It has been established that an efficient management of river basins
will assist significantly in solving the problems of poor agricultural
productivity in any country. To realize this fact, a lot of countries
in the last decade had placed high premium on River Basin development.
Agricultural projects under this scheme which involve arable crop production
utilized mainly the flood plain and valley bottom soils and to some extent
the adjoining upland. Unfortunately of late most of these river basins
lies within regions where rainfall are often inadequate in amount and
erratic in timing (Singh, 2000), thus necessitating irrigation in order
to satisfy the moisture requirements of the crops needed to meet the demands
for food and fiber.
Most of the river basin soils are productive, which necessitated their
cultivation all year round to a host of high value agronomic and horticultural
crops (Singh and Babaji, 1989). Farming on the River Basins depends on
rains in the wet season (rice cultivation) and residual soil moisture
in the dry season (Horticultural crops). To alleviate the problem of moisture
stress during the prolonged gaps between rains as well as in the dry season,
supplementary irrigation is provided by lifting the water from perennial
surface water bodies, shallow wells and tube wells. Although the irrigation
is useful for sustaining agricultural production in any locality, it is
imperative that the soils to be use must be evaluated. If there is no
proper land evaluation for irrigation agriculture technical problems may
arise later that would affect both soil quality and crop production adversely.
Determining the suitability of land for irrigation requires a thorough
evaluation of soil properties, the topography of the land within the field
and the quality of water to be used for irrigation. Few researchers Akinbile
et al. (2007), Adefisan et al. (2007) and Olowolafe and
Patrick (2001) have all investigated on effective factors to be considered
when evaluating soils for irrigation. irrigation systems and water supplies.
A basic understanding of soil/water/plant interaction will help irrigator
efficiently manage their crops, soils, irrigation systems and water supplies.
Unfortunately published information on the properties of River basin soils
Considering how large hectares of river basin soils which are agriculturally
productive for irrigation agriculture within Nigeria and are mistakenly
used for construction (residential and industrial), there is therefore
the need to stop such a trend and conserve the river basin soils. Such
river basin soils when fully harnessed and exploited can be used for irrigation
agriculture in medium and large scale and will alleviate the present food
shortage in the country and consequently alleviate poverty.
The potential uses of these soils are many and it is the objective of
this research study to explore what is known about these soils with respect
to their properties and their evaluation for irrigation land use. It is
hoped that the information to be generated from this study will equip
potential users with pedological information that is essential for the
sustainable management of these soils for sustainable agriculture and
environment within the study area.
Therefore, the major objective of this study is to evaluate the suitability
of Asu river basin soils for irrigation agriculture. The result of this
study is expected to serve as a guide in the choice of crops and management
practice for different parts of the study area.
MATERIALS AND METHODS
Description of Study Area
The study areas are located in Anambra state (around Nsukka and Afikpo)
in south eastern Nigeria. The approximate geographic coordinates are 6°
5I`N, 7° 23` E. These areas are located around the eastern part of
the cross River and the area lies between Okposi and A fikpo. The area
covers about 120.89 ha of land.
The climate of the basin lies within the humid, subtropical climate with
distinct wet and dry season. The wet season spans from March to November
and dry season from December to February. Means annual total rainfall
ranges from 1800 to 2200 mm. The rainy season has two peaks (July and
September) with a break in August. Temperature is generally high throughout
the year with little deviation from the mean annual of 26 °C February-April
as the hottest months. Humidity is high. The average index of mean relative
humidity is never than 70%.
The geology of the Asu River Basin is dominated by shale which grades
laterally into Amasiri sandstone. The vegetation of the area falls within
the lowland rainforest zone and in the moist forest at low medium attitude
zone. Land use around the area is basically agriculture. Crops such as
Yam, Cassava, Maize, Cocoyam and Vegetable (Fluted pumpkins, Okro, red
peppers, bitter leaf and melons) are grown.
Field Study and Laboratory Analysis
The major soil types were mapped using the approach developed by the
soil survey manual (Soil Survey Staff, 2003). Four soil types were identified
after the Rigid grid soil survey. Four soil profiles were dug and described
according to the FAO (2006). Field descriptions and sample collections
were made during the dry season to ensure freedom from groundwater disturbance.
Soil samples collected during the detailed soil survey investigation
were air-dried and made to pass 2 mm sieve utilizing those that are less
than 2 mm for laboratory analysis. The IITA procedures for soil and plant
analysis (IITA, 1979) were followed in analyzing the soil samples.
Land Evaluation Procedure
Land Suitability for Irrigation
To evaluate the land suitability for irrigation the parametric evaluation
system of Sys (1985) was applied, using soil and land characteristics.
