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Characterization of selected sandstone-derived soils in the ecological zones of Nigeria
Geoderma, 13 (1975) 3 3 1 - - 3 4 7 © Elsevier Scientific Publishing C o m p a n y , A m s t e r d a m - - Printed in The Netherlands
CHARACTERIZATION OF SELECTED SANDSTONE-DERIVED SOILS IN THE ECOLOGICAL ZONES OF NIGERIA
J.A. O G U N W A L E l, T.I. A S H A Y E 2, C.T.I. O D U I and A.A.A. F A Y E M P Department of Agronomy, University of Ibadan, Ibadan (Nigeria) 2 Institute of Agricultural Research and Training, Moor Plantation, Ibadan (Nigeria) (Received October 19, 1973; revised version accepted November 27, 1974)
ABSTRACT Ogunwale, J.A., Ashaye, T.I., Odu, C.T.I. and Fayemi, A.A.A., 1975. Characterization of selected sandstone-derived soils in the ecological zones of Nigeria. G e o d e r m a , 13: 331--347. Sandstone-derived soils constitute a b o u t 18% of the land surface o f Nigeria. These soils o c c u r in all the major ecological zones and c o n s e q u e n t l y are f o u n d under a wide range of rainfall and vegetation. The m o r p h o l o g y and c o m p o s i t i o n o f a group o f profiles selected to represent soils o f the several zones have therefore been studied. The characteristics of the profiles can in part be related to the ecological zones and in part to the parent rock. S o m e characteristics are n o t clearly related to either. Both pH and base saturation decrease with increasing rainfall. The highest values are in the low rainfall z o n e and the lowest in the high rainfall zone. Soils o f the m e d i u m rainfall zone are marked at d e p t h by greater a c c u m u l a t i o n s o f silicate clays, iron oxides, and a l u m i n i u m oxides than soils o f b o t h low and high rainfall regions. Kaolinite is the d o m i n a n t clay mineral in all o f the soils. Minor p r o p o r t i o n s of o t h e r minerals are present, and t h e y s e e m to increase slightly with decreasing rainfall. Textures o f all profiles strongly reflect the nature o f the parent materials. The sand fractions are i m p o r t a n t in all parts of all profiles and are d o m i n a n t t h r o u g h o u t s o m e of them. INTRODUCTION
Soils derived from sandstones cover about 160,000 km 2 (65,000 square miles), which is about 18% of the surface area of Nigeria. These soils, reported by Vine (1954) to be chemically poor, are planted to a wide range of tree crops such as kola, off-palm and rubber in the southern parts of Nigeria, whilst in the northern parts they support industrial and food crops such as cotton, sugar-cane, guinea-corn and millet. Earlier pedological studies in Nigeria (Doyne and Watson, 1933; Vine, 1949; Nye, 1955a; Smyth and Montgomerry, 1962) have centered on cocoa-supporting softs found on metamorphic and igneous rocks rather than the softs derived from sandstones. Reports on the properties of soils derived from sandstones (Vine, 1956; Moss, 1960; Klinkenberg, 1963; Jungerius and Levelt, 1964; Valette and Adeyemo, 1964; Hansell, 1965) do not bring out relationships between the chemical, physical and mineralogical properties of the soils.
332 The objectives of this paper are to present the chemical, mechanical and mineralogical properties of selected sandstone-derived soils of Nigeria and to relate these properties to the distribution of the soils, climate and vegetation. Materials and m e t h o d s
Soil samples were collected from sites located within sandstone areas according to a geological map of Nigeria (Carter, 1964). Sample sites were selected on the basis of geological formations, ecological zones, and rainfall, as given in Table I. The sites are shown in Fig.1. /S
/ (j'"
-, .8~
/ ~-~'~-/
~
MOKWA
r~( /f
~o I,PE"U L_ "
~ .... / BENIN
..r
-4
0 4° i
8° i
2 4 0 KM.
4%
12° i
Fig.1. A map of Nigeria showing soil sampling sites. The profiles were described according to the Food and Agricultural Organization handbook - - " G u i d e to the study of soil in the field" (FAO, 1965). Soil samples were air-dried and passed through a 2-mm sieve. These air-dried samples were used in the chemical, mechanical and mineralogical analysis. Each soil sample was analysed for calcium, magnesium, potassium, sodium, exchangeable acidity, available phosphorus (Bray and Kurtz P~) and organic matter following the method outlined by Jackson (1958). CEC (cation exchange capacity) was determined by the saturation method (Jackson, 1958). Iron and aluminium were extracted with sodium dithionite-citrate solution (McKeague and Day, 1966). Iron was determined on an atomic absorption spectrophotometer. Aluminium was determined colorimetrically using the alizarin red-S method of Shapiro and Brannock (1962). Mechanical analysis was done by the method of Kilmer and Alexander (1949). The pretreatment method of Brewer (1964) was followed in mineralogical analysis. Fractionation and preparation of slides for X-ray analysis were carried out as outlined by Jackson (1968). X-ray diffraction pattems were obtained for Mg-saturated and glycerolsolvated clay samples. Similar diffraction patterns were obtained for K-saturated and glycerol-solvated clay samples, after heating to 550°C.
Ikenne
Benin
Iperu--Agege
Benin--Alagba
Sombreiro Warrideltaic
Benin
Abeokuta
Nupe
wet lowland rain forest
rain forest
dry lowland
southern Guinea Savanna
200--300
150--200
125--150
100--125
Sources: Klinkenberg (1963), Carter (1964), Hopkins (1965), and Reyment (1965).
Agbaru--Etinan
Nupe
Bima
Bima
Mokwa--Danggappe
175--100
northern Guinea Savanna
Gombe
Gombe
Annual rainfall range
Gombe--Gombe
Ecological zone
(cm)
Geological formation
Sandstone type
Soil sampling localities and series
Geological and ecological data on some sandstone areas of Nigeria ~
TABLE I
Quaternary (PliocenePleistocene)
Quarternary (Pliocene-Pleistocene)
Cretaceous (Senonian)
Crecaceous (Senonian)
Cretaceous (Senonian--Albian)
Tertiary (Paleocene)
Sandstone age
C~ C~ e~
334 Each silt sample was p a c k e d in an a l u m i n i u m b o x m o u n t for X-ray analysis. A Philip's d i f f r a c t o m e t e r using a graphite m o n o c h r o m a t o r , Cuka radiation, and a s c a n n i n g speed o f 2°/min was e m p l o y e d . MORPHOLOGY OF SELECTED PROFILES
Gombe series (Gombe) The pit site was close t o the V e t e r i n a r y Office on the left-hand side o f t h e G o m b e - - K a f a r a t i road. T o p o g r a p h y was gently sloping t o the s o u t h e a s t . T h e well-drained soil profile was d r y o n t o p and m o i s t in the d e e p e s t h o r i z o n . Parkia clappertoniana, Cassia sp., Ctenium sp. and Sida sp. were the m o s t c o m m o n plants. The p r e s u m e d p a r e n t material is the G o m b e s a n d s t o n e . Colours given are f o r m o i s t c o n d i t i o n s . Depth (cm) 0-- 20 20-- 70
70-- 90
90--140
140--165
Profile description dark brown (7.5YR 4/4) sand; weak medium subangular blocky structure; friable; common fine roots; few termite nests; few medium vesicular pores; clear wavy boundary; pH 6.2 yellowish red (5YR 4/6) fine sand; weak medium subangular blocky structure: friable; common medium subangular blocky quartz grains; few manganiferous concretions; few medium roots; few medium vesicular pores; smooth clear boundary; pH 5,6 yellowish red (5YR 4/6) sandy loam with about 20% light yellowish brown (10YR 6/4) mottles; moderate medium subangular blocky structure; friable; common medium quartz grains; few medium manganiferous concretions; few coarse vesicular pores; smooth clear boundary; pH 5.7 red (2.5YR 4/6) sandy clay loam with about 5% light yellowish brown (10YR 6/4) mottles; moderate medium/coarse subangular blocky structure; firm; common fine/medium quartz grains; few coarse roots; common medium and few coarse interstitial pores; sharp wavy boundary; pH 5.1 red (2.5YR 4/6) sandy clay loam; gravelly layer on top of reddish brown sandstone grit; pH 4.8
Danggappe series (Mokwa) The soil profile was l o c a t e d o n the M o k w a agricultural farm. T o p o g r a p h y was flat. The well-drained soil profile was fairly d r y w h e n described. Natural vegetation had been cleared and grasses o f weed t y p e s were growing a r o u n d the pit. Gmelina arborea was the d o m i n a n t tree p l a n t e d a r o u n d the pit, a n d t h e c o m m o n grass was Brachiaria deflexa. The p r e s u m e d p a r e n t material is the N u p e sandstone. Colours given are for m o i s t c o n d i t i o n s . Depth (cm) 0--18
Profile description dark reddish brown (2.5YR 2/4) sand; moderate medium subangular blocky structure; slightly hard when dry; common medium grass roots; few termite nests; few medium tubular pores; wavy sharp boundary; pH 6.1
335
18-- 48
48-- 95
95--145
145--288
