study area. Zone 1 or the controlled zone is the zone where the erosion control
measure is constructed and successful. Zone 2 or the uncontrolled zone is also
affected by river bank erosion but without a control measure. The third zone is a

forest zone about 80 years old and located in a relatively undisturbed environment
free from any erosion menace. In order to evaluate the effectiveness of the control
strategy adopted in the area, an analysis of variance (ANOVA) test was used to
compare the mean values of soil physical and chemical properties in the three zones.
In addition, the mean values of the soil properties in zones 1 and 2 were expressed as
a percentage of the forest zone equilibrium in order to assess the extent of soil
depletion caused by the erosion. The ANOVA results indicate that there is a
significant difference in eight out of the 11 soil properties analysed (atp 2 0.05) in the
three zones. In addition, the analysis also reveals that there is an improvement in the
soil nutrient status as a result of the erosion control strategy adopted in the study
area. For instance, all the soil nutrient properties attained over 80% of the forest
equilibrium level in the areas under the erosion control measure (zone 1). By
contrast, only three soil properties attained up to 50% of the forest equilibrium level
in zone 2. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
Abstract: This

I nt roduc t ion

Land degradation directly affects agriculture by diminishing crop yields, food supplies and water resources. However, other sectors of the economy and
the environment

are seriously affected as well
(Hauck, 1985). Today, there are problems of land
degradation in all regions of the world, but, in the
developing countries, there is the additional issue of
*Department of Geography, University of Ibadan, Ibadan,
Nigeria.
tNigerian Institute of Social and Economic Research,
Ibadan, Nigeria.

the capacity of the existing land resources to provide
food for the rapidly growing population. This,
according to McNamara (1990) arises because the
majority of the people of the Sub-Saharan Africa are
dependent on the land for their living. But the productive capacity of the land is being restricted by the
pressure of rapidly growing numbers of people who
do not have the wherewithal to put back into the land
what they are forced to take from it. Thus, since the
late 1970s the various regimes that have ruled in
Nigeria have not only stressed the importance of
agriculture to the overall development of the nation

but in addition have designed various programmes to
105

Geoforum/Volume

106

promote self-sufficiency in food production. These
programmes include the National Accelerated Food
Production Programme, Operation Feed the Nation,
the Green Revolution Programme and the establishment of the River Basin Development Authorities.
Furthermore, between 1981 and 1985 alone, the government committed more than US$763.9 million to
the agricultural sector in recognition of the need to
bridge the demand-supply gap in food production.
Despite these programmes and the huge fiscal outlay,
the performance of the agricultural sector has not
been impressive. While the total amount of cereals
(millet, guinea corn, maize and rice) and root crops
(yams, cassava and cocoyam) produced in Nigeria
rose gradually between 1972 and 1976, production

declined sharply and steadily between 1978 and 1983
when most of the aforementioned programmes were
launched and executed. As shown in Table 1, cereal
production has since picked up but this is not the case
for roots crops, the total production of which has
remained low.
One reason for this unimpressive performance is the
continued relative neglect of the environment as a
vital component of the supply side of the food
equation. Spurred by rapid population growth and

Table 1. Estimated
output of food crops in Nigeria from
1972-1973 to 1988 compared with 1961-1965 (1000 metric
tonnes)*

Year
1961-1965
1972-1973
1974

1975
1976
1976-1979
1979-1980
1980-1981
1981-1982
1982-1983
1983-1984
1984-1985
1985-1986
1987
1988

