Agricultural Land Use and Soil Degradation in a Part of Kwara State, Nigeria OLUWOLE AMEYAN* and O. OGIDIOLU
Summary Because of the alarming rate of increase in population all over tropical Africa, and the consequent need to grow more food, several writers have suggested the practice of continuous or permanent cultivation in place o f the traditional bush fallowing system. This suggestion has been made without recognising the natural vulnerability of tropical soils and the associated problems of actual soil degradation, especially in situations where fertilizer inputs are limited. This study examines the effects of different land use practices on actual soil degradation in a part of Kwara State, Nigeria. This involves comparing the physical and chemical properties of the soils in areas under continuous cultivation, fallow and forests, and using the technique of factor analysis to isolate indices which best describe these phenomena. The results show that the main effects of continuous cultivation in the area examined were to increase the acidity of the soil, that soil organic matter content was likely to double after 10 years of faUow conditions, and that continuous cultivation was capable of reducing the cationexchange capacity of soils by at least one-third. In general, the soils of the area of study display marked variability, especially with respect to their chemical properties. This is mainly due to variations in soil organic matter content, which in itself is due to differences in agricultural land use practices. Factor analysis of the soil properties generated four main indices of actual * Dr J. Oluwole Ameyan, the senior author, is on the staff of the University of norin. Mr O. Ogidiolu is at the Department of Geography, Ondo State University, Ado-Ekifi, Nigeria.
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Department of Geography, University of llorin, llorin, Nigeria
soil degradation, of which organic matter is the most important. Some implications of the results are examined, particularly in relation to generating an awareness of actual soil degradation and land use planning.
Introduction
Soil degradation is a world-wide phenomenon. In tropical Africa this phenomenon is being hastened by reduction in fallow periods, and the shift from the conventional, bush fallowing system to permanent cultivation caused by population pressures on land use. Although a phenomenon of common occurrence, soil degradation has been given diverse definitions and interpretations in different environmental situations. Some definitions have viewed "soil degradation" as synonymous with "weathering". For example, Kovda (1977) defined soil degradation as the wasting or wearing down of soil by epigenetic agents. From a soil conservation point of view, Dudal (1981) defined soil degradation as the loss-of-land productivity, qualitatively or quantitatively, through many processes, such as soil erosion, overgrazing, cultivation, leaching, waterlogging, and pollution of the land. A similar definition, but with emphasis on crop production given by Nye and Greenland (1960), Faniran and Areola (I978), and Leow and Gardiner (1982) viewed soil degradation as the gradual or complete reduction in soil fertility with resultant decline in crop productivity. This gradual reduction in soil fertility may either be through the physical removal of the soil by erosion, or 285
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Fig. 1 Map of Kwara State, Nigeria, showing the location of Iludun-Oro. Dot on the inset map shows location of Tiudun-Oro within Nigeria. gradual decline in soil fertility without actual loss of the soil, or a combination of both. It should be clear from the above that soil degradation is a complex concept, brought about by the interaction of many interwoven processes. The concept can only be given correct and meaningful interpretation in specific context in relation to aims, scope and content of study. Riquer (1970) distinguished between two main forms of soft degradation - potential and actual degradation. Potential soil degradation results from the action of three main physical factors soil, climate and relief, without interference of human factors or the protection of natural vegetation. Thus potential soil degradation represents the vulnerability of the soil to 286
degradation on account of its physical properties. Actual soil degradation, on the other hand, is the degradation which actually occurs under certain types of land use and natural vegetation. It is an expression of the natural vulnerability of the soil, modified by human factors and the protective effects of vegetation. This study seeks, as a first stage, to examine the relative degree of actual soil degradation resulting from some agricultural land use practices in a part of Kwara State, Nigeria. As a second stage, indices which best explain soil degradation under these land use practices are developed using a multivariate stochastic technique. The Environmentalist