dusky red (10R 3/4) sandy loam; strong medium subangular blocky structure; hard when dry; few medium roots; common termite nests (25--30 cm long and 15 cm wide); few coarse stones; few medium vesicular pores; smooth clear boundary; pH 5.8 dark red (10R 3/6) coarse sandy clay loam; strong coarse subangular blocky structure; hard when dry; few medium fibrous roots; c o m m o n termite nests; few medium vesicular pores; smooth diffuse boundary; pH 5.0 dark red (10R 3/6) coarse sandy clay; strong coarse subangular blocky structure; hard when dry; few termite nests; few medium vesicular pores; thin clay coatings on termite nests and old root channels; diffuse smooth boundary; pH 4.3 similar to the above horizon (95--145 cm) except for the presence of common medium quartz grains between depths of 210 and 288 cm
Agege series (Iperu) The soil profile was located at Iperu on land surveyed for sugar-cane production assessment on the left-hand side of the Ibadan--Lagos road. The topography was fairly flat to gently sloping to the south. The moderately well-drained soil profile was moist when described. The most c o m m o n plant species were Alchornea cordifolia, Millettia thonningii, Holarrhena floribunda, and grasses belonging to the Andropogoneae family. The land was planted to cassava intermixed with some pepper. The presumed parent material is Ikenne sandstone. Colours given are for moist conditions. Depth (cm) 0-- 18 18-- 43
43-- 70
70-- 90
90--180
Profile description brown (7.5YR 4/2) loamy sand; weak medium granular structure; abundant earthworm casts; c o m m o n medium roots; sharp wavy boundary; pH 6.0 dark brown (7.5YR 4/4) sandy clay; weak medium granular structure; friable when moist; few medium reddish brown concretions (2--8 m m diameter); few horizontal e m p t y animal burrows (2--3 cm diameter); few medium soft roots; smooth clear boundary; pH 4.8 strong brown (7.5YR 5/6) fine sandy clay with about 5% yellowish brown (10YR 5/8) mottles; moderate medium subangular blocky structure; friable when moist; c o m m o n hard spheroidal and subangular blocky brownish red concretions; few quartz gravels; few medium decaying roots; clear irregular boundary; pH 4.5 yellowish brown (10YR 5/6) fine sandy clay with about 25% yellowish red (5YR 5/6) mottles; strong medium subangular blocky structure; friable when moist, slightly sticky when wet; c o m m o n hard medium reddish brown spheroidal/subangular blocky concretions; few quartz gravels; few decaying medium roots; gradual smooth boundary; pH 4.4 light yellowish brown (10YR 6/4) fine sandy clay with about 45% coarse yellowish red (5YR 5/8) mottles; strong medium subangular blocky structure; friable when moist, slightly sticky when wet; c o m m o n medium/coarse reddish brown subangular blocky concretions; very few fine roots; pH 4.5
336
Alagba series (Benin) T h e soil p r o f i l e was l o c a t e d in t h e o l d o i l - p a l m p l a n t a t i o n o f t h e N i g e r i a n I n s t i t u t e f o r Oil Palm R e s e a r c h a t Benin. T h e t o p o g r a p h y was a l m o s t fiat o n a hill t o p . T h e c o u n t r y is rolling. T h e w e l l - d r a i n e d soil was m o i s t w h e n des c r i b e d . T h e m o s t c o m m o n p l a n t s p e c i e s was Elaeis guineensis (oil p a l m ) . T h e p r e s u m e d p a r e n t m a t e r i a l is Benin s a n d s t o n e . C o l o u r s given are f o r m o i s t conditions. Depth (cm) O-- 12 12-- 58
58-- 83 83--117
117--152
192--252
252--308
Profile description dark reddish brown (5YR 3/3) loamy sand; very weak fine granular structure; loose when moist; abundant fine roots; clear wavy boundary; pH 4.7 reddish brown (2.5YR 4/4) sandy loam; weak fine subangular blocky structure; very friable when moist; abundant fine fibrous roots; clear wavy boundary; pH 4.2 dark red (10R 3/6) sandy clay loam; weak medium subangular blocky structure; friable when moist; few medium roots; clear irregular boundary; pH 3.5 dark red (10R 3/6) sandy clay loam; moderate medium subangular blocky structure; friable when moist; common medium fibrous roots; thin clay coatings in former root channels; few medium vesicular pores; clear smooth boundary; pH 3.5 dark red (10R 3/6) sandy clay loam; moderate medium subangular blocky structure; friable when moist; common medium fibrous roots; few white-ant channels; very thin clay coatings in former root channels; few coarse vesicular pores; smooth diffuse boundary; pH 3.5 dark red (10R 3/6) sandy clay loam; moderate coarse subangular blocky structure; friable when moist; few medium root channels; thin clay coatings in root channels; common medium vesicular pores; diffuse smooth boundary; pH 3.5 dark red (10R 3/6) sandy clay loam; moderate coarse subangular blocky structure; friable when moist; few medium fibrous roots; thin clay coatings in root channels; few medium vesicular pores; pH 3.9
Etinan series (Agbaru) T h e soil p r o f i l e was l o c a t e d in t h e o l d f a r m s e t t l e m e n t a t A g b a r u , o n t h e l e f t - h a n d side of t h e A g b a r u - - U g h e l l i r o a d . T h e t o p o g r a p h y was a l m o s t flat. T h e c o u n t r y is g e n t l y rolling. T h e w e l l - d r a i n e d soil was m o i s t w h e n d e s c r i b e d . T h e n a t i v e v e g e t a t i o n was c l e a r e d b e f o r e c i t r u s was p l a n t e d on t h e soil. T h e m o s t c o m m o n p l a n t s p e c i e s w e r e c i t r u s a n d Calopogonium mucunoides. T h e Calopogonium sp. was u s e d as c o v e r c r o p t o p r e v e n t e r o s i o n . T h e p r e s u m e d p a r e n t m a t e r i a l is a y e l l o w s a n d s t o n e . C o l o u r s given are f o r m o i s t c o n d i t i o n s . Depth (cm) 0--
5
Profile description black (7.5YR 2/0) sandy loam; very weak fine granular structure; very friable when moist; few earthworms; common fine fleshy roots; clear wavy boundary; pH 4.5
337
5-- 10 10-- 20
20-- 55
55-- 95
95--115
115--180
dark brown (7.5YR 4/2) sand; very weak fine granular structure; loose when moist; few earthworms; common fine roots; sharp wavy boundary; pH 4.4 grayish brown (10YR 3/2) loamy sand; weak fine granular structure; very friable when moist; few earthworms; common fine roots; clear wavy boundary; pH 4,3 brown (7.5YR 5/4) loamy sand with about 15% strong brown (7.5YR 5/6) mottles; weak fine granular structure; very friable when moist; few medium fibrous roots; common medium vesicular pores; diffuse wavy boundary; pH 4.5 strong brown (7.5YR 5/6) loamy sand with about 20% brown (7.5YR 5/4) mottles; weak medium subangular blocky structure; friable when moist; common medium fibrous roots; common coarse interstitial pores; diffuse wavy boundary; pH 4.3 strong brown (7.5YR 5/6) fine sandy loam with about 25% strong brown (7.5 YR 6/6) mottles; moderate medium subangular blocky structure; friable when moist; few medium fibrous roots; common coarse vesicular pores; diffuse smooth boundary; pH 4.2 strong brown (7.5YR 5/6) loamy sand with about 10% strong brown (7.5YR 6/6) mottles; moderate medium subangular blocky structure; friable when moist; few medium fibrous roots; common coarse and few medium vesicular pores; pH 4.0
RESULTS AND DISCUSSION
Morphology The relationships of colour gradations to drainage conditions among softs of a catenary association are well known (Buntley and Westin, 1965). Better drained soils tend to have redder hues and higher chromas than less welldrained soils, other things being equal. This trend is evident among the profiles studied in Nigeria, although they do n o t form a catenary association. The deeper horizons of the well-drained Gombe, Danggappe, and Alagba profiles have hues of 5YR, 2.5YR, and 10R, whereas comparable horizons of the moderately well-drained Agege profile have hues of 7.5YR and 10YR. The more reddish colours are associated with better drainage. The Gombe, Danggappe, and Alagba profiles occur in the low and intermediate rainfall zones. These three profiles have redder colours in their deeper horizons than does the Etinan profile, which is found in the zone of high rainfall {annual average of 250 cm). All four profiles are well drained, but the moisture regime in the Etinan soil includes longer intervals of saturation or near saturation. The Etinan profile is also sandier in the deeper horizons than the other three, and the difference in texture may contribute to the yellower colour. All six profiles have sandy surface layers and increasing proportions of clay with depth. The Agege and Danggappe profiles, which occur in the zones of intermediate rainfall, are appreciably higher in clay in the deeper horizons than the other four (Fig.2). The profiles in both the driest and wettest zones are lower in clay in the deeper horizons. The occurrence of clay coatings in former root channels at depth in the
338 AVERAGE ,~" CLAY IN I0
20
30
40
PROFILE 50
60
50
150
~
f o
250
Fig. 2. Average profile clay in the sandstone-derived soils of Nigeria.