Cereals (millet, corn,
maize. rice)
7333
5775
7878
8245
8402

5575
5797
6885
7214
7682
9172
10,061
11,185
10,586
11.381

25 Number l/1994

the increased affluence of urban dwellers, the various
cropland expansion projects have led to the acceleration of land degradation, particularly soil erosion. It
is partly in recognition of this fact that the National
Ecological Fund was established. Projects and programmes to either control or ameliorate ecological
disasters have been financed from this fund. One such
programme is the River Bank Erosion Control Project of the Rivers State of Nigeria. The Rivers State
Government of Nigeria implemented 16 individual

soil erosion control schemes during the 1980-1985
plan period, including the Sagbama River Bank Erosion Control Project. While the desirability of these
erosion control measures cannot be contested, there
has been little research into their effectiveness, particularly as they affect soil properties and agricultural
productivity in general.
Therefore, this study is designed to find out the extent
to which the erosion and flood control measures
embarked upon by the Rivers State Government
have been effective in conserving the soil resources of
the areas using the Sagbama project as a case study.
Specifically, the study is a comparative analysis of the
physical and chemical properties of soils in zones with
and without erosion control in the study area. In
addition, the study relates the mean values of the soil
properties in the two zones to the equilibrium level of
an 80-year-old and relatively undisturbed forest zone
in the area.

Root crops (yam,
cassava. cocoyam)

21,217
10,830
26,997
27,508
28,280
7556
6711
6262
6139
7042
6014
6116
6773
6726
6929

The Study Area

About 66% of the Rivers State of Nigeria lies within
the Niger Delta, an area with great socio-economic

potential but highly vulnerable to erosion and flooding. According to Pathak (1985) and Fubara (1987)
more than 0.7 million ha of land which otherwise
could have been used for agriculture and human
settlement have been rendered useless as a result of
the flooding and erosion associated with continuously
reshaping channels. Flooding in the Niger Delta normally lasts from 3 to 5 months annually, submerging
agricultural lands, fishing sites, human settlements
*Sources: (i) FAO Production
Year Book, 1976; (ii) FOS
and even forest lands. When the floods recede.
Annual Abstracts
of Statistics 1985; (iii) CBN Annual
coastal erosion, the collapse of river banks occurs,
Report and Statement
of Accounts,
1988; and (iv) Oyeand changes occur in the physical and chemical
nuga, V. B. (1991) [in zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
Nigeria 2010, A.D. and Beyond.
properties of soils.
p. 311.

Geoforu~olume

25 Number 111994

The study area, Sagbama, is located at a bend on the
Forcados creek and is the headquarters of the Sagbama Local Government Area of Rivers State of
Nigeria (Figure 1). It has an estimated population of
43,256 according to the 1991 provisional census result. The annual rainfall in the area varies between
2500 and 3500 mm, with a wet season between March
and November and a dry season between December
and February.

107
topography of the study area is flat to gently undulating with slopes rarely exceeding 3%.
Shifting cultivation is the predominant agricultural
system practised in the area. Plantain is the major
crop grown, while yams, melon, cocoyams and vegetables are minor crops. Most farming operations are
carried out manually.

Selecting

Recent classification of the vegetation of Rivers State
(see Figure 2) recognizes five main vegetation and
ecological zones. These are: the dry flat land, dry land
with abundant swamp, mangrove swamps, beaches
and bars. Sagbama is located in the dry land with
abundant swamp zone. In general, six ‘main’ geological formations cover the Rivers State. Sagbama is
located on the Alluvial Meander Belts formation and
the deposits are of recent type (Holocene). The

an

Erosion

Control

Option

for

Sagbama
According to Zincon International (1981) several
erosion control measures, such as concrete walls,
sheet piling, groynes, artificial nourishment and slope
revetment, were considered for application on the
eroding banks of Sagbama. The slope revetment
option was chosen based on a decision analysis of
several factors such as soil properties and con-

Figure zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
1. Map of Rivers State of Nigeria showing the study area.