Study Area The study area was Uudun-Oro, some 53 km south east of Ilorin, the capital of Kwara State, Nigeria (Fig. 1). The area is underlain mainly by stratified metamorphosed sedimentary rocks of quartzites, quartz schist, and epidiorites, with occasional outcrops of gneiss, undifferentiated migmatites and pegmatites. The soils are generally classified as ferruginous tropical soils on acid crystalline rock (d'Hoore, 1964). Okunola (1969), however, classified the soils of the area into two main types - the Oro and Ijan soil series. The area of study lies within the Oro soil series, characterised by brown or reddish-brown gravels and mottled clay containing ironstone, manganese concretions, flakes of mica and fragments of quartz. Agriculture in the area is dominated by shifting cultivation, crop rotation and mixed fanning. Although fallow periods vary from site to site, areas close to the village of nudun-Oro are intensely fanned, the length of the fallow period generally increases with distance from the settlement. The area receives between 1250 mm and 1500 mm of rainfall during a seven month rainy season. Temperature is high throughout the year, ranging from a mean monthly minimum of about 16°C in January, to a mean monthly maximum of about 36°C in March. Except in some protected sites, the vegetation is largely secondary in nature. Common species include Parkia clappertoniana, Afzelia africana, Andropogon gayanus, and a large number of shrubs and fobs of the families
Araceae, Compositae and Orchidaceae. Methods Three land areas with differing land use practices lying less than 0.5 km from each other were chosen for study. These were (a) an area of land which had been under continuous cultivation for about five years (hereafter called cultivated land), (b) a 10- year fallow land, and (c) a protected forest land. Within each of the land use categories 20 sampling sites were randomly located and soil samples collected from the topsoils (0-15 cm depth). The soil samples collected were analysed for particle size distribution by the hydrometer method. Soil pH was determined potentiometrically in water (soil:water ratio of 1:2), exchangeable sodium, calcium and potassium were determined by flame photometry and
Volume 9, Number 4 (1989)
exchangeable magnesium by atomic absorption spectrophotometer. Exchangeable acidity was determined by a residual carbonate method (Akroyd, 1957), cation exchange capacity (CEC) by the summation method (Chapman, 1965), total nitrogen by the Kjeldahl method, available phosphorus by the Bray extraction method, and organic carbon by the wet oxidation method. Organic matter was obtained by multiplying soil organic carbon content by 1.724.
Statistical Analyses The mean value and the coefficient of variation (CV) of the data on each soil property were calculated, and one way analysis of variance used to compare the mean values of soil properties between the land use categories. Although the above procedures provided an insight into the basic patterns underlying the variability of soil degradation in the different land use categories, the combined data were also analysed to highlight the cause and nature of the correlation pattern between the various soil properties as revealed by factor analysis. Knowledge of this kind may lead to (i) an understanding of the main underlying factors of actual soil degradation in the type of environment under study, and (ii) an appreciable reduction in the sampling effort required to characterise soil degradation by restricting analyses and mapping to easily measured indicator variables. In principle, factor analysis is useful in identifying recognizable relationships among different variables contained in an extensive data set or correlation matrix. Such an identification is facilitated by the extraction of statistically significant "factors", each of which is capable of reproducing an appreciable proportion (eigenvalue) of the overall variance as reflected by the entire normalised data set. Results Table 1 summarises the laboratory data on the cultivated, fallow and forest lands, giving the mean, the range of values and the coefficient of variation for each soil property. There is a progressive decrease in soil acidity from about 7.5 in forest land, through 6.6 in fallow land, to 6.2 in the cultivated lands. This implies that the soil under continuous cultivation in the area of study is slightly acidic, while fallow and forest lands arc neutral in reaction. Thus, one of the effects of continuous cultivation in this