Danggappe and Alagba profiles is taken as evidence of the downward transfer of some clay. Such clay coatings were observed at a depth of 1.5 m in the Danggappe profile and below 2 m in the Alagba profile. In Paleudults of the Upper Coastal Plain of North Carolina, U.S.A., clay coatings are present in the regolith at depths of 2 and even 3 m, although none occur in overlying horizons (Cady and Daniels, 1968). Similar occurrences of clay films at depth have been reported near Washington, D.C. (Simonson, 1970) and in soils derived from granite in Nigeria (Nye, 1955a). These clay coatings line partings in the regolith well below B horizons, indicating that silicate clays can be moved to considerable depths. The larger amounts of clay in the deeper horizons, combined with the presence of clay coatings in some profiles, form the basis for recognition of argiUic horizons in the Gombe, Dadin Kowa, and Danggappe soils. In the Alagba profile, the clay films occur below 2 m and are therefore n o t considered diagnostic. Chemical p roper ties
The pH values in water (1:2 soil:water ratio) of the surface layers are a full unit lower in the profiles under high rainfall than in the profiles under low or moderate rainfall (Table III). A similar difference exists between the surface layers and the deeper ones of the soils of the low rainfall areas b u t n o t in the high rainfall areas. The pH values in water were greater than those in 1N KC1, indicating a lack of positive charges in these soils (Bennema and Vettori, 1960). The CEC (cation exchange capacity expressed in me/100 g of soil material) ranges from 1.9 to 17.3 in the surface layers (Table III). It ranges from 1.5 to 8.1 in the subsurface layers, although most values are between 3 and 5. The highest value of 17.3 me is associated with an organic matter content of 7% in the surface layer of the Etinan profile. In contrast, a CEC of 1.9 me in the surface layer, as observed in the G o m b e profile, is especially low for such horizons.
339
TABLE II Mechanical analyses of six sandstone-derived soils of Nigeria Depth (cm)
Sand (%)
Silt (%)
Clay (%)
Texture
0.2 0.3 5.2 1.7 0.6
6.10 10.0 17.2 21.1 31.4
sand sand sandy loam sandy clay loam sandy clay loam
2.2 1.8 4.1 3.3 5.2
8.3 10.3 12.0 30.1 27.7
sand loamy sand loamy sand sandy clay loam sandy clay loam
90.5 84.5 57.8 58.7 54.7 55.5
0.7 0.4 7.4 1.0 0.6 0.6
8.1 16.2 34.8 40.4 45.8 44.4
sand sandy sandy sandy sandy sandy
81.0 55.7 43.3 44.2 43.3
7.7 6.8 8.7 12.7 17.0
11.3 37.5 48.0 43.1 39.7
loamy sand sandy clay clay clay clay loam
4.4 1.7 1.4 0.8 0.7 0.5 0.5 1.8
9.8 18.4 21.0 27.1 29.1 31.6 32.9 32.9
loamy sand sandy loam sandy clay loam sandy clay loam sandy clay loam sandy clay loam sandy clay loam sandy clay loam
22.3 4.3 5.4 4.4 4.3 4.4 8.8
8.1 4.4 7.4 8.3 12.8 17.1 10.8
sandy loam sand loamy sand loamy sand loamy sand sandy loam loamy sand
Go mbe series 0-- 20 20-- 70 70-- 90 90--140 140--165
94.4 90.4 77.7 77.2 69.1
DadinKowa series 0-- 18 18-- 35 35-- 68 68--133 133--170
89.5 87.9 83.9 66.6 67.2
Danggappe series 0-- 18 18-- 48 48-- 95 95--145 145--210 210--288
loam clay loam clay clay clay
Agege series 0-- 18 18-- 43 43-- 70 70-- 90 90--180
Alagba ser~s 0-- 12 12-- 58 58-- 83 83--117 117--152 152--192 192--252 252--308
85.8 79.9 77.6 72.1 70.3 67.9 66.7 65.4
Etinan series 0-- 5 5-- 10 10-- 20 20-- 55 55-- 95 95--115 115--180
69.7 91.3 87.3 87.3 83.0 78.5 80.4
6.2 5.6 5.7 5.1 4.8
pH H20
6.1 5.4 4.6 4.4 4.9
0 - - 18 1 8 - - 43 4 3 - - 70 7 0 - - 90 90-180
Agege series
0-- 18 1 8 - - 48 4 8 - - 95 95--145 145--210 210--288
6.0 4.8 4.5 4.4 4.5
6.1 5.8 5.0 4.3 4.3 4.3
Danggappe series
0-18 1 8 - - 35 3 5 - - 68 68--133 133--170
Dadin Kowa series
0 - 20 20--70 7 0 - - 90 90--140 140--165
Gombe series
Depth (cm)
5.4 4.0 3.6 3.7 3.8
5.3 5.0 4.4 3.7 3.6 4.0
5.4 4.6 3.8 3.6 4.1
5.8 5.0 5.0 4.6 4.3
pH 1 N KCI
Ca 2 ÷
5.4 7.1 8.1 7.6 6.8
3.2 2.3 4.2 4.3 4.4 4.4
3.2 2.3 2.0 6.4 5.5
1.9 1.5 2.4 3.1 4.9
3.3 1.8 1.0 1.1 0.8
2.0 1.0 0.9 0.7 0.9 1.2
1.7 0.8 0.4 1.6 1.8
1.2 0.6 0.8 1.0 1.3
( m e / 1 0 0 g soil)
CEC
1.1 1.6 1.3 1.0 0.4
0.7 0.7 1.0 0.7 0.5 0.7
0.6 0.6 0.5 0.9 1.1
0.3 0.2 0.6 0.6 0.9
Mg: ÷
C h e m i c a l p r o p e r t i e s o f six s a n d s t o n e - d e r i v e d soils o f Nigeria
T A B L E III
0.01 0.01 0.01 0.02 0.01
--0.01 0.01 -0.01
---0.01 0.01
0.02 0.01 0.01
--
--
Na ÷ H ~
0.02 0.01 0.01 0.02 0.02
0.01 0.02 0.02 0.01 0.01 0.01
2.3 3.1 5.4 5.7 5.8
0.3 0.3 1.7 1.7 2.0 1.4
1.8 1.9 0.8 1.6 1.2
0.8 0.7 1.1 1.6
0.01 0.01 0.01 0.03 0.03
0.3
0.01 0.01 0.01 0.08
.......
0.01
K•
81.0 47.1 28.8 27.5 17.9
84.3 75.2 45.8 31.7 30.8 43.8
73.2 59.7 42.6 39.6 41.8
49.1 61.8 50.5 46.7
78.1
(%)
Base saturation
1.8 0.7 0.7 0.7 0.5
0.66 0.28 0.31 0.21 0.17 0.17
0.66 0.36 0.18 0.28 0.26
0.56 0.16 0.13 0.13 0.18
O.M. (%)
1.23 1.66 1.86 4.40 9.27
1.00 1.97 3.66 3.92 4.23 4.52
0.19 0.29 0.32 1.14 1.29
0.17 0.35 0.69 0.77 1.43
Fe:O3 (%)
0.46 0.63 0.64 1.21 1.96
0.15 0.29 0.36 0.55 0.50 0.49
0.06 0.06 0.10 0.15 0.12
0.09 0.03 0.12 0.23 0.17
AI20~ (%)
4.6 2.2 0.8 0.9 0.8
3.6 0.9 0.3 0.5 0.4 0.4
2.0 1.2 1.3 1.2 1.4
2.5 1.1 0.7 0.5 0.3
Available phosphate (ppm)
O
50
series
o15 510 lo20 2055 5595 95-115 115-180
Etinon -cries
4.5 4.4 4.3 4.5 4.3 4.2 4.0
4.7 4.2 3.5 3.5 3.5 3.4 3.5 3.9
PH Hz0
III (continued)
o- 12 1258 5883 83-117 117-152 152-192 192-252 252-308
Alogba
(cm)
Depth
TABLE
4.2 3.9 3.7 3.8 3.7 3.6 3.6
4.1 3.5 3.4 3.4 3.3 3.3 3.4 3.5
PH 1N KC1
17.3 2.7 5.3 1.9 3.6 3.5 3.4
4.4 3.5 3.0 3.0 3.7 3.3 3.1 3.5
CEC ~ (me/100 ____-
4.2 0.5 0.4 0.3 0.2 0.2 0.2
1.1 0.5 0.3 0.3 0.3 0.2 0.3 0.6
g soil)
Ca’+
1.4 0.2 0.1 0.1 0.1 0.1 0.1
0.5 0.1 0.1 0.1 0.1 0.1 0.1 0.2
Mg”
0.01
0.05 -
0.01 0.01 0.01 0.02
0.01 0.01 -
Na+
0.04 0.01 0.01 0.01 0.01 0.01 0.01
0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.02
K+
8.2 2.2 4.0 1.7 2.9 3.2 2.3
2.7 2.5 2.2 2.8 3.1 2.6 2.7 2.5
H’
33.0 22.4 9.7 18.7 7.4 8.9 9.4
36.7 14.8 10.2 10.6 8.6 8.1 10.3 28.9
Base saturation
7.0 1.3 2.0 0.6 0.6 0.4 0.2
1.9 0.8 0.6 0.4 0.3 0.3 0.2 0.2
0.69 0.18 0.47 0.61 0.83 1.19 1.16
1.29 1.83 2.63 2.57 3.23 3.32 3.00 3.43
Fe@, (%)
_~
13.1 3.4 3.8 1.8 1.3 1.1 0.9
1.3 0.7 0.5 0.5 0.2
0.25 0.23 0.15 0.23 0.11
0.36 0.02 0.14 0.12 0.31 0.35 0.30 __
4.9 2.4 3.8
(pm)
Available phosphate
0.08 0.20 0.25
(%)
40,
342
C E C 5 IO
qN 15
M. EQ.~IOOGM. OLAY 20 25 50
40
8O (D
120 z
Y I'-n
160 200
240 280
Fig.3. Depth functions of clay CEC for six sandstone-derived soils of Nigeria.