108

GeoforumNolume

1’1
q
El

q

Figure 2. Map showing

ditions, flow conditions including water level, velocity distribution and the pattern of river discharge in
the neighbourhood of the area to be protected. Performance factors were also taken into account including service life, serviceability of the control measures
and public needs. In addition, the extent to which the
construction would affect the balance of the physical
environment, the desire to minimize the impact on
the economic life of the people and the need for easy
access from the river to the town at any time of the
year were also considered.
The slope revetment option used in Sagbama was
designed in such a way as to re-shape the river banks
to a slope of 1:3-l :5 using sand dredged from nearby
pits in the river bed. The new sand embankment was
then covered with a sand-filled mattress or revetment
made of polypropylene (synthetic fibre). Two layers
of green polypropylene material were sewn together

vegetation/ecological

25 Number l/1994

-

Dry flat land
Dry land with abundant
swamp zones
Freshwater
Mangrove

swamp
swamp

zones.

at intervals of approximately 0.3 m in the direction
perpendicular to the river current. The hollow spaces
between the thread lines were filled with dredged
sand. Thus, the revetment, apart from having a natural green appearance, also allowed vegetation to
grow soon after its construction and the roots thus
provide an additional strength. zyxwvutsrqponmlkjihgfedcbaZ

Me tho do lo g y

The choice of Sagbama as the study area was guided
by two factors. First, the settlement is one of the areas
in Rivers State where river bank erosion is very acute
due to its location at a bend on the Forcados creek.
Second, unlike the other 15 erosion controlled projects embarked upon in the state where the hydraulic
sand-filled design described above was 100% successful, that of Sagbama was only partially successful.

Geoforum/Volume

25 Number l/1994 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
109

Thus, at Sagbama, it is possible to obtain areas within
the settlement covered by the erosion control design
and areas still under the menace of river bank erosion
for comparative purposes.
The soil sampling was carried out at three different
zones in the Sagbama area: these are referred to as
the controlled zone (CZ) or zone 1, the uncontrolled zone (UZ) or zone 2, and forest zone (FZ) or
zone 3. The location of the three zones in and
around Sagbama is as shown in Figure 3. From each
zone, six sampling plots of about 1 ha in size were
randomly located. All the 12 plots for the CZ and
UZ fall within existing plantain farms managed by
peasant farmers. According to the farmers they have
never applied chemical or organic fertilizers on the
farms. The six plots for the FZ were used as the
control plots because they are all located in a relatively undisturbed environment free from the erosion menace and have been fallow for about 80
years.
Soil samples were collected from five randomly
located points within each sampling plot. They were
collected from two pre-determined depths of O-30
and 30-60 cm (subsequently referred to as topsoil and
subsoil). The soil samples were air-dried and passed
through a 2-mm sieve and analysed for particle size

composition by hydrometer methods (~ouyoucos,
1926), for organic carbon by the chromic acid digestion method (Walkley and Black, 1934), and for total
nitrogen by the Kjeldahl method. Soil exchangeable
calcium, potassium and sodium were determined by
flame photometry, while exchangeable magnesium
was determined by atomic absorption spectrophotometry. Soil pH was determined potentiometrically in
0.01 M calcium chloride solution using a soil-tosolution ratio of 1:2 (Peech, 1965).
The data on the yield of food crops were collected
directly from the farmers’ records in the CZ and UZ.
In all, 80 farmers from the CZ and 8.5 farmers from
the UZ have yield records for the three food crops
analysed in the study. The crops are yam, cassava and
rice. The average yield for each crop in each zone was
computed and the difference between the zones compared using the Students t-test.

Results and Discussion
Tables 2-4 show the mean, standard deviation and
coefficient of variation of soil properties in the three
zones being compared. Generally, the soils have
relatively small sand fractions and higher clay and silt
fractions, with the clay component increasing with
soil depth. The coefficients of variation reveal that

Forcados River

Farm land and back swamp area
Farm land

cz

Road
= controlled

zone

FZ = forestzone
UZ = uncontrolled

Figure 3. Map

showing

Sagbama Township
and
NEDECO (1982).

the

sampling

zones.