These characteristics concern environmental factors, drainage properties,
soil physical and chemical properties. They are rated and used to calculate
the suitability index for irrigation (Ci) according to the formula:
Ci = Ax B/100 xC/100xD/100xE/100xF/100
||Suitability index for irrigation
||Soil texture rating
||Soil depth rating
Suitability classes are defined considering the value of the suitability
index (Table 1). For this study, gravity or surface
irrigation that require impermeable soil with drip and localized irrigation
and are less regarding to these characteristics were analysed. The soil
characteristics were used according to different ratings specifying each
type of irrigation (Table 2-7).
|| Suitability index for the irrigation suitability indices
|Source: Sys, 1995
|| Textural classes rating for irrigation suitability
|Source: Sys 1985
|| Soil depth rating for irrigation
|| CaCO3 status rating for Irrigation
|| Electro-conductivity rating for irrigation
|| Drainage classes rating for irrigation
|| Slope Rating for irrigation
The data generated from the field for each pedon was then used to calculate
the suitability index for irrigation (Ci) using the equation below for
the four pedons.
Ci = A x B/100 C/100xD/100 x E/100x F/100
For example; to calculate the Ci (surface irrigation)for Ihuibe 1 using
the equation we obtained the result as follow.
Statistical analysis of the suitability rating for irrigation methods
(surface and drip) was compared together using rank correlation coefficient
to see whether there is a correlation between the two methods as evaluated
for irrigation using Sys (1985) method. For rank correlation coefficient
the formula used are
||Differences of pairs
||No. of pairs
RESULTS AND DISCUSSION
Land and Soil Characteristics of Study Area
The summary of the Land and soil characteristics of the study area
(pedons) is shown in Table 8. All the soils are deep
(soil depth > 100 cm). This is good enough and will allow good penetration
of roots of crops. For soil depth rating, all peonds were rated 100 for
both surface (gravel) and drip irrigation (Table 9,
10). Soil depth refers to the thickness of the soil
materials which provide structural support, nutrients and water for plants.
The depth to a contrasting soil layer of sand and gravel can affect irrigation
management decision if the depth to this layer is less than 3 feet, the
rooting depth and available soil water for plants is decreased. Soils
with less available water for plants require more frequent irrigations.
The soil textures of the soils except Ameta 2 were considered good for
surface (gravity) irrigation. However Ameta 2 soil texture was considered
good for drip irrigation in spite of the loamy sand nature of the soil
(Table 8) and it was rated 85% for suitability index
for irrigation. Coarse textured soils such as sands and gravel usually
have high infiltration rates. The infiltration rates of medium and fine
textured soils such as loams, silts and clays are lower than those of
coarse textured soils and more dependent on the stability of the soil
aggregates. Water and plant nutrient losses may be greater on coarse textured
soils, so the timing and quantity of chemical and water applications is
particularly critical on these soils.
|| Land and soil characteristics of pedons
|| Land and soil characteristics rating and capability
index for drip/loc tlized irrigation of the pedons
|S1 = Highly Suitable, S2 = Moderately Suitable, N1 =
Currently not Suitable, EC = Electrical conductivity (mmho cm-1)
Ec = mmho cm-1 rating
||Land and characteristics rating and capability index
for surface/gravity irrigation of the pedons
|S1 = Highly Suitable, S2 = Moderately Suitable, N1 =
currently not Suitable, EC = Electrical Conductivity (mmho cm-1),
Ec = mmho cm-1 rating
When the CaCO3 (calcium carbonate) was rated all the soils
were considered suitable for irrigation use. All the soils were rated
100 (Table 8, 9, 10).
Electrical conductivity values for all the soils are low (Table
8). The values recorded for all the soils are less than 4 mm ho/cm
1 recommended as critical values for saline soils. Therefore
there is no reason to be afraid that the soil might be saline. All the
soils were rated 100 with reference to Electrical conductivity, meaning
that all the soils are suitable for irrigation with reference to the salinity
content of the soils
A large proportion of the soil (Ihuibe 1, Ihuibe 2 and Ameta 2) were
imperfectly drained and were rated 90 (drip irrigation) and 80 (surface/gravity
irrigation) for irrigation suitability. This is considered to be suitable
for irrigation agriculture. With reference to slope all the soils were
rated 95 (Table 10) for surface (gravity) irrigation.
This is considered suitable for surface irrigation however when all the
soils were rated for drip irrigation, they were found to be highly suitable
(S1). This go to say slope may not be a critical factor in drip/localized
irrigation when compared with surface irrigation because of erosion problem.