The CEC tends to increase with increasing clay c o n t e n t {Table II), but there is not a consistent trend. Possibly, the presence of organic matter in some horizons and slight differences in clay mineralogy offset greater quantities of clay. The reddest profiles {Danggappe and Alagba soils) have essentially the same CEC's per 100 g of clay, as shown in Fig.3. The CEC of the clay fraction in these two profiles is lower than it is in the other four. Subsurface horizons of those four, the Gombe, Dadin Kowa, Agege, and Etinan profiles, have some micaceous minerals in their clay fractions which may account for higher CEC of the clay. CEC's of all of these softs are relatively low, especially as compared to soils of comparable textures outside the tropics. Levels of exchangeable cations other than H ÷ are low or very low in all six profiles {Table III). The maxima in me/100 g are 4.2 for Ca 2÷, 1.6 for Mg2, 0.05 for Na÷, and 0.04 for K ÷. The high value for Ca 2+ is in the surface layer of the Etinan profile which occurs under heavy rainfall and vegetation. Recycling of that element by vigorous forest growth could be responsible for the a m o u n t of Ca ~+ at the surface. Below the immediate surface layer, the amounts of exchangeable Ca 2+ are 0.5 me or less. Levels of Mg2÷ are mostly less than 1 me, although a few are slightly above t h a t figure. Levels of K ÷ and Na ÷ are also low, which would be consistent with extensive leaching of soils derived from parent materials that were initially low in bases. The amounts of exchangeable Ca2÷ exceed those of H ÷ in the Gombe and Dadin Kowa profiles, occurring in the zone of low rainfall. Hydrogen exceeds Ca2+ in some horizons of the Danggappe profile, in all but the surface layer of the Agege profile, and in all parts of the Alagba and Etinan profiles. Thus, there is a shift from slight dominance of bases in soils of the drier zones to
343
clear dominance of H ÷ in the wetter zones. The relationships between H ÷ and Ca 2÷ are also expressed in percentage base saturation, as shown in Table III. It tends to decrease from profile to profile in going from the zone of low to the zone of high rainfall. Like the a m o u n t of exchangeable Ca 2÷, base saturation is also at its m a x i m u m in the surface layer of each profile and at a minim u m in one o f the subsurface horizons. Recycling of Ca :÷ by the vegetation adds t h a t element to the surface layers continually. A m o u n t s of free iron oxides in the surface layers range from 0.17 to 1.29% and those of aluminium oxides from 0.06 to 0.46% (Table III). Amounts in the deeper horizons are greater and range from 0.32 to 9.27% for iron oxides and from 0.10 to 1.96% for aluminium oxides. The maxima for both iron and aluminium oxides were in the deepest sampled layer of the Agege profile which was marked by c o m m o n concretions. Both forms of oxides tended to be the highest in the soils that were red, highest in clay, high in concretions, or some combination of two or more of the three. Contents o f available P ranged from 2.0 to 13.1 ppm in the surface layers and from 0.3 to 3.8 ppm in subsurface horizons, as shown in Table III. The m a x i m u m in the surface layer, like that of exchangeable Ca 2÷, is associated with the high level o f organic matter in the Etinan profile. Levels of available P are, on the whole, low.
Mineralogical properties The clay fraction {~ 2 pm) is predominantly made up of kaolinite in all the soils. Appreciable amounts of mica, vermiculite, quartz, vermiculite--smectite and mica--smectite interstratified materials are also present in the clay fractions in differing amounts {Table IV). The silt fraction is dominated by quartz (4.26A, 3.34A). Kaolinite, feldspar
TABLE IV M i n e r a l o g i c a l c o m p o s i t i o n o f t h e c l a y a n d silt f r a c t i o n s o f six s a n d s t o n e - d e r i v e d soils o f Nigeria Soil series name
Kaolinite
Interstratified material (mica-smectite)
Mica
Quartz
Feldspar
Gombe Dadin Kowa Danggappe
+++ +++ +++ +++ +++ +++
** ** ** *** *** *
** ** * ** * **
+++ +++ ++ ++ ++ +++
+ ++ *** * * **
Agege Alagba Etinan
N o t e : +++, 4 0 - - 8 0 % ; ++, 2 0 - - 4 0 % ; +, 1 0 - 2 0 % ; ***, 5 - - 1 0 % ; **, 3 - - 5 % ; *, 1 - - 3 % . T h e c o l u m n s f o r q u a r t z a n d f e l d s p a r w e r e a s s e s s m e n t s f r o m silt f r a c t i o n s .
344
(6.5A, 3.2A ) and mica also occur in differing amounts in the silt fraction. The silt fractions of the Gombe and Dadin Kowa profiles contain between 13 and 38% feldspar as shown in Table IV. The silt fractions of the sandstonederived soils from southern Nigeria contain less than 10% feldspar, as shown in Fig.4. Feldspar levels are thus appreciably lower in soils of the southern part of the country. This could reflect greater weathering under higher rainfall. The difference could also be related to aeolian additions of dust from the Sahara Desert to soils of northern Nigeria. More than likely, both greater weathering
CLAY
SILT
3
Ld MG* GLY, 25°C 0
GOMBE
co K*GLY,550°C
',""
~>-
,I,
MG* GLY, 25°C
bO Z
AGEGE
'~
W
Z
~D O
MG* GLY, 25°C
! I ~ '
co
J
I !
°~
< K* GLY,550°c
ALAGBA I
3 34
L
4.26
i
356
72 ,~ N G S T R
OM
72
S P A C I N G
Fig.4. X-ray diffractograms for silt and clay fractions of some sandstone-derived soils of Nigeria.
345
under higher rainfall and greater additions of feldspathic minerals in the drier part of the c o u n t r y have affected the present character of the soils. CLASSIFICATION OF PROFILES
Tentative classification of the sandstone-derived soil profiles in two systems is offered in Table V. The Gombe and Dadin Kowa profiles have been placed into the order of Alfisols in the 7th Approximation (Soil Survey Staff, 1967) and into the group of Nitosols in the FAO/UNESCO system (FAO, 1970). The Danggappe profile is placed into the order of Ultisols and the group of Ferralsols. The Agege profile is classified as an Oxisol and as a Ferralsol. The Alagba and Etinan profiles, both o f which are extremely acid and strongly leached, are classed as Oxisols, but the former is placed in the group of Acrisols and the latter in t h a t of Ferralsols in the FAO/UNESCO system. Placements are carried down to the subgroup and great soil group levels in Table V. TABLE V Tentative Classification of six sandstone-derived soils of Nigeria Local series name
7th Approximation
FAO/UNESCO
Gombe Dadin Kowa Danggappe Agege Alagba Etinan
Ochric Haploxeralfs Ultic Haploxeralfs Typic Paleustults Typic Eutrustox Tropeptic Haplorthox Psammentic Acrohumox
Rhodic Nitosols Dystric Nitosols Rhodic Ferralsols Plinthic Ferralsols Haplic Acrisols Xanthic Ferralsols
CONCLUSIONS
These preliminary studies of a selected group o f sandstone-derived soils in Nigeria suggest that some of their characteristics are related to rainfall or ecological zones and others to the parent materials. Still others seem largely independent o f both. Both pH and base saturation decrease with increasing rainfall. The highest values were found in soils of the low rainfall zone and lowest ones in the high rainfall zone. The d o m i n a n t clay mineral was kaolinite in all of the soils. Proportions of other minerals in the clay fraction had a slight tendency to be higher in the soils o f low rainfall zones, but this was n o t a consistent trend. On the other hand, the silt fraction did have a consistent tendency to be more feldspathic in soils of the drier regions. The sand fraction was a major one in all of the soils, regardless of rainfall zones, and a direct reflection o f the sandy parent materials.
