Pedology of Near Gully Sites and its Implications on the Erodibility of Soils in Central South Eastern Nigeria
Near gully sites in southeastern Nigeria were investigated from April to July 2005 to ascertain their erodibility potentials. Soils were sandy and of low Silt-Clay Ratio (SCR) but were eroding. Exchangeable Sodium Percentage (ESP), carbon content, silt, clay and Calcium-Magnesium Ratio (CMR) tended to influence erodibility of these soils. They correlated significantly with Dispersion Ratio (DR) (p≤0.05). Exchangeable sodium percentage was higher in pedons near gully sites (ESP = 1.4-5.1) compared with results of non-gully site (ESP = 0.1-0.3). Calcium-magnesium ratio (CMR) was generally narrow (0.3-3.0). Organic matter content was very low in pedons near gully sites (less than 11%). Cation exchange capacity was lower in sites affected by gullying (CEC = 3.50-7.00 cmol kg-1). Low base saturation (less than 40%) shows very strong leaching of basic cations in the study area.
Soil erosion is a predominating soil degradative agent in southeastern Nigeria and is believed to be related to soil properties among other non-soil contributors. Currently, the spate of soil erosion which comes in various forms is so aggravated that it threatens life and property in the sub-region. In addition to non-human factors, such as climate man has been implicated as facilitating soil erosion by inappropriate land use practices (Eremie, 1990). All these manifest in reduced productivity of agricultural lands (Oti, 2002) and increased poverty (Mbagwu and Obi, 2003).
Indices have been proposed to assess potential soil loss in soils of southeastern Nigeria. Obi et al. (1989) observed that in southeastern Nigeria, the soil aggregate stability technique was the least satisfactory index for determining erodibility of soils. Mbagwu (1986) evaluated the relative erodibility values of soils formed on a toposequence by use of various erodibility indices and observed that clay ratio, dispersion ratio and dispersion index gave better estimation of soil loss. Igwe et al. (1995) reported that Dispersion Ratio (DR), Wishmeiers erodibility (K), Clay Dispersion Index (CDI) and Clay Flocculation Index (CFI) ranked higher in predicting soil loss. However, they noted that organic carbon and Fe2O3 control flocculation and deflocculation properties of these soils in southeastern Nigeria.
In all these, the pedology of soils proximal to sites of various forms of soil erosion has not been widely studied. The major objective of this study was to investigate influence of pedological processes on soils near gully sites as such may be instrumental to the formation of adjoining gullies and consequent reduction in cultivable land.
MATERIALS AND METHODS
The study area comprises Imo and Abia States of southeastern Nigeria lying
between latitudes 4°401 and 8°151 North and longitudes
6°401 and 8°151 East (Federal Department of Agricultural
Land Resources, 1985). The geological materials are mainly alluvial deposits,
coastal plain sands (Benin formation), shale (Bende-Ameki formation), upper
coal measures (Nsukka formation) and falsebedded sandstones (Orajaka, 1975).
Its relief is between 50-400 m (Ofomata, 1987). Geomorphologically, it is a
lowland with north-lying hills (Ofomata, 1975). The study site is in the humid
tropics with annual rainfall reaching 3000 mm in some parts and having rainfall
intensities up to 200 mm h-1 (Obi and Asiegbu, 1980). There are two
main categories of vegetation which are being rapidly deforested by increasing
human population and attendant activities. Farming is a major socio-economic
activity, with soil fertility regenerated by bush fallows whose length is drastically
reduced due to demographic pressure (Onweremadu, 1994).
Field studies are conducted from April to July 2005 in the study area. Six
sites, namely Mbaise, Umuahia, Bende, Okigwe, Orlu and Owerri were chosen after
a reconnaissance visit. Gullies were found in the first 5 locations while Owerri
was used as a control site for the study. Pedons were dug at the edges of the
gullies and the control site. These pedons were described according to the FAO
Guidelines for profile pit description (1998) while soil classification of was
done according to the rules set out in soil taxonomy (Soil Survey Staff, 2003).