Slope is important to soil formation and management because of its influence
on runoff, soil drainage, erosion, use of machinery and choice of crops.
IRRIGATION SUITABILITY EVALUATION
The suitability of the soils was assessed for irrigation following the
method of Sys 1985. The assessment of soils for irrigation involves the
using of properties that are permanent in nature that cannot be change
or modified without exorbitant cost. Such properties include depth, drainage,
texture and slope. These properties are known to constitute some kind
of hindrance to irrigation crop production. Chemical properties that are
usually considered (e.g., fertility) can be changed with minor improvement.
The processing of the parametric evaluation system for gravity and drip/localized
irrigation using the Sys (1985) method gave the irrigation suitability
results in tables 9 and 10. From
Table 10, 41.40% the area (Ameta 1) have been classified
as highly suitable (S1) for surface irrigation about 45.97% of the study
area (Ihuibe 1 and Ihuibe 2) are moderately suitable (S2) for surface
irrigation. The limiting factor that lowers these soils to S2 is mainly
the soil drainage status (Table 8). Also in some cases,
the slope class also handicaps the use for surface irrigation purpose
as seen in the case of soil of Ihuibe 1 and Ihuibe 2 (Table
8) with slope of 2-3%. Simple slopes of 1% or less are commonly used
for gravity (Surface) irrigation. Simple and complex slopes greater than
1% should only be irrigated with sprinkler or drip systems. Center pivot
sprinkler irrigation systems can operate on slopes up to 15% but simple
slope greater than 9% are not generally recommended. To accommodate an
irrigation application method such as gravity or sprinkler systems, the
slope in a field can be modified by land smoothing. However, land smoothing
may cause yield reduction for one to three growing seasons. The place
where topsoil was removed are most likely to have yield reductions. Special
management of these areas through increased fertilizer and organic matter
application may be required for accelerated recovery.
About 12.62% of the study area (Ameta 2) is classified as currently not
suitable (N1) for surface gravity irrigation. The limiting factor is mainly
soil texture (Loamy sand) which was rated 55% for this soil. Soil texture
is relevant to permeability, infiltration and water holding capacity of
the soil. As we know water and plant nutrient losses may be greater on
coarse textured soils, so the timing and quantity of chemical and water
applications is particularly critical on these soils. Surface irrigation
requires heavier soils. For drip and localized irrigation (Table
9), a good proportion (41.40%) of the area (Ameta1) are highly suitable
(S1) while 58.60% of the area (Ihuibe1, Ihuibe2 and Ameta2) are classified
as moderately suitable (S2). The Ameta 2 that was classified as currently
not suitable (N1) under surface irrigation is now classified as moderately
suitable (S2) for drip/Localized irrigation. This is because soil texture
is not regarded as a critical factor for drip irrigation. The agreement
between surface and drip irrigation suitability rating was compared using
rank correlation. The results show that the two was significant with R
value of 0.60 (p<0.01).This shows that there is agreement in the rating
ability of the Sys (1985) method of rating the soils for irrigation agriculture.
The comparison of the two types of irrigation revealed that it would
be more beneficial to irrigate by drip or localized type, as the latter
mode improves all the suitability to the irrigation purposes (e.g., the
Ameta 2 Soil). The drip irrigation is recommended for a sustainable use
of this natural resource.
CONCLUSIONS AND RECOMMENDATIONS
An irrigation evaluation of soils of Asu River Basin was carried out.
The result obtained reveals that the soils are suitable for both surface/gravity
and drip/localized irrigation at various rating levels (Highly and moderately
suitable). The parameters investigated (Soil texture, soil depth, CaCO3,
Electrical Conductivity (EC), Soil Drainage and Slope) all fall within
the tolerable limit for irrigation purpose. As a result of the agricultural
potential of these soils (River Basin soils), the land should therefore
be conserved when not in use against permanent use e.g., residential,
building and industrial uses. With the current potential of these soils
for irrigation, if fully harnessed and properly utilized, it will increase
crop productivity and consequently alleviate poverty.
From the result of the study, the following recommendations are suggested
||It is better to irrigate the land using the drip/localized
method since it improves the suitability to the irrigation purposes
and also ensure the sustainable use of the land for irrigation agriculture.
||As a result of the competing demand of land for agriculture urban
and other land use activities within the study area, land use plan
is therefore necessary to conserve and protect this suitable agricultural
land against other land uses.
||The quality of water to be use for irrigating
the soils must be evaluated appropriately. Further studies can be carried
out on water quality requirement for irrigation agriculture for this area.