346
ACKNOWLEDGEMENTS
The authors wish to express their gratitude to Dr. Klinkenberg and Mr. Valette of Ahmadu Bello University, Zaria, for their assistance during field trips to the northern states of Nigeria. The authors are grateful to the members of the field office division of the Institute of Agricultural Research and Training, University of Ife, Moor Plantation branch in Ibadan, for their assistance during field trips to Agege and Iperu. Thanks also go to Dr. A.U. Salami and Dr. D.O. Ataga for their assitance during field trips to the Mid-Western State of Nigeria. The authors are indebted to Professor J.B. Dixon of Texas A & M University for guiding the first author in the mineralogical analysis of the soils.
REFERENCES Bennema, J. and Vettori, L., 1960. The influence of the carbon-sesquioxide ratios in the pH of Latosols. Trans. Int. Congr. Soil Sci., 7th, Madison, Wis., IV: 244--250 Brewer, R., 1964. Fabric and Mineral Analysis of Soils. Wiley, New York, N.Y., 419 pp. Buntley, G.J. and Westin, F.C., 1965. A comparative study of development colour in a Chestnut--Chernozem--Brunizemsoil climosequence. Soil Sci. Soc. Am. Proc., 29(5): 579--582 Cady, J.G. and Daniels, R.B., 1968. Genesis of some very old soils -- The Paleudults. Trans. Int. Congr. Soil Sci., 9th, Australia, IV: 103--112 Carter, J.D., 1964. A Geologic Map of Nigeria. Geological Survey Division, Federal Ministry of Mines and Power, Lagos, Nigeria Doyne, H.C. and Watson, W.A., 1933. Soil formation in Southern Nigeria (The "Ilepa" Profile). J. Agrie. Sci., 2 3 : 2 0 8 - - 2 1 5 FAO, 1965. Guidelines for Soil Description. FAO, Rome, 53 pp. FAO, 1970. Soil Map of the World, FAO/UNESCO Project. Key to Soil Units for the Soil Map of the World. FAO, Rome, 16 pp. Hansell, J.R.F., 1965. The Ofada area. MANR Soil Survey Report (Moor Plantation, Ibadan), 6 0 : 4 5 pp. Hopkins, B., 1965. Forest and Savanna, Heinemann, London, 112 pp. Jackson, M.L., 1958. Soil Chemical Analysis. Prentice Hall, Englewood Cliffs, N.J., 498 pp. Jackson, M.L., 1968. Soil Chemical Analysis -- Advanced Course. Published by the author, Madison, Wis. Jungerius, P.D. and Levelt, T.W.M., 1964. Clay mineralogy of soils over sedimentary rocks in Eastern Nigeria. Soil Sci., 97(2): 89--95 Kilmer, V.J. and Alexander, L.T., 1949. Methods of making mechanical analysis of soils. Soil Sci., 6 8 : 1 5 - - 2 4 Klinkenberg, K., 1963. The soils of the middle Gongola Region. Institute of Agriculture, Samaru, Zaria, Nigeria, Bull. N o . 2 1 : 6 5 pp. McKeague, J.A. and Day, J.H., 1966. Dithionite- and oxalate-extractable Fe and Al as aids in differentiating various classes of soils. Can. J. Soil Sci., 4 6 : 1 3 - - 2 2 Moss, R.P., 1960. A Soil Geography of a Part of South-Western Nigeria. Thesis, University of London, 162 pp. Nye, P.H., 1955a. Some soil-forming processes in the humid tropics: II. Development of the upper-slope member of the catena. J. Soil Sci., 6 : 5 1 - - 6 2 Nye, P.H., 1955b. Some soil forming processes in the humid tropics: III. Laboratory studies in the development of a typical catena over granite gneiss. J. Soil Sci., 6(1): 63--73
347
Reyment, R.A., 1965. An Aspect of the Geology of Nigeria. University of Ibadan Press, Ibadan. Shapiro, L. and Brannock, W.W., 1962. Rapid analysis of silicate, carbonate and phosphate rocks. Geol. Surv. Bull., 1144-A25-26. U.S. Government Printing Office, Washington D.C. Simonson, R.W., 1970. Loss of nutrient elements during soil formation. In: O.P. Engelstad (Editor), Nutrient Mobility in Soils: Accumulation and Losses. Soil Sci. Soc. Am., Spec. Publ., N o . 4 : 2 1 - - 4 5 Smyth, A.J. and Montgomerry, R.F., 1962. Soils and Land use in Central Western Nigeria. Western Nigeria Government Press, Ibadan, 265 pp. Soil Survey Staff, 1967. Supplement to Soil Classification System. Soil Conservation Service, U.S.D.A., Washington, D.C., 207 pp. Valette, P.J. and Adeyemo, A., 1964. Soil Survey Report of Mokwa, Bida and Idah Divisions. Samaru Progress Report, 105 pp. Vine, H., 1949. Nigerian soils in relation to parent materials. Comm. Bur. Soil Sci. Tech. Comm, 4 6 : 2 2 - - 2 9 Vine, H., 1954. Latosols of Nigeria and some related soils. Inter-Afr. Soils Conf., 2nd, Document 15, Vol. 1, pp.295--368 Vine, H., 1956. Studies of soil profiles at the WAIFOR main station and at some other sites of Oil Palm Experiments. J. West Aft. Oil Palm Res., 1(4): 11--58
CHARACTERIZATION OF SELECTED SANDSTONE-DERIVED SOILS IN THE ECOLOGICAL ZONES OF NIGERIA
J.A. O G U N W A L E l, T.I. A S H A Y E 2, C.T.I. O D U I and A.A.A. F A Y E M P Department of Agronomy, University of Ibadan, Ibadan (Nigeria) 2 Institute of Agricultural Research and Training, Moor Plantation, Ibadan (Nigeria) (Received October 19, 1973; revised version accepted November 27, 1974)
ABSTRACT Ogunwale, J.A., Ashaye, T.I., Odu, C.T.I. and Fayemi, A.A.A., 1975. Characterization of selected sandstone-derived soils in the ecological zones of Nigeria. G e o d e r m a , 13: 331--347. Sandstone-derived soils constitute a b o u t 18% of the land surface o f Nigeria. These soils o c c u r in all the major ecological zones and c o n s e q u e n t l y are f o u n d under a wide range of rainfall and vegetation. The m o r p h o l o g y and c o m p o s i t i o n o f a group o f profiles selected to represent soils o f the several zones have therefore been studied. The characteristics of the profiles can in part be related to the ecological zones and in part to the parent rock. S o m e characteristics are n o t clearly related to either. Both pH and base saturation decrease with increasing rainfall. The highest values are in the low rainfall z o n e and the lowest in the high rainfall zone. Soils o f the m e d i u m rainfall zone are marked at d e p t h by greater a c c u m u l a t i o n s o f silicate clays, iron oxides, and a l u m i n i u m oxides than soils o f b o t h low and high rainfall regions. Kaolinite is the d o m i n a n t clay mineral in all o f the soils. Minor p r o p o r t i o n s of o t h e r minerals are present, and t h e y s e e m to increase slightly with decreasing rainfall. Textures o f all profiles strongly reflect the nature o f the parent materials. The sand fractions are i m p o r t a n t in all parts of all profiles and are d o m i n a n t t h r o u g h o u t s o m e of them. INTRODUCTION
Soils derived from sandstones cover about 160,000 km 2 (65,000 square miles), which is about 18% of the surface area of Nigeria. These soils, reported by Vine (1954) to be chemically poor, are planted to a wide range of tree crops such as kola, off-palm and rubber in the southern parts of Nigeria, whilst in the northern parts they support industrial and food crops such as cotton, sugar-cane, guinea-corn and millet. Earlier pedological studies in Nigeria (Doyne and Watson, 1933; Vine, 1949; Nye, 1955a; Smyth and Montgomerry, 1962) have centered on cocoa-supporting softs found on metamorphic and igneous rocks rather than the softs derived from sandstones. Reports on the properties of soils derived from sandstones (Vine, 1956; Moss, 1960; Klinkenberg, 1963; Jungerius and Levelt, 1964; Valette and Adeyemo, 1964; Hansell, 1965) do not bring out relationships between the chemical, physical and mineralogical properties of the soils.