Soil samples were collected from pedons based on the degree of horizon differentiation
and slope was measured using Abney level. All sample points were located on
a gentle to undulating topography.
Soil samples were air-dried and sieved using 2 mm sieve mesh. Particle size
distribution was determined by hydrometer method (Gee and Bauder, 1986). Soil
pH was estimated by the method of Hendershot et al. (1993). Soil organic
carbon was measured by Walkley and Black method (Nelson and Sommers, 1982).
Total nitrogen was obtained by microkjeldahl method (Bremner and Mulvaney, 1982).
Exchangeable bases and cation exchange capacity was determined by with neutral
1N NH4OAc (Jackson, 1962). Dispersion ratio (Middleton, 1930) was
used to assess erodibility of studied soils. Dispersion Ratio (DR) was computed
Soil data were subjected to correlation analysis using SAS version 8.2 (SAS
RESULTS AND DISCUSSION
Table 1 and 2 summarize physical and
chemical properties of studied pedons. Generally, soils were sandy. This implies
that these soils are least susceptible to erosion (Dupriez and Deleener, 1992)
as they have abundant macropores capable of allowing entry and movement of water
into the deeper layers of the pedon. Contributing, Obi and Asiegbu (1980) reported
low transportability of sand-sized fractions. These notwithstanding, deep gullies
were developed in the sites except at Owerri. All soils studied are erodible,
having Dispersion Ratio (DR) greater than 15% (Table 1), although
Owerri site is currently not eroding. However, these non-eroding status of Owerri
site could be attributed to lower volume of runoff water that enters the site.
Selected chemical properties of soils of the studied sites
CEC = Cation Exchange Capacity, B.S. = Base Saturation,
ESP = Exchangeable Sodium Percentage, C = Carbon, OM = Organic Matter,
TN = Total Nitrogen
Morgan (1978) suggested the use of potential soil loss, stating that an increasing
value trend to erodibility but results of the study show low values (Table
1), implying that soils have been exposed to weathering for a long period.
Values (SCR<1.2) suggest high weatherability of soils (van Wambeke, 1962)
which according to Morgan (1978) means low erodibility, but soils were actually
The foregoing clearly shows that sandy textural classes may not have influenced
erodibility (Bazzoffi and Mbagwu, 1986) as much as organic components of the
soils. Organic matter content of soils was low and C/N ratio narrow (Table
2) pointing to high mineralization (Eshett et al., 1989). This suggests
poor aggregation and high vulnerability to erosion. Organic matter has the capacity
to chelate metal ions, such as Fe3+ and Al3+ but these
chelates are temporarily withdrawn from soil system leading to pronounced erosion
(Tan, 1978). Igwe et al. (1995) observed that dispersion and flocculation
are influenced by status of metal-organic complexes in tropical soils.
Values of Ca-Mg ratio (Table 2) show increased leaching and
this varied among soils as least values were found in soils of Owerri site.
Inter-and intra-pedal variabilities are possibly due to differential land use
and rapidity of pedogenesis, respectively. The low values of Ca/Mg in the entire
study point to high levels of loss of these basic cations and this leads to
phenomenal decrease in Ca and P (Landon, 1984). The above effect is worst in
acidic soils (Oti, 2002).
Exchangeable Sodium Percentage (ESP) was higher in near gully sites than in
non-gully sites (Table 2). Values of ESP were higher in lower
horizons of pedons near gully sites especially in Mbaise and Umuahia. This could
be responsible for the deepening nature of these gullies. Earlier, Imeson and
Kwaad (1980) used the ESP to estimate the dispersive nature of lower horizons
proximal to gully sites, concluding that lower horizons close to gully sites
are more dispersive than those away from gullies.
Lower values of CEC experienced in pedons near gullies are suggestive of predominance of 1:1 type clay minerals (Igwe et al., 2002) which are more dispersive that 2:1 clay mineral like montmorrillonite. This means that aggregative power of clays depends on type.