287
Table 1 Summary of data for topsoils of three land use categories, nudun-Oro, Nigeria. Cultivated land Range CV (%)
Fallow land Range CV (%)
Soil property
]
pH Exch. Ca++ me 100 g-I Exch. K+ me 100 g-r
6.15 2.26 0.55
5.8-6.6 1.4-3.0 0.5-1.4
4 23 96
6_57 5.44 0.71
Exch. Na+ me 100 g-1 gq Exch. acidity CEC me 100 g-I Organiccarbon(%) Organicmatter(%) Total nitrogen(%) C:N ratio AvailableP (ppm) Sand (%) Silt(%) Clay (%)
0.04 0.82 0.08 3.75 0.68 1.24 0.06 113 4.16 59.99 19.68 2232
0.03-0.07 0.4-1.4 0.04-0.34 2.1-4.8 0.17-1.06 03-1.8 0.02-0.09 8.5-12.5 0.5-2_5 44.4-68A 13.3-23.3 18.3-28.3
36 44 115 24 47 39 38 10 175 10 22 16
0.04 1.08 0.06 732 132 2.27 0.43 3.03 8.79 59.99 15.28 24.72
Exch. Mg ++ me 100
environment is to increase the acidity of the soil. The fertility status of soils under the different land use practices, in terms of organic matter, total nitrogen and the exchangeable cations shows that organic matter had the lowest mean value (about 1.24 percent) in the cultivated lands. This is mainly due to the destruction and mineralization of soil organic matter as a result of continuous cultivation and Cropping. After 10 years of fallow, however, the organic matter content had more or less doubled in the fallow lands. This result is fairly consistent with those reported by Fauck (1963) in which there was a 30 percent decrease in organic matter after two years of cultivation. The moderately high organic content of soils under forests may be attributed to the relatively abundant supply of litter by the trees. This level of organic matter status is probably responsible for the fairly high base status of soils under forests, especially of calcium and magnesium. Exchangeable calcium and magnesium account for over 83 percent, 89 percent and 94 percent o f the exchangeable cations of cultivated, fallow and forest soils respectively. The soils under the different land use categories appear to have sufficient potassium levels, and even the cultivated sites are not close to the 0.11 me 100 g-1 considered critical for arable crops in Nigeria. The cation exchange capacity is about 3.8 me 100 g-1 under cultivation. This value is more than doubled in fallow and more than tripled in forest lands. In terms of soil degradation therefore, cultivation is capable of
288
~
6.0-7.8 2.6-13.2 0.2-1.6
9 11 76
0.03-0.07 34 0.4-1.4 72 0.02-0.11 52 3.5-17.0 64 03-2.1 50 0-5-3.6 49 0.09-0.68 47 2.4-3.5 11 0.6-2.3 109 38.4-66.4 1 11.3-27.3 33 20.3-34.3 17
Forest land Range CV (%) 7.47
7.0-7.8
9.62 5.2-12.8 0.67 0.5-1.0 0.06 0.04-0.12 1.82 0.8-4.6
3
27 24 42 59
0.06
0.04-0.11
41
12.25 1.98 3.41 1.33 1.44 10.66 58.99 15.88 24.72
6.8-18.1 1.8-2.1 3.0-3.6 0.08-0.11 0.6-2.0 4.8-23.6 48.4-66.4 11.3-25.2 22.3-28.3
29 6 6 18 22 49 10 33 8
reducing the cation exchange capacity of soils by at least one-third. Textural characteristics under the different land use categories are predominantly sandy, with the sand fraction having a mean value of about 80 percent for the cultivated land, and 60 percent for the fallow and forest lands. The sandy nature of the soils may be attributed to many factors, such as d o w n w a r d eluviation o f clay, chemical destruction of kaolinite in topsoils and perhaps soil erosion which affects mainly the fine soil particles (Aim, 1970; Juo and Lal, 1977; Ameyan, 1988). There is a progressive increase in clay content from the cultivated to the fallow and forest lands, with the mean value stabilising at 24.72 percent which appears to be the equilibrium level for soil clay content in this environment. The higher clay content of soils under fallow and forests appear to suggest a reduction in clay eluviation under these land use practices. The degree of variation as shown by the CV values exhibited by the soil properties under the different land use categories, as expected, shows that perhaps with the exception of silt, the soil physical properties show less variation than the chemical properties. In agreement with results from other areas in N i g e r i a (Areola, 1984; Ameyan, 1985), the pH is the least variable of the soil properties. Organic matter varies least in forest lands, but the highest variability of this property as recorded for the fallow lands may be attributed to differing d e g r e e o f plant regeneration, and hence, of litter accumulation. Of the e x c h a n g e a b l e cations, exchangeable
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Table 2 Factor loadings, eiganvaluesand variances for the first four factors. Ur~otated factor loadings are shown in brackets. Variable