Source:

zone

Geoforum/Volume

110
Table 2. Variability

of soil properties
trolled zone*

Soil property
Sand (%)
Silt (%)
Clay (%)
PH
Organic

matter

Total nitrogen
Phosphorus

(%)
(%)

(ppm)

Exchangeable

Ca’+ (ppm)

Exchangeable

Mg’+ (ppm)

Exchangeable

Na+ (ppm)

Exchangeable

Kf (ppm)

T
S
T
S
T
S
T
S
T
S
T
S
T
S
lS
T
S
T
S
T
S

in the erosion

x

SD

12.60
10.20
42.60
41.00
44.80
48.80
3.10
4.60
2.56
1.85
0.14
0.09
2.46
2.36
4.84
7.03
3.90
8.10
0.15
0.09
1.36
0.85

3.83
3.65
3.43
4.21
2.25
2.34
0.40
0.31
0.24
0.18
0.02
0.01
0.37
0.34
0.23
0.12
0.48
0.71
0.02
0.01
0.07
0.04

con-

;;
30.39
35.78
8.05
10.27
5.03
4.80
9.76
8.91
9.38
9.73
14.29
11.11
15.04
14.41
4.75
5.97
12.31
8.77
13.33
11.11
5.15
4.71

silt/clay content

of the soils implies

pH values

of the topsoils

matter.

Sand (“6)
Silt (%)
Clay (%)
PH
Orgamc

matter

Total nitrogen
Phosphorus

(% )

(70)

(ppm)

Exchangeable

Ca’+ (ppm)

Exchangeable

Mg’+ (ppm)

Exchangeable

Na+ (ppm)

Exchangeable

Kt (ppm)

T
S
T
S
T
S
T
S
T
S
T
S
T
S
T
S
T
S
T
S
T
S

SD

22.90
17.80
40.00
37.20
37.10
15.00
3.84
1.82

5.07
3.6-t
4.35
3.85
7.05
6.65
0.77
0.89
0.31
0.28
0.05
0.03
0.82
0.76
1.08
1.08
0.91
0.62
0.08
0.05
0.25
0.21

Soil organic

1.oo
0.X’)
0.09
0.06
2.00
1.86
1.96
1.47
0.09
0.67
0.13
0.09
0.42
0.35

matter

and total

‘;T = topsoil. S = subsoil. .U = mean value. SD = standard
deviation, CV = coefficient of variation.

nitrogen

the UZ for organic matter and total nitrogen,

than

are

respect-

ively. The coefficients of variation values for organic
matter and total nitrogen in the UZ are fairly high,
being generally more than 30%.
Unlike the
exchangeable

CZ and FZ, the mean values of the
cations are lower in the UZ. This may
to the lower

clay and organic

of soil properties
land zone*

Soil property
7’ I4
L_.
20.45
10.88
10.35
IO.00
14.78
20.05
1x.37
31.00
31.46
55.56
50.00
-tl .oo
30.86
55.10
73.47
91.93
92.54
61.54
55.56
59.52
46.67

lower

generally
moderate
and decrease with soil depth.
Nevertheless,
the mean values of the two soil properties in the CZ are more than 2.3 and 1.5 times that of

Table 4. Variability

s

are generally

those of the subsoils. This may be due to the production
of organic acids by decomposing
organic

Table 3. Variability

Soil property

that the soils have

high water and nutrient holding capacities. The mean
pH values of the soils range between 3.84 and 4.82,
indicating
that the soils are moderately
acidic. The

be attributed

in the uncontrolled

l/1994

the particle size composition
of the soil exhibits little
spatial variation,
with the exception
of sand in the
CZ. This is expected since sand is the predominant
soil fraction used in constructing
the erosion control
embarkment
in the study area. Nevertheless,
the high

*T = topsoil, S = subsoil, F = mean value, SD = standard
deviation, CV = coefficient of variation.

of soil properties
zone

25 Number

Silt (%)
Clay (%)
PH
Organic

matter

Total nitrogen
Phosphorus

in the forest/fallow

s

Sand (“lo)

(‘%)
(%)

(ppm)

Exchangeable

Ca”

(ppm)

Exchangeable

Mg’+ (ppm)

Exchangeable

Na+ (ppm)

Exchangeable

Kt (ppm)

T
S
T
S
T
S
T
S
T
S
T
S
T
S
T
S
T
S
T
S
T
S

matter

17.70
16.80
42.30
36.75
45 .oo
46.35
4.15
4.26
2.74
1.x9
0.17
0.13
2.42
2.02
4.36
4.24
3.77
2.92
0.14
0.16
1.29
1.08