332 The objectives of this paper are to present the chemical, mechanical and mineralogical properties of selected sandstone-derived soils of Nigeria and to relate these properties to the distribution of the soils, climate and vegetation. Materials and m e t h o d s
Soil samples were collected from sites located within sandstone areas according to a geological map of Nigeria (Carter, 1964). Sample sites were selected on the basis of geological formations, ecological zones, and rainfall, as given in Table I. The sites are shown in Fig.1. /S
/ (j'"
-, .8~
/ ~-~'~-/
~
MOKWA
r~( /f
~o I,PE"U L_ "
~ .... / BENIN
..r
-4
0 4° i
8° i
2 4 0 KM.
4%
12° i
Fig.1. A map of Nigeria showing soil sampling sites. The profiles were described according to the Food and Agricultural Organization handbook - - " G u i d e to the study of soil in the field" (FAO, 1965). Soil samples were air-dried and passed through a 2-mm sieve. These air-dried samples were used in the chemical, mechanical and mineralogical analysis. Each soil sample was analysed for calcium, magnesium, potassium, sodium, exchangeable acidity, available phosphorus (Bray and Kurtz P~) and organic matter following the method outlined by Jackson (1958). CEC (cation exchange capacity) was determined by the saturation method (Jackson, 1958). Iron and aluminium were extracted with sodium dithionite-citrate solution (McKeague and Day, 1966). Iron was determined on an atomic absorption spectrophotometer. Aluminium was determined colorimetrically using the alizarin red-S method of Shapiro and Brannock (1962). Mechanical analysis was done by the method of Kilmer and Alexander (1949). The pretreatment method of Brewer (1964) was followed in mineralogical analysis. Fractionation and preparation of slides for X-ray analysis were carried out as outlined by Jackson (1968). X-ray diffraction pattems were obtained for Mg-saturated and glycerolsolvated clay samples. Similar diffraction patterns were obtained for K-saturated and glycerol-solvated clay samples, after heating to 550°C.
Ikenne
Benin
Iperu--Agege
Benin--Alagba
Sombreiro Warrideltaic
Benin
Abeokuta
Nupe
wet lowland rain forest
rain forest
dry lowland
southern Guinea Savanna
200--300
150--200
125--150
100--125
Sources: Klinkenberg (1963), Carter (1964), Hopkins (1965), and Reyment (1965).
Agbaru--Etinan
Nupe
Bima
Bima
Mokwa--Danggappe
175--100
northern Guinea Savanna
Gombe
Gombe
Annual rainfall range
Gombe--Gombe
Ecological zone
(cm)
Geological formation
Sandstone type
Soil sampling localities and series
Geological and ecological data on some sandstone areas of Nigeria ~
TABLE I
Quaternary (PliocenePleistocene)
Quarternary (Pliocene-Pleistocene)
Cretaceous (Senonian)
Crecaceous (Senonian)
Cretaceous (Senonian--Albian)
Tertiary (Paleocene)
Sandstone age
C~ C~ e~
334 Each silt sample was p a c k e d in an a l u m i n i u m b o x m o u n t for X-ray analysis. A Philip's d i f f r a c t o m e t e r using a graphite m o n o c h r o m a t o r , Cuka radiation, and a s c a n n i n g speed o f 2°/min was e m p l o y e d . MORPHOLOGY OF SELECTED PROFILES
Gombe series (Gombe) The pit site was close t o the V e t e r i n a r y Office on the left-hand side o f t h e G o m b e - - K a f a r a t i road. T o p o g r a p h y was gently sloping t o the s o u t h e a s t . T h e well-drained soil profile was d r y o n t o p and m o i s t in the d e e p e s t h o r i z o n . Parkia clappertoniana, Cassia sp., Ctenium sp. and Sida sp. were the m o s t c o m m o n plants. The p r e s u m e d p a r e n t material is the G o m b e s a n d s t o n e . Colours given are f o r m o i s t c o n d i t i o n s . Depth (cm) 0-- 20 20-- 70
70-- 90
90--140
140--165
Profile description dark brown (7.5YR 4/4) sand; weak medium subangular blocky structure; friable; common fine roots; few termite nests; few medium vesicular pores; clear wavy boundary; pH 6.2 yellowish red (5YR 4/6) fine sand; weak medium subangular blocky structure: friable; common medium subangular blocky quartz grains; few manganiferous concretions; few medium roots; few medium vesicular pores; smooth clear boundary; pH 5,6 yellowish red (5YR 4/6) sandy loam with about 20% light yellowish brown (10YR 6/4) mottles; moderate medium subangular blocky structure; friable; common medium quartz grains; few medium manganiferous concretions; few coarse vesicular pores; smooth clear boundary; pH 5.7 red (2.5YR 4/6) sandy clay loam with about 5% light yellowish brown (10YR 6/4) mottles; moderate medium/coarse subangular blocky structure; firm; common fine/medium quartz grains; few coarse roots; common medium and few coarse interstitial pores; sharp wavy boundary; pH 5.1 red (2.5YR 4/6) sandy clay loam; gravelly layer on top of reddish brown sandstone grit; pH 4.8
Danggappe series (Mokwa) The soil profile was l o c a t e d o n the M o k w a agricultural farm. T o p o g r a p h y was flat. The well-drained soil profile was fairly d r y w h e n described. Natural vegetation had been cleared and grasses o f weed t y p e s were growing a r o u n d the pit. Gmelina arborea was the d o m i n a n t tree p l a n t e d a r o u n d the pit, a n d t h e c o m m o n grass was Brachiaria deflexa. The p r e s u m e d p a r e n t material is the N u p e sandstone. Colours given are for m o i s t c o n d i t i o n s . Depth (cm) 0--18
Profile description dark reddish brown (2.5YR 2/4) sand; moderate medium subangular blocky structure; slightly hard when dry; common medium grass roots; few termite nests; few medium tubular pores; wavy sharp boundary; pH 6.1
335
18-- 48
48-- 95
95--145
145--288
dusky red (10R 3/4) sandy loam; strong medium subangular blocky structure; hard when dry; few medium roots; common termite nests (25--30 cm long and 15 cm wide); few coarse stones; few medium vesicular pores; smooth clear boundary; pH 5.8 dark red (10R 3/6) coarse sandy clay loam; strong coarse subangular blocky structure; hard when dry; few medium fibrous roots; c o m m o n termite nests; few medium vesicular pores; smooth diffuse boundary; pH 5.0 dark red (10R 3/6) coarse sandy clay; strong coarse subangular blocky structure; hard when dry; few termite nests; few medium vesicular pores; thin clay coatings on termite nests and old root channels; diffuse smooth boundary; pH 4.3 similar to the above horizon (95--145 cm) except for the presence of common medium quartz grains between depths of 210 and 288 cm
Agege series (Iperu) The soil profile was located at Iperu on land surveyed for sugar-cane production assessment on the left-hand side of the Ibadan--Lagos road. The topography was fairly flat to gently sloping to the south. The moderately well-drained soil profile was moist when described. The most c o m m o n plant species were Alchornea cordifolia, Millettia thonningii, Holarrhena floribunda, and grasses belonging to the Andropogoneae family. The land was planted to cassava intermixed with some pepper. The presumed parent material is Ikenne sandstone. Colours given are for moist conditions. Depth (cm) 0-- 18 18-- 43
43-- 70
70-- 90
90--180
Profile description brown (7.5YR 4/2) loamy sand; weak medium granular structure; abundant earthworm casts; c o m m o n medium roots; sharp wavy boundary; pH 6.0 dark brown (7.5YR 4/4) sandy clay; weak medium granular structure; friable when moist; few medium reddish brown concretions (2--8 m m diameter); few horizontal e m p t y animal burrows (2--3 cm diameter); few medium soft roots; smooth clear boundary; pH 4.8 strong brown (7.5YR 5/6) fine sandy clay with about 5% yellowish brown (10YR 5/8) mottles; moderate medium subangular blocky structure; friable when moist; c o m m o n hard spheroidal and subangular blocky brownish red concretions; few quartz gravels; few medium decaying roots; clear irregular boundary; pH 4.5 yellowish brown (10YR 5/6) fine sandy clay with about 25% yellowish red (5YR 5/6) mottles; strong medium subangular blocky structure; friable when moist, slightly sticky when wet; c o m m o n hard medium reddish brown spheroidal/subangular blocky concretions; few quartz gravels; few decaying medium roots; gradual smooth boundary; pH 4.4 light yellowish brown (10YR 6/4) fine sandy clay with about 45% coarse yellowish red (5YR 5/8) mottles; strong medium subangular blocky structure; friable when moist, slightly sticky when wet; c o m m o n medium/coarse reddish brown subangular blocky concretions; very few fine roots; pH 4.5
