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EVALUATION OF WATER RETAINING AND CONDUCTING
CHARACTERISTICS, AGGREGATE STABILITY AND BULK DENSITY OF
COCOA GROWING SOILS IN IKWUANO, ABIA STATE, NIGERIA
*Oguike, P. C. and Henshaw, I. E.
Department of Soil Science and Meteorology
Michael Okpara University of Agriculture, Umudike, Abia State, Nigeria
Abstract
Efficient utilization of water and nutrients by plants requires good rooting systems which are enhanced by soil physical reference to 15-30 cm depth, water retention and BD were similar while Pt and
Ksat varied. The more clayey soil from
Nkalunta conducted less water and retained properties. In this study, some physical properties of cocoa growing soils from more than soils of the other 3 communities.
Keywords: Water retention, porosity, aggregate, cocoa, Ikwuano.
Umuariaga, Itunta, Nkalunta and Amuru communities were evaluated. Auger and core samples were collected from 10 cocoa plantations in each of the 4 communities.
Free range survey technique was used to randomly collect samples for the purpose of compositing to represent the average conditions in the locations with similar cropping and management history. The auger samples (0 – 30 cm depth) were used for particle size analysis, mean weight diameter (MWD), and clay dispersion index (CDI). The core samples, collected at
0-15 and 15-30 cm depths, were used to determine field capacity (FC), permanent wilting point (PWP), bulk density (BD) and saturated hydraulic conductivity (Ksat).
Available water content (AWC) was deduced from the difference between FC and PWP whereas total porosity (Pt) was computed from BD value assuming a particle density of 2.65 g cm -3 . Data collected were subjected to analysis of variance (ANOVA) and differences between means were detected by Fisher’s least significant difference at 5% probability level (P≤0.05). At the micro aggregation level, soil from Nkalunta was significantly (P≤0.05) better (%CDI=12.7) than Umuariaga (%CDI=28.0) but was statistically similar to Itunta and Amuru soils. Mean weight diameter across 4 communities although values indicated poor macro aggregation. At 0-15cm depth, AWC and
FC were was each statistically similar similar across the communities, whereas significant (P≤0.05) differences were observed in PWP, BD, Pt, and Ksat. In all the 4 communities, with
*Corresponding author: vincento_inter@yahoo.co.uk
+2348035518350
Introduction
Cocoa is a major crop in Nigeria (Ololade et al ., 2010), and on a global scale, it succeeds on many different kinds of soils
(Ibiremo et al ., 2010).
The forest-like nature of cocoa plantation intercepts solar radiation and helps to reduce soil temperature thereby slowing down rate of organic matter decomposition as well as serving as carbon dioxide sink.
This phenomenon mitigates the negative warming effects of climate change.
However, drought induced by climate change affects tree species (Spore 2012).
Heat sensitive cocoa plants, under high temperature, require enough water during growing periods otherwise, the pods will be restricted (Spore, 2012). Therefore, water retention characteristics influenced by Pt,
BD, Ksat and infiltration, must be optimized in areas where cocoa is grown.
These physical properties also influence root development and ease of cultivation after rainfall or irrigation.
A deep-rooted plant with tap root system, cocoa tree requires deep and well drained soils (Okpeke, 2005) with high infiltration rates and saturated hydraulic conductivity.
According to Ololade et. al . (2010), soils supporting cocoa tree contain significant percentage of sand in some areas while high proportions of silt and clay dominate in other areas.
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Cocoa tree, sensitive to drought, can only withstand water logging for a short period.
Therefore, soils sustaining cocoa tree must have both good moisture retention and good drainage properties. To achieve these, deep loamy soil is preferred while shallow soils should be avoided. A major factor affecting cocoa production in Nigeria is dearth of information on soil characteristics of cocoa growing areas due to limited research (Fasina et. al ., 2007). Scanty literature exists on the chemical properties
(Oyewole et al ., 2012; Aikpokpodion,
2012; Ibiremo et. al ., 2010; Aweto and
Obe, 1993) while information on physical properties of cocoa growing soils is virtually non existent.
The objectives of the study are: (i) to evaluate the physical properties of cocoa growing soils and (ii) to provide information on soil physical properties for land evaluation system for cocoa production.