Results of correlation analyses of erodibility and soil properties are shown
in Table 3. Exchangeable Sodium Percentage (ESP) was correlated
significantly with DR (R = 0.89; p≤0.05) and the same trend was followed by
soil carbon (R = 0.88; p≤0.05). Other significant correlation coefficients
were found between silt and DR (R = 0.82, p≤0.05), clay and DR (R = 0.38,
p≤0.05), CMR and DR (R = -0.73; p≤0.05). The results show that ESP, soil
carbon, silt, clay and CMR could be used for prediction of erodibility potential
of soils of the site. Similar findings have been made by some researchers (Igwe
et al., 1995).
A correlation matrix of indicators of erodibility and other
SCR = Silt-clay ratio, DR, Dispersion Ratio, CMR = Calcium-Magnesium
Ratio, ESP = Exchangeable Sodium Percentage, * = Significant at p = 0.05,
** = Significant at p = 0.01, ns = non Significant
Classification of studied soils
Soils were classified as shown in Table 4, indicating
that soils are highly weathered. The pedogenic status of these soils is attributed
to the combined and interactive effects of climate and organisms in the region
plus the localized impact of topography and lithology.
Soils are sandy and would seem to resist soil erosion but have chemical nature predisposing them to soil erosion. Non-soil factors like climate promote inability of these soils to resist erosion.
Exchangeable Sodium Percentage (ESP), carbon, silt, clay and CMR are highly correlated with erodibility of soils. Values of SCR were low against the popular belief that highly erodible soils have high SCR. Finally soils data should be subjected to Principal Component Analysis (PCA) to help find out the most influential pedological factors in erodibility of soils.
I remain satisfied with the technical assistance I received from Staff of Department of Soil Science, University of Nigeria, Nsukka as well as those of Erosion Institute, Federal University of Technology, Owerri all in Nigeria.
1: Bazzoffi, P. and J.S.C. Mbagwu, 1986. A structural stability ranking of some soils from North central Italy by the water stability index and sixteen other indices. Annali, 17: 89-98.
2: Bremner, J.M. and G.S. Mulvaney, 1982. Nitrogen Total. In: Methods of Soil Analysis Part 2: Chemical and Microbiological Properties, Page, A.L., R.H. Miller and D.R. Keeney (Eds.). ASA, Madison, WI., USA., pp: 595-624
3: Dupriez, H. and P. Deleener, 1992. Ways of Water: Run off Irrigation and Drainage. The Macmillan Press Ltd., London
4: Eremie, S.W., 1990. The vertiver technology, soil and water conservation for Nigeria's small farmers. A Paper Presented at the Workshop on Soil Erosion/Landslide and Rural Development for self-reliance held at the University of Nigeria, pp: 17.
5: Eshett, E.T., J.A. Omuete and A.S.R. Juo, 1989. Soil chemical properties and mineralogy in relation Southeastern Nigeria. J. Agric. Sci. Camb., 112: 377-386.
6: Anonymous, 1985. The reconnaissance soil survey of Imo Nigeria (1:250,000). Soils Rep., pp: 133.
7: FAO., 1998. Guidelines for Soil Profile Descriptions. 2nd Edn., Food and Agriculture Organization, Rome, pp: 66
8: Gee, G.W. and J.W. Bauder, 1986. Particle Size Analysis. In: Methods of Soil Analysis, Klute, A. (Ed.). Part 1. Am. Soc. Agron., Madison, WI., pp: 91-100
9: Herdershot, W.H., H. Lalande and M. Duquette, 1993. Soil Reaction and Exchangeable Acidity. In: Soil Sampling and Methods of Analysis, Carter, M.R. (Ed.). Lewis Publishers, Boca Raton, FL., pp: 141-145
10: Igwe, C.A., F.O.R. Akamigbo and J.S.C. Mbagwu, 1995. The use of some soil aggregate indices to assess potential soil loss in soils of South-Eastern Nigeria. Int. Agrophys., 9: 95-100.