336
Alagba series (Benin) T h e soil p r o f i l e was l o c a t e d in t h e o l d o i l - p a l m p l a n t a t i o n o f t h e N i g e r i a n I n s t i t u t e f o r Oil Palm R e s e a r c h a t Benin. T h e t o p o g r a p h y was a l m o s t fiat o n a hill t o p . T h e c o u n t r y is rolling. T h e w e l l - d r a i n e d soil was m o i s t w h e n des c r i b e d . T h e m o s t c o m m o n p l a n t s p e c i e s was Elaeis guineensis (oil p a l m ) . T h e p r e s u m e d p a r e n t m a t e r i a l is Benin s a n d s t o n e . C o l o u r s given are f o r m o i s t conditions. Depth (cm) O-- 12 12-- 58
58-- 83 83--117
117--152
192--252
252--308
Profile description dark reddish brown (5YR 3/3) loamy sand; very weak fine granular structure; loose when moist; abundant fine roots; clear wavy boundary; pH 4.7 reddish brown (2.5YR 4/4) sandy loam; weak fine subangular blocky structure; very friable when moist; abundant fine fibrous roots; clear wavy boundary; pH 4.2 dark red (10R 3/6) sandy clay loam; weak medium subangular blocky structure; friable when moist; few medium roots; clear irregular boundary; pH 3.5 dark red (10R 3/6) sandy clay loam; moderate medium subangular blocky structure; friable when moist; common medium fibrous roots; thin clay coatings in former root channels; few medium vesicular pores; clear smooth boundary; pH 3.5 dark red (10R 3/6) sandy clay loam; moderate medium subangular blocky structure; friable when moist; common medium fibrous roots; few white-ant channels; very thin clay coatings in former root channels; few coarse vesicular pores; smooth diffuse boundary; pH 3.5 dark red (10R 3/6) sandy clay loam; moderate coarse subangular blocky structure; friable when moist; few medium root channels; thin clay coatings in root channels; common medium vesicular pores; diffuse smooth boundary; pH 3.5 dark red (10R 3/6) sandy clay loam; moderate coarse subangular blocky structure; friable when moist; few medium fibrous roots; thin clay coatings in root channels; few medium vesicular pores; pH 3.9
Etinan series (Agbaru) T h e soil p r o f i l e was l o c a t e d in t h e o l d f a r m s e t t l e m e n t a t A g b a r u , o n t h e l e f t - h a n d side of t h e A g b a r u - - U g h e l l i r o a d . T h e t o p o g r a p h y was a l m o s t flat. T h e c o u n t r y is g e n t l y rolling. T h e w e l l - d r a i n e d soil was m o i s t w h e n d e s c r i b e d . T h e n a t i v e v e g e t a t i o n was c l e a r e d b e f o r e c i t r u s was p l a n t e d on t h e soil. T h e m o s t c o m m o n p l a n t s p e c i e s w e r e c i t r u s a n d Calopogonium mucunoides. T h e Calopogonium sp. was u s e d as c o v e r c r o p t o p r e v e n t e r o s i o n . T h e p r e s u m e d p a r e n t m a t e r i a l is a y e l l o w s a n d s t o n e . C o l o u r s given are f o r m o i s t c o n d i t i o n s . Depth (cm) 0--
5
Profile description black (7.5YR 2/0) sandy loam; very weak fine granular structure; very friable when moist; few earthworms; common fine fleshy roots; clear wavy boundary; pH 4.5
337
5-- 10 10-- 20
20-- 55
55-- 95
95--115
115--180
dark brown (7.5YR 4/2) sand; very weak fine granular structure; loose when moist; few earthworms; common fine roots; sharp wavy boundary; pH 4.4 grayish brown (10YR 3/2) loamy sand; weak fine granular structure; very friable when moist; few earthworms; common fine roots; clear wavy boundary; pH 4,3 brown (7.5YR 5/4) loamy sand with about 15% strong brown (7.5YR 5/6) mottles; weak fine granular structure; very friable when moist; few medium fibrous roots; common medium vesicular pores; diffuse wavy boundary; pH 4.5 strong brown (7.5YR 5/6) loamy sand with about 20% brown (7.5YR 5/4) mottles; weak medium subangular blocky structure; friable when moist; common medium fibrous roots; common coarse interstitial pores; diffuse wavy boundary; pH 4.3 strong brown (7.5YR 5/6) fine sandy loam with about 25% strong brown (7.5 YR 6/6) mottles; moderate medium subangular blocky structure; friable when moist; few medium fibrous roots; common coarse vesicular pores; diffuse smooth boundary; pH 4.2 strong brown (7.5YR 5/6) loamy sand with about 10% strong brown (7.5YR 6/6) mottles; moderate medium subangular blocky structure; friable when moist; few medium fibrous roots; common coarse and few medium vesicular pores; pH 4.0
RESULTS AND DISCUSSION
Morphology The relationships of colour gradations to drainage conditions among softs of a catenary association are well known (Buntley and Westin, 1965). Better drained soils tend to have redder hues and higher chromas than less welldrained soils, other things being equal. This trend is evident among the profiles studied in Nigeria, although they do n o t form a catenary association. The deeper horizons of the well-drained Gombe, Danggappe, and Alagba profiles have hues of 5YR, 2.5YR, and 10R, whereas comparable horizons of the moderately well-drained Agege profile have hues of 7.5YR and 10YR. The more reddish colours are associated with better drainage. The Gombe, Danggappe, and Alagba profiles occur in the low and intermediate rainfall zones. These three profiles have redder colours in their deeper horizons than does the Etinan profile, which is found in the zone of high rainfall {annual average of 250 cm). All four profiles are well drained, but the moisture regime in the Etinan soil includes longer intervals of saturation or near saturation. The Etinan profile is also sandier in the deeper horizons than the other three, and the difference in texture may contribute to the yellower colour. All six profiles have sandy surface layers and increasing proportions of clay with depth. The Agege and Danggappe profiles, which occur in the zones of intermediate rainfall, are appreciably higher in clay in the deeper horizons than the other four (Fig.2). The profiles in both the driest and wettest zones are lower in clay in the deeper horizons. The occurrence of clay coatings in former root channels at depth in the
338 AVERAGE ,~" CLAY IN I0
20
30
40
PROFILE 50
60
50
150
~
f o
250
Fig. 2. Average profile clay in the sandstone-derived soils of Nigeria.
Danggappe and Alagba profiles is taken as evidence of the downward transfer of some clay. Such clay coatings were observed at a depth of 1.5 m in the Danggappe profile and below 2 m in the Alagba profile. In Paleudults of the Upper Coastal Plain of North Carolina, U.S.A., clay coatings are present in the regolith at depths of 2 and even 3 m, although none occur in overlying horizons (Cady and Daniels, 1968). Similar occurrences of clay films at depth have been reported near Washington, D.C. (Simonson, 1970) and in soils derived from granite in Nigeria (Nye, 1955a). These clay coatings line partings in the regolith well below B horizons, indicating that silicate clays can be moved to considerable depths. The larger amounts of clay in the deeper horizons, combined with the presence of clay coatings in some profiles, form the basis for recognition of argiUic horizons in the Gombe, Dadin Kowa, and Danggappe soils. In the Alagba profile, the clay films occur below 2 m and are therefore n o t considered diagnostic. Chemical p roper ties
The pH values in water (1:2 soil:water ratio) of the surface layers are a full unit lower in the profiles under high rainfall than in the profiles under low or moderate rainfall (Table III). A similar difference exists between the surface layers and the deeper ones of the soils of the low rainfall areas b u t n o t in the high rainfall areas. The pH values in water were greater than those in 1N KC1, indicating a lack of positive charges in these soils (Bennema and Vettori, 1960). The CEC (cation exchange capacity expressed in me/100 g of soil material) ranges from 1.9 to 17.3 in the surface layers (Table III). It ranges from 1.5 to 8.1 in the subsurface layers, although most values are between 3 and 5. The highest value of 17.3 me is associated with an organic matter content of 7% in the surface layer of the Etinan profile. In contrast, a CEC of 1.9 me in the surface layer, as observed in the G o m b e profile, is especially low for such horizons.