Factor 1
Factor 2
Factor 3
Factor 4
pH Exch. Ca++me 100 g-1 Exeh. K+ me 100 g'Z Exeh. Na+ me 100 g-1 Exch. Mg++ me 100 gd Exeh. acidity CEC me 100 g-1 Organic carbon (%) Organic matter (%) Total nitrogen (%) C:N ratio Available P (ppm) Sand (%) Silt (%) Clay (%) Eigenvalue Total variance (%) Cumulativevariance
0.831 (-0.861) 0.900 (0.953) 0.149 (0.323) 0.568 (0.697) 0.154 (0.397) 0.073 (0.072) 0.795 (0.938) 0.948 (0.889) 0.943 (0.888) 0.915 (0.862) 0.684 (-0.671) 0.618 (0.500) 0.007 (-0.326) 0.020 (0.237) 0.002 (0.247) 6.532 43.5 43.5
0.018(0.122) 0.245 (0.019) 0.607 (0.469) 0.538 (0.279) 0.184 (0.388) 0.001 (0.228) 0.319 (0.164) 0.054 (0.292) 0.067 (0.279) 0.011 (0.329) 0.009(0.335) 0.015 (0.307) 0.891(0.902) 0.518 (0.775) 0.850 (0.559) 2.788 18.6 62.1
0.408 (0.089) 0.189 (0.077) 0.014 (0.045) 0.043 (0.162) 0.882 (0.305) 0.067 (0.762) 0.457 (0.039) 0.027 (0.111) 0.033 (0.119) 0.054 (0.016) 0.200 (0.440) 0.350 (0.209) 0.283 (0.045) 0.393 (0.345) 0.023 (0.451) 1.380 8.5 70.7
0.106 (-0.322) 0.087 (0.001) 0.092 (0.272) 0.095 (0.341) 0.044 (0.662) 0.793 (-0.047) 0.071 (0.203) 0.006 (0.112) 0.017 (0.120) 0.113 (0.035) 0.500 (0.054) 0.050 (0.345) 0.261 (0.139) 0.621 (-0.814) 0.263 (0.419) 1.097 7_3 78.0
p o t a s s i u m exhibits the h i g h e s t d e g r e e o f variability in cultivated lands. This decreases gradually through fallow (CV = 76 percen0 to the f o r e s t l a n d s ( C V = 24 percent). Available p h o s p h o r u s also exhibits h i g h e r d e g r e e o f variation in cultivated than in fallow and forest lands. It would appear therefore that the main variations in actual soil degradation in the study area are brought about by variations in the land use practices. Statistical technique of one way analysis of variance shows that variations in soil properties between the land use types are highly significant (one percent probability level) for pH, the e x c h a n g e a b l e c a t i o n s , c a t i o n e x c h a n g e capacity, o r g a n i c matter, total n i t r o g e n and available phosphorus. Variations in soil textural characteristics are, however, not significant.
Factor Analysis of the Soil Properties H a v i n g a n a l y s e d the relative degree o f soil d e g r a d a t i o n u n d e r the d i f f e r e n t l a n d use c a t e g o r i e s , the factors r e s p o n s i b l e m a y be determined by factor analysing the combined soil data for the different land use categories. Four orthogonal (uncorrelated) factors with eigenvalues exceeding unity were extracted from the original data set. The factor loadings and eigenvalue structure are shown in Table 2. The first factor extracts about 43 percent of the total variance, and the first four factors out of fifteen account for about 78 percent of the total variation
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in the soil p r o p e r t i e s . F a c t o r l o a d i n g s are considered salient whenever their absolute values exceed 0.30 (Gorsuch, 1974). I n t e r p r e t a t i o n s o f the u n r o t a t e d f a c t o r l o a d i n g s are c o m p l e x and p r e c i s e p h y s i c a l meaning cannot be attached to the factor axes. Hence, the following interpretation of the factors is based on the varimax rotated factor loadings. T h e first factor, a c c o u n t i n g for the h i g h e s t percentage of the total variance has the highest positive loadings for organic matter and total nitrogen. Other variables with high loadings on this factor are pH (0.831), exchangeable calcium (0.900), exchangeable s o d i u m (0.568), cation e x c h a n g e c a p a c i t y (0.795) and available phosphorus (0.618). The initial correlation matrix generated among all the soil variables prior to factor analysis showed that organic matter has the highest number of other properties significantly associated with it, including total nitrogen (r = 0.99), clay (r = 0.55), exchangeable calcium (r = 0.48), e x c h a n g e a b l e p o t a s s i u m (r = 0.49), exchangeable sodium (r = 0.75), exchangeable magnesium (r = 0.60), C:N ratio (r = 0.54) and cation exchange capacity (r = 0.60). Because of the significant influence of organic matter on other properties with high loadings on factor 1, this factor is considered as the organic matter factor. The second factor has the highest loadings on sand (0.891) and clay (0.850). Other properties w i t h h i g h l o a d i n g s o n this factor i n c l u d e
289