Materials and Methods
Sampling location
Soil samples were collected from Ikwuano local government area, Abia state, located at latitudes 5
7 0 34’ to 7 0
0 26’ to 5
36
’
0 29
’
N and longitudes
E in southeastern Nigeria with rainforest vegetation type. The area occupies about 281 km 2 with a mean annual rainfall of 2238 mm, minimum and maximum temperatures of 23 and 32 0 C, respectively. Relative humidity is in the range of 63-80% (NRCRI, 2003).
Soil Sample Collection
Auger and core samples were collected from 10 cocoa plantations in each of 4 communities in Ikwuano comprising
Umuariaga, Itunta, Nkalunta and Amuru.
Auger samples from various parts of each plantation were collected at 0-30cm depth, composited and used for the determination of particle size distribution, CDI and
MWD.
Three replicate core samples were collected at two depths (0-15 and 15-30cm) and used to determine FC, PWP, BD and Ksat.
Total porosity (Pt) was computed from BD value assuming a particle density (PD) of
2.65 kg m -3 thus:
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Pt =1 – (BD/PD) x 100 …. (1)
Available water content (AWC) was deduced from the difference between FC and PWP as follows:
AWC = FC – PWP …. (2)
Laboratory Analysis
The auger samples were air-dried and a portion was sieved with a 5.6mm diameter sieve size and used for the determination of
MWD. The other portion, passed through a
2-mm diameter sieve size, was used to determine particle size distribution and
CDI. The core samples were saturated in water and used for determination of BD,
Ksat, FC and PWP.
Particle size analysis was by Bouyoucos hydrometer method described by Gee and
Bauder (1986) with calgon (sodium hexa metaphosphate) as dispersing agent. The samples were also dispersed in water to enable the computation of CDI (Igwe et al .,
1995) thus:
... (3)
Mean weight diameter (MWD) was determined by wet-sieving method of
Kemper and Rosenau (1986). Saturated hydraulic conductivity (Ksat) was by the constant head permeameter method.
Darcy’s equation as explained by Youngs
(2001) was used for the computation of
Ksat as shown:
… (4)
Where Q is water discharge (cm 3 ), L is length of soil column (cm), A is cross sectional area of the soil column (cm 2 ),
H is the head pressure difference causing flow (dimensionless) and T is the time of flow (sec.)
Water retained at FC and PWP were determined using the saturation water percentage (SP) based estimation models of
Mbagwu and Mbah (1998) as follows:
FC = 0.79 (SP) – 6.22 (r = 0.972) … (5)
PWP = 0.51 (SP) – 8.65 (r = 0.949) .. (6)
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Available water content (AWC) was deduced as shown in equation 2. Bulk density (BD) was determined using core method as described by Anderson and Ingram (1993). Total porosity (Pt) was calculated as shown in equation 1.
Statistical Analysis
Data were subjected to analysis of variance (ANOVA). Fisher’s least significant difference
(LSD
0.05
) was used to detect differences between means.
Results and Discussions
Particles size analysis.
Distribution of soil particle sizes in the 4 communities studied is shown in Table 1.
Although the means were not statistically analyzed, soil from Umuariaga contained higher sand fraction than the other 3 with Nkalunta containing the least. The order was Umuariaga
(50.7%) > Amuru (43.5%) > Itunta (43.2%) > Nkalunta (41.2%).
For the silt fraction, soil from Nkalunta (51.6%) contained more than the others followed by
Amuru (34.0%), Umuariaga (33.2%) and Itunta (31.6%) in descending order. Varying clay contents were observed in the 4 communities with soil from Nkalunta containing more than the others. Not withstanding the variations in particle size distribution, the texture was generally loam.
Table 1. Texture of soils under cocoa plantation.
Location
Umuariaga
Itunta
Nkalunta
Amuru
% sand
50.7
43.2
41.2
43.5
% silt
33.2
31.6
51.6
34.0
% clay
20.1
25.4
30.2
22.5
Texture
Loam
Loam
Clay loam
Loam
The higher sand content in Umuariaga soil could be attributed to their being derived from unconsolidated sand deposits, formed over coastal plain sand and sedimentary rocks
(Obigbesan et.al
.,1981). Textures of soils are related to their parent materials (Akamigbo and Asadu, 1983) which accounted for the similarity of particle size distribution observed in the soils studied. Igwe et al .(1999) made similar observations when they reported that soils derived from different geologic formations varied in particle size distribution.