SD
4.34
4.21
3.75
2.59
F-I.33
5.34
0.36
0.28
1.21
1.02
0.07
0.03
0.41
0.29
0.32
0.24
0.38
0.26
0.04
0.06
0.07
0 03

(?,
34.96
25.06
8.87
7.05
11.84
11.28
X.68
6.57
44.16
53.97
41.18
25.00
16.94
14.36
7.34
5.66
10.08
8.90
28.57
37.50
5.43
2.78

*T = topsoil, S = subsoil. X = mean value, SD = standard
deviation, CV = coefficient of variation.

GeoforumNolume

25 Number 111994

111

content of the soils in such a zone (Pitty, 1979;
Aweto, 1985) as well as the combined effects of the
high rainfall intensity and river erosion which tends to
leach and wash away the cations. Generally, the
coefficient of variation values for the exchangeable
cations are moderate, ranging between 4 and 28% in
the CZ and FZ but highly varied in the UZ.

Comparison between the soils in the three zones
In order to ascertain whether there is any significant
difference in the soil physical and chemical properties
in the three zones, an analysis of variance (ANOVA)
was carried out to compare the mean values of the
topsoil properties between the three zones. The
analysis was limited to the topsoil because in the
tropics the soil nutrient reserves are often concentrated in the surface horizon (Lal, 1985).
Table 5 shows the ANOVA results obtained when the
soil properties of the three zones were compared.
Most of the properties determining the nutrient status
of soils, and thereby their agricultural productivity,
differ markedly between the three zones at p > 0.05
with the exception of the calcium and sodium contents of the soils. This suggests that, all other factors
being equal, crop yields from farms located in the
three zones will differ and are likely to be higher in
the CZ and FZ because of the high mean values of the
soil nutrients. The highly significant (at the 0.01 level)
results obtained for the sand, silt and clay contents
Table 5. Results of the ANOVA

(despite the fact that the soils are from the same
geological formation) is probably due to the type of
erosion control strategy adopted in the study area, as
explained earlier.
In order to critically evaluate the effects of the erosion control measure on the nutrient status of the
soils, the data were further analysed by expressing the
mean values of the soil properties in the CZ and UZ
as a percentage of the mean values obtained in the
FZ. The assumption is that since the FZ has been left
undisturbed for a fairly long time (about 80 years) the
levels of soil properties obtained in this zone can be
regarded as the maximum level attainable in the
ecosystem. Thus, the higher the percentage obtained
for the soil properties in each zone, the closer the
build up of the soil properties to the climax vegetation
situation. The soil properties considered for this
analysis are those that are directly related to the
fertility of the soils. These are the organic matter,
total nitrogen, phosphorus, calcium, magnesium,
sodium and potassium content, all of the topsoil. As
expected, most of the soil properties in the CZ
recorded a generally higher percentage than those of
the UZ. For instance, the fact that the organic matter
level in the CZ is closer to that of the FZ (see Table 6)
is expected since it has long been established that the
organic matter is not only important in reducing
erodibility (Higgit, 1991) but also reduces splash
detachment (Ekwue, 1990). Table 6 also reveals that
the mean values of such soil properties as the available phosphorus. calcium, magnesium, potassium

test to compare the mean values of topsoil properties
zones*

in the three

Mean value
Soil variables
Sand (%)
Silt (%)
Clay (%)
P”
Organic matter (%)
Total nitrogen (%)
Phosphorus
(ppm)
Calcium (ppm)
Magnesium (ppm)
Sodium (ppm)
Potassium (ppm)
*CZ = erosion
p < 0.5.

controlled

cz

uz

FZ

12.60
42.60
44.80
4.10
2.56
0.14
2.46
4.84
3.90
0.15
1.36

22.90*
40.00*
37.10*
3.84
1.00*
0.09*
2.00*
1.96
0.99*
0.13
0.42*

12.70
42.30
45.00
4.15
2.74
0.17
2.42
4.36
3.77
0.14
1.29

zone, UZ = controlled

zone, FZ=

Calculated
F-value
50.92
4.60
22.61
1.00
48.68
19.37
6.30
0.96
124.73
0.36
343.57