Direct Link |
11: Igwe, C.A., F.O.R. Akamigbo and J.S.C. Mbagwu, 2002. Soil moisture retention characteristics in relation to erodibility and texture of some soils of Southeastern Nigeria. E. Afr. Agric. J., 68: 17-21.
Direct Link |
12: Imeson, A.C. and F.J.P.M. Kwaad, 1980. Gully types and gully predication. Geografisch, 14: 430-441.
13: Jackson, M.L., 1962. Soil Chemical Analyses. Prentice-Hall Inc., New York, pp: 498
14: Landon, J.R., 1984. Booker Tropical Manual: A Handbook for Soil Survey and Agricultural Land Evaluation in the Tropics and Subtropics. Longman, New York
15: Mbagwu, J.S.C., 1986. Erodibility of soils formed on a catenary toposequence in southeastern Nigeria as evaluated by different indexes. East. Afr. Agric. For. J., 52: 74-80.
16: Mbagwu, J.S.C. and M.C. Obi, 2003. Land degradation, agricultural productivity and rural poverty: Environmental implications. Proceedings of the 28th Annual Conference of Soil Science Society of Nigeria, November 4-7, 2003, Umudike, pp: 1-11
17: Middleton, H.E., 1930. Properties of soil which influence soil erosion. US Department of Agric. Tech. Bull., pp: 178.
18: Morgan, R.P., 1978. Field studies of rainsplash erosion. Earth Surf. Proc., 3: 295-299.
19: Nelson, D.W. and L.E. Sommers, 1983. Total Carbon, Organic Carbon and Organic Matter. In: Methods of Soil Analysis, Part 2: Chemical and Microbiological Properties Page, A.L., R.H. Miller and D.R. Keeney (Eds.)., 2nd Edn., ASA and SSSA, Madison, WI., USA, pp: 539-579
CrossRef | Direct Link |
20: Obi, M.E. and B.O. Asiegbu, 1980. The physical properties of some eroded soils Southeastern Nigeria. Soil Sci., 130: 39-48.
21: Obi, M.E., F.K. Salako and R. Lal, 1989. Relative susceptibility of some southeastern Nigeria soils to erosion. Catena, 16: 215-225.
22: Ofomata, G.E.K., 1975. Landform Regions. In: Nigeria in Maps: Eastern States, Ofomata, G.E.K. (Ed.). Ethiope Publishing House, Benin, pp: 33-34
23: Ofomata, G.E.K., 1987. Soil Erosion in Nigeria. The Views of a Geomorphologies Inaugural Lecture Series No. 7 University of Nigeria, Nsukka, Nigeria, pp: 3-33
24: Onweremadu, E.U., 1994. Investigation of soil and other related constraints to sustained agricultural productivity of soil of Owerri agricultural zone in Imo State, Nigeria. M.Sc. Thesis, University of Nigeria, Nsukka, Nigeria, pp: 164.
25: Orajaka, S.O., 1975. Geology. In: Nigeria in Maps: Eastern States, Ofomata, G.E.K. (Ed.). , Ethope Publishing House, Benin, pp: 5-7
26: Oti, N.N., 2002. Discriminant functions for classifying erosion degraded lands at Otamiri, Southeastern Nigeria. Agron. Sci., 3: 34-40.
Direct Link |
27: SAS, Institute, 2001. SAS user's guide: Statistics. Ver. 8.2, Cary, N.C.
28: Anonymous, 2003. Keys to Soil Taxonomy. 9th Edn., United States Department of Agriculture, National Resources Conservation Services, USA., pp: 332
29: Tan, K.H., 1978. Formation of metal-humic acid complexes by titration and their characterization by differential thermal analysis and infra-red spectroscopy. Soil Biol. Biochem., 10: 123-129.
30: Van Wambeke, A.R., 1962. Criteria for classifying tropical soils by age. J. Soil Sci., 13: 124-132.
CrossRef | Direct Link |