Aggregate stability
Stability of aggregates did not vary across the communities except that the soil from
Umuariaga significantly (P < 0.05) showed the least micro aggregate stability (Table 2).
Similarity in aggregation and aggregate stability indicated similar aggregation mechanism
(Strickland et al ., 1988).
Table 2. Mean weight diameter and clay flocculation index
Location MWD (mm) % CDI
Umuariaga
Itunta
1.51
1.30
28.0
17.3
Nkalunta
Amuru
LSD (
0.05
)
1.42
1.34
0.33
12.7
18.3
6.71
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Although highest MWD value of 1.51mm was recorded for soil from Umuariaga, macro aggregate stability was low across the communities. At the micro aggregation level, stability was also low with soil from Nkalunta significantly (P < 0.05) better that the others.
The low micro and macro aggregate stabilities of soils under the cocoa plantations were contrary to expectation as their OM contents
(not analyzed) were anticipated to be enhanced due to litter fall. The observed low micro and macro aggregate stabilities in this study were therefore inconsistent with the results reported by Oguike and Mbagwu
(2009) who observed increased aggregate
Pt of the soils studied at 0-15 and 15-30 cm depths, respectively. Water retained at FC and PWP across the communities were statistically similar though, soil from
Nkalunta retained more water than the others with Umuariaga soil retaining the least.
However, at 0-15cm depth, with reference to
PWP, soil from Nkalunta significantly (P <
0.05) retained more water than soil from
Umuariaga. The trend in all cases indicated an inverse relationship between water retention and BD.
At both depths, soil from Nkalunta was more porous than the others. At 0-15cm depth, with reference to Pt, soil from Nkalunta was stability of soils under forest and bush fallow lands.
These soils appear to be dominated by micro aggregates following the observed slow Ksat probably as a result of dominant microspores.
This assumed micro aggregation was probably due to soil disturbance during land preparation for cocoa cultivation when macro significantly (P < 0.05) more porous than
Umuariaga and Itunta soils. At 15-30cm depth, soil from Nkalunta was also significantly (P < 0.05) more porous than soil from Umuariaga but was statistically similar to Itunta and Amuru soils.
The slow Ksat of these soils suggested a preponderance of micro pores due to their aggregates were destroyed and reduced to smaller aggregating units. The micro aggregates were expected to be stable
(Nichols, 2011).
Water retention, bulk density, total porosity and saturated hydraulic conductivity
Tables 3 and 4 show the water retaining and fine textures which reflected in their AWC.
Slow Ksat of the soils negated its relationship with low BD and enhanced Pt and is inconsistent with the report of Okpeke (2005).
This observation was also not consistent with the reports of other researchers (Mbagwu al.
,1983; Igwe
2006). et al.
, 1995; Oguike, conducting properties as well as the BD and
Table 3. Water retention, bulk density, porosity and saturated hydraulic conductivity.
(0-15cm)
Location % FC % PWP % AWC BD (g m -3 ) %Pt Ksat (cm sec -1 ) et et al.
,
Umuariaga 39.0 18.0 21.0
Itunta
Nkalunta
Amuru
40.1
42.3
40.3
20.1
21.0
20.0
20.0
21.3
20.3
0.90
0.80
66.0 0.06
69.8 0.05
0.70 74.0 0.03
0.75 72.0 0.05
0.11 4.14 0.02 LSD (
0.05
) 4.82 2.5 4.7
Table 4. Water retention, bulk density, porosity and saturated hydraulic conductivity.(15 -30cm)
Location % FC % PWP
Umuariaga 40.3 20.0
Itunta 40.1 20.1
% AWC
20.3
20.0
22.6
BD (g m
1.4
1.2
-3 ) %Pt Ksat (cm sec
47.2
54.7
0.03
0.04
-1 )
Nkalunta
Amuru
LSD (
0.05
)
43.0 20.4
42.0 20.2
4.04 2.02
21.8
3.01
1.0
1.2
0.4
62.3 0.02
54.7 0.04
14.0 0.01
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Conclusion
Soil with both good water retention and drainage properties favours cocoa production. Therefore, the loam and clay loam textures of the Ikwuano soils on which cocoa is grown are desirable.
Cocoa plantations, similar to and resembling secondary forests, provide very long-lasting planted- fallow periods capable of reducing climate-induced soil degradation in the rainforest agro ecology thereby optimizing and preserving soil physical properties.
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