Significance
level
0.01
0.05
0.01
NS
0.01
0.01
0.05
NS
0.01
NS
0.01

forest zone, NS = not significant

at

GeoforumNolume

112
Table 6. Soil nutrient levels in the erosion controlled and
uncontrolled
zones expressed as a percentage of the forest/
fallow zone*
uz

cz

Soil variable

*CZ = erosion

36.4

03.3
82.3
101.7
111.0
103.5
107.1
105.-i

Organic matter
Total nitrogen
Phosphorus
Calcium
Magnesium
Sodium
Potassium
controlled

.57
I. 9
82.7
44.‘)
16.3
92.9
32.6

zone. UZ = uncontrolled

25 Number

l/1994

out to compare the average yield of the three crops in
each zone. The yields of cassava and yam in the zyxwvutsrqpon
CZ
are about 1.5 and 2.0 times those in the UZ, respectively. In addition,

there

is a significant

difference

at

p 2 0.05 in the yield of these two crops in the two
zones. The non-significant
difference
recorded
for
the third food crop, rice, may be attributed to the fact
that rice is grown

as swamp

rice in the study area:

therefore, the crop can do well in both the CZ and the
UZ provided water is always available.

zone.
Conclusion

and sodium

are even higher in the CZ than in the FZ

as indicated by a more than 100% value obtained for
the soil properties.
This is probably due to the fact
that most of these soil nutrients
the standing

vegetation

By contrast,

only three

are immobilized

by

found in the FZ.
soil properties

sodium,

available

phosphorus

and total

recorded

up to 50% of the levels obtained

In order

to test the effect

in the UZnitrogenin the FZ.

of the low soil nutrient

According
to Larson et al. (1983) losses in crop
productivity
can be directly attributed to replaceable
and irreplaceable
soil properties lost during the process of erosion.

The replaceable

soil attributes

are

essentially
the plant nutrients.
such as the phosphorus,
calcium,
magnesium,
potassium,
organic
matter and the nitrogen contents of the soil. On the
other hand, the irreplaceable
attributes
involve reduction in plant rooting depth and water-holding
capacity which, according to Abrol
Gbadegesin
(1989), are influenced

ef al. (1968) and
by the textural

values recorded
in the UZ on agricultural
productivity in the study area, a comparative
analysis of

composition
and organic matter contents of the soil.
However,
as noted by La1 (1985), due to the com-

farmers’ yield records for three food crops were
carried out. In all, the yield records of 80 and 85
peasant farmers, respectively
from the CZ and UZ,
were used in computing
the average yield for each

plexity of the interacting factors involved it is difficult
to establish
a direct cause-and-effect
relationship

crop in each zone. Ail the farmers, who were cultivating between
1 and 2 ha, were employing
broadly
similar traditional farming methods; any difference in
average yields would, therefore.
be the result of soil
property variations and not a function of differential
farming

on

Nevertheless,
as shown in this study, the control
measure does appear to have improved the nutrient
status of the soils in the area, as evidenced by the high
levels of the ‘replaceable’
soil attributes
in the zone
where the control measure
was constructed.
The
most seriously
affected soil properties
highlighted

practices.

Table 7 shows the results of the Students

Table 7. Results

between erosion and degradation
in soil quality
the one hand and crop yield on the other.

f-test carried

in this study

are

of the Student’s t-test to compare yield of selected
controlled and uncontrolled
zones’

the

‘replaceable’

soil attributes.

food crops in the erosion

Yield
(kg/ha)
Type of
crop

CZ

Yam
Cassava
Rice

4544
3300
1100

*CZ = erosion

controlled

uz
2251
2347
100.5

Absolute
difference
2290
953
95

zone. UZ = uncontrolled

SE

t

Significance
level

148.6
67.5
62.6

5.10
14.12
1.52

0.01
0.01
NS

zone, NS = not significant

at p < 0.05.