exchangeable potassium (0.607), exchangeable sodium (0.538) and cation exchange capacity (0.319). The initial correlation matrix revealed strong positive association between clay and exchangeable potassium (r = 0.38), exchangeable sodium (r = 0.53) and cation exchange capacity (r = 0.65), and between silt and exchangeable potassium (r = 0.72). Hence, areas with high levels of fine soil fractions are rich in sodium, potassium and cation exchange capacity. Conversely, as expected, a general increase in soil sand content also implies significant decline in exchangeable sodium and potassium and cation exchange capacity. On the basis of these considerations, factor 2 is termed the soil textural factor. The third factor has high positive loadings on exchangeable magnesium (0.882), pH (0.408), cation exchange capacity (0.457), available phosphorus (0.350) and silt (0.393) and is considered the effective cation exchange capacity factor. Finally, the fourth factor is almost totally produced by exchangeable acidity, hence it is called the exchangeable acidity factor. Therefore the main variations in soil degradation in this environment appear to derive from variations in organic matter content and the associated variations in soil nutrient levels, as well as differences in soil textural characteristics brought about by differences in clay eluviation under different land use practices. Conclusions In this study soil degradation in terms of gradual decline in soil fertility has been examined by comparing soil properties under three main land use categories. The reduction in soil fertility associated with cultivation and cropping has also been estimated. It has been shown that the marked variability of soil properties, especially with respect to soil chemical properties arise mainly from differences in land use practices. Four basic factors - organic matter, soil texture, effective cation exchange capacity, and soil exchangeable acidity - appear to be the dominant indices of soil degradation in this environment. Additional indices are obviously present, but are more difficult to identify as such by factor analysis. Preparation of maps showing the risks of soil degradation and its rate is one of the more pragmatic ways to generate the required awareness of this phenomenon and encourage sound land use planning. The results from this study suggest that the first division which may be 290
most usefully made in the mapping of soil degradation in this environment is on the basis of soil organic matter characteristics. References Aim, P.M. 1970. West African Soils. Oxford University Press, Oxford. Akroyd, T.N.N. 1957. Laboratory Testing in Soil Engineering. Soil Mechanic Ltd. Chelsea; Marshal Press Ltd, London. Ameyen, O. 1985. Variability of soils developed on migmatites in a part of the middle-belt of Nigeria.
Applied Geography, 6, 309- 323. Ameyen, O. 1988. The soil factor in crop production: An exploratory study in a humid tropical environment.
Agric. Systems, 26 (1), 51-64. Areola, O. 1984. The characteristicsand fertilityslams of the soils of the old cocoa farms of Ibadan region, Nigeria. Malaysian Journal of Tropical Geography, 10, 1-11. Chapman, H.D. 1965. Cation-exchangecapacity, pp.851-901. In: Black, G.A. (ed) Methods of SoU Analysis, 2. d'Hoore, LL. 1964. Soil Map of Africa, 1:500000. Explanatory Monograph Comm. for Tech. Co-op. in Africa. Joint Project No.ll, Lagos. Dudal, R. 1981. An evaluation of conservation needs. In: Morgan, R.P.C. (ed.),Soil Conservation Problems and Prospects, pp.3-12. John Wiley and Sons, New York. Faniren, A. and Areola, O. 1978. Essentials of Soil Study (with special reference to tropical areas). Heineman, London. Fauck, R. 1963. The sub-group of leached ferruginous tropical soils with concretions. African Soils, 5, 407-429. Gorsuch, R.L. 1974. Factor Analysis. W.B. Saunders Co., Philadelphia" PA. Juo, A.S.R. and LaL R. 1977. The effects of fallow and continuous cultivation on the chemical and physical properties of an Alf'tsol in western Nigeria. Plant and Soil, 47, 567-584. Kovda" V.A. 1977. Soil loss: an overview. Agroecosystem, 3, 205- 224. Leow, K.S. and Gardiner, T. 1982. Soil degradation as a physical constraint for land use planning with special reference to northern Nigeria. In: Nutalaya" P. et aL
(eds,), Soil, Geology and Landforms: Impact on Land Use Planning in Developing Countries, Association of Geoscientists for International Development, A.8.1-A.8.11. Nye, P.H. and Greerdend, D3. 1960. The soil under shifting cultivation. Comm. Burr. Soil Tech. Comm., VoL51, Harpenden, England. Okunola, Y.A. 1969. The reconnaissance survey of Offa and Omu- aran area in Kwara state of Nigeria
(mimeo). Riquer, J. 1977. Land resource degradation. In: Land Resource for Population of the Future. Report of an FAO/UNFA expert consultant. FAO, Rome.
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