GeoforumNolume

25 Number l/1994

113

especially in the zone not covered by erosion control
measures. Although such attributes are, according to
Larson et al. (1983), replaceable, their replacement
involves a cost. Therefore, subsistence and commercial farming can only be sustained in the uncontrolled
zone with the liberal application of fertilizers as the
soils found in this zone have relatively low levels of
soil nutrients when compared with the other two
zones. In addition, the low level of organic matter
content of soils in this zone can be combated by the
adoption of the biological erosion control methods of
crop residue mulches and a reduced tillage system.
However, as observed by Olokesusi et al. (1992),
though the structural erosion control measures
appear to be effective in improving the fertility status
of the soils, the synthetic fibre used is showing signs of
rapid deterioration. It is thus envisaged that in the
next 20-30 years, if the entire control process is not
repeated and extended to the uncontrolled areas, the
amount of soil loss due to river bank erosion will be
quite significant and this will eventually reduce the
agricultural production of the area.

Aweto, A. 0. (1985) Soil cation exchange capacity dynamics under bush fallow in south-western
Nigeria,
Geoforum,

16,85- 92.

Bouyoucos,
G. S. (1926) Estimation
of the colloidal
materials soils, Science, 64, 362.
Fubara, D. M. (1987) The menace of flood erosion and
environmental
disasters combat plan, In: Disasters in
Nigeria: Soil Erosion. Federal Ministry of Science and
Technology,
Lagos.
Gbadegesin,
A. (1989) The influence of organic matter and
texture on some moisture characteristics
of soils in the
savanna area of south-western
Nigeria, Savanna, 10,5866.

Hauck, F. W. (1985) Soil erosion and its control in developing countries,
In: Soil Erosion and Conservation, pp.
718-728, S. A. El-Swaify et al. (Eds). Soil Conservation
Society of America.
Lal, R. (1985) Soil erosion and its relation to productivity in
tropical soils, In: Soil Erosion and Conservation, pp.
237-247. S. A. El-Swaify et al. (Eds). Soil Conservation
Society of America.
Lal, R., Kang, B. T., Moorman, F. R., Juo, A. S. R. and
Moomaw, J. C. (1975) Soil management
problems and
possible solutions in western Nigeria, In: Soil M anagement in Tropical America, pp. 372-408, E. Bornemeiza
and A. Alvarodo (Eds). North Carolina State University, Raleigh.
Larson, W. E., Pierce, F. J. and Dowdy, R. H. (1983) The
threat of soil erosion to long-term production,
Science,
219,45% 465.

Finally, it is important to note that tackling the McNamara, R. S. (1990) African’s development
crisis:
agricultural
stagnation,
population
explosion
and
enproblem of soil erosion in developing countries has
vironmental degradation,
Address to the Africa Leaderbecome a matter of great urgency. Fortunately, the
ship Forum OTA, Nigeria, 21 June 1990.
awareness of the need to cope with the menace is NEDECO (1982) Sagbama Sandfilling Project. NEDECO,
Port Harcourt .
growing. However, to carry out soil conservation
Pathak,
N. K. (1985) Erosion control in Rivers Statework comprehensively and successfully, greater exchallenge for appropriate
technology,
Nigeria Sunday
pertise is needed as well as more political commitTimes, September,
8-9.
ment in the execution of the programme. Soil Peech, M. (1965) Hydrogen-ion
activity, In: M ethods of
conservation should be comprehensive,
including
Soil Analysis, Vol. 2, pp. 914-926, C. A. Black (Ed.).
Pitty, A. F. (1975) Geography and Soil Properties, 1st
prevention, reclamation and productivity functions. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG
Acknowledgements- The

committee
Research
Societies
carry out
provided

authors are grateful to the Sub-

on African Agriculture
of the Social Sciences
Council and the American Council of Learned
for awarding the research team a fellowship to
this study. The funds for the award have been
by the Ford and Rockefeller Foundations.

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