339
TABLE II Mechanical analyses of six sandstone-derived soils of Nigeria Depth (cm)
Sand (%)
Silt (%)
Clay (%)
Texture
0.2 0.3 5.2 1.7 0.6
6.10 10.0 17.2 21.1 31.4
sand sand sandy loam sandy clay loam sandy clay loam
2.2 1.8 4.1 3.3 5.2
8.3 10.3 12.0 30.1 27.7
sand loamy sand loamy sand sandy clay loam sandy clay loam
90.5 84.5 57.8 58.7 54.7 55.5
0.7 0.4 7.4 1.0 0.6 0.6
8.1 16.2 34.8 40.4 45.8 44.4
sand sandy sandy sandy sandy sandy
81.0 55.7 43.3 44.2 43.3
7.7 6.8 8.7 12.7 17.0
11.3 37.5 48.0 43.1 39.7
loamy sand sandy clay clay clay clay loam
4.4 1.7 1.4 0.8 0.7 0.5 0.5 1.8
9.8 18.4 21.0 27.1 29.1 31.6 32.9 32.9
loamy sand sandy loam sandy clay loam sandy clay loam sandy clay loam sandy clay loam sandy clay loam sandy clay loam
22.3 4.3 5.4 4.4 4.3 4.4 8.8
8.1 4.4 7.4 8.3 12.8 17.1 10.8
sandy loam sand loamy sand loamy sand loamy sand sandy loam loamy sand
Go mbe series 0-- 20 20-- 70 70-- 90 90--140 140--165
94.4 90.4 77.7 77.2 69.1
DadinKowa series 0-- 18 18-- 35 35-- 68 68--133 133--170
89.5 87.9 83.9 66.6 67.2
Danggappe series 0-- 18 18-- 48 48-- 95 95--145 145--210 210--288
loam clay loam clay clay clay
Agege series 0-- 18 18-- 43 43-- 70 70-- 90 90--180
Alagba ser~s 0-- 12 12-- 58 58-- 83 83--117 117--152 152--192 192--252 252--308
85.8 79.9 77.6 72.1 70.3 67.9 66.7 65.4
Etinan series 0-- 5 5-- 10 10-- 20 20-- 55 55-- 95 95--115 115--180
69.7 91.3 87.3 87.3 83.0 78.5 80.4
6.2 5.6 5.7 5.1 4.8
pH H20
6.1 5.4 4.6 4.4 4.9
0 - - 18 1 8 - - 43 4 3 - - 70 7 0 - - 90 90-180
Agege series
0-- 18 1 8 - - 48 4 8 - - 95 95--145 145--210 210--288
6.0 4.8 4.5 4.4 4.5
6.1 5.8 5.0 4.3 4.3 4.3
Danggappe series
0-18 1 8 - - 35 3 5 - - 68 68--133 133--170
Dadin Kowa series
0 - 20 20--70 7 0 - - 90 90--140 140--165
Gombe series
Depth (cm)
5.4 4.0 3.6 3.7 3.8
5.3 5.0 4.4 3.7 3.6 4.0
5.4 4.6 3.8 3.6 4.1
5.8 5.0 5.0 4.6 4.3
pH 1 N KCI
Ca 2 ÷
5.4 7.1 8.1 7.6 6.8
3.2 2.3 4.2 4.3 4.4 4.4
3.2 2.3 2.0 6.4 5.5
1.9 1.5 2.4 3.1 4.9
3.3 1.8 1.0 1.1 0.8
2.0 1.0 0.9 0.7 0.9 1.2
1.7 0.8 0.4 1.6 1.8
1.2 0.6 0.8 1.0 1.3
( m e / 1 0 0 g soil)
CEC
1.1 1.6 1.3 1.0 0.4
0.7 0.7 1.0 0.7 0.5 0.7
0.6 0.6 0.5 0.9 1.1
0.3 0.2 0.6 0.6 0.9
Mg: ÷
C h e m i c a l p r o p e r t i e s o f six s a n d s t o n e - d e r i v e d soils o f Nigeria
T A B L E III
0.01 0.01 0.01 0.02 0.01
--0.01 0.01 -0.01
---0.01 0.01
0.02 0.01 0.01
--
--
Na ÷ H ~
0.02 0.01 0.01 0.02 0.02
0.01 0.02 0.02 0.01 0.01 0.01
2.3 3.1 5.4 5.7 5.8
0.3 0.3 1.7 1.7 2.0 1.4
1.8 1.9 0.8 1.6 1.2
0.8 0.7 1.1 1.6
0.01 0.01 0.01 0.03 0.03
0.3
0.01 0.01 0.01 0.08
.......
0.01
K•
81.0 47.1 28.8 27.5 17.9
84.3 75.2 45.8 31.7 30.8 43.8
73.2 59.7 42.6 39.6 41.8
49.1 61.8 50.5 46.7
78.1
(%)
Base saturation
1.8 0.7 0.7 0.7 0.5
0.66 0.28 0.31 0.21 0.17 0.17
0.66 0.36 0.18 0.28 0.26
0.56 0.16 0.13 0.13 0.18
O.M. (%)
1.23 1.66 1.86 4.40 9.27
1.00 1.97 3.66 3.92 4.23 4.52
0.19 0.29 0.32 1.14 1.29
0.17 0.35 0.69 0.77 1.43
Fe:O3 (%)
0.46 0.63 0.64 1.21 1.96
0.15 0.29 0.36 0.55 0.50 0.49
0.06 0.06 0.10 0.15 0.12
0.09 0.03 0.12 0.23 0.17
AI20~ (%)
4.6 2.2 0.8 0.9 0.8
3.6 0.9 0.3 0.5 0.4 0.4
2.0 1.2 1.3 1.2 1.4
2.5 1.1 0.7 0.5 0.3
Available phosphate (ppm)
O
50
series
o15 510 lo20 2055 5595 95-115 115-180
Etinon -cries
4.5 4.4 4.3 4.5 4.3 4.2 4.0
4.7 4.2 3.5 3.5 3.5 3.4 3.5 3.9
PH Hz0
III (continued)
o- 12 1258 5883 83-117 117-152 152-192 192-252 252-308
Alogba
(cm)
Depth
TABLE
4.2 3.9 3.7 3.8 3.7 3.6 3.6
4.1 3.5 3.4 3.4 3.3 3.3 3.4 3.5
PH 1N KC1
17.3 2.7 5.3 1.9 3.6 3.5 3.4
4.4 3.5 3.0 3.0 3.7 3.3 3.1 3.5
CEC ~ (me/100 ____-
4.2 0.5 0.4 0.3 0.2 0.2 0.2
1.1 0.5 0.3 0.3 0.3 0.2 0.3 0.6
g soil)
Ca’+
1.4 0.2 0.1 0.1 0.1 0.1 0.1
0.5 0.1 0.1 0.1 0.1 0.1 0.1 0.2
Mg”
0.01
0.05 -
0.01 0.01 0.01 0.02
0.01 0.01 -
Na+
0.04 0.01 0.01 0.01 0.01 0.01 0.01
0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.02
K+
8.2 2.2 4.0 1.7 2.9 3.2 2.3
2.7 2.5 2.2 2.8 3.1 2.6 2.7 2.5
H’
33.0 22.4 9.7 18.7 7.4 8.9 9.4
36.7 14.8 10.2 10.6 8.6 8.1 10.3 28.9
Base saturation
7.0 1.3 2.0 0.6 0.6 0.4 0.2
1.9 0.8 0.6 0.4 0.3 0.3 0.2 0.2
0.69 0.18 0.47 0.61 0.83 1.19 1.16
1.29 1.83 2.63 2.57 3.23 3.32 3.00 3.43
Fe@, (%)
_~
13.1 3.4 3.8 1.8 1.3 1.1 0.9
1.3 0.7 0.5 0.5 0.2
0.25 0.23 0.15 0.23 0.11
0.36 0.02 0.14 0.12 0.31 0.35 0.30 __
4.9 2.4 3.8
(pm)
Available phosphate
0.08 0.20 0.25
(%)
40,
342
C E C 5 IO
qN 15
M. EQ.~IOOGM. OLAY 20 25 50
40
8O (D
120 z
Y I'-n
160 200
240 280
Fig.3. Depth functions of clay CEC for six sandstone-derived soils of Nigeria.
The CEC tends to increase with increasing clay c o n t e n t {Table II), but there is not a consistent trend. Possibly, the presence of organic matter in some horizons and slight differences in clay mineralogy offset greater quantities of clay. The reddest profiles {Danggappe and Alagba soils) have essentially the same CEC's per 100 g of clay, as shown in Fig.3. The CEC of the clay fraction in these two profiles is lower than it is in the other four. Subsurface horizons of those four, the Gombe, Dadin Kowa, Agege, and Etinan profiles, have some micaceous minerals in their clay fractions which may account for higher CEC of the clay. CEC's of all of these softs are relatively low, especially as compared to soils of comparable textures outside the tropics. Levels of exchangeable cations other than H ÷ are low or very low in all six profiles {Table III). The maxima in me/100 g are 4.2 for Ca 2÷, 1.6 for Mg2, 0.05 for Na÷, and 0.04 for K ÷. The high value for Ca 2+ is in the surface layer of the Etinan profile which occurs under heavy rainfall and vegetation. Recycling of that element by vigorous forest growth could be responsible for the a m o u n t of Ca ~+ at the surface. Below the immediate surface layer, the amounts of exchangeable Ca 2+ are 0.5 me or less. Levels of Mg2÷ are mostly less than 1 me, although a few are slightly above t h a t figure. Levels of K ÷ and Na ÷ are also low, which would be consistent with extensive leaching of soils derived from parent materials that were initially low in bases. The amounts of exchangeable Ca2÷ exceed those of H ÷ in the Gombe and Dadin Kowa profiles, occurring in the zone of low rainfall. Hydrogen exceeds Ca2+ in some horizons of the Danggappe profile, in all but the surface layer of the Agege profile, and in all parts of the Alagba and Etinan profiles. Thus, there is a shift from slight dominance of bases in soils of the drier zones to
343
clear dominance of H ÷ in the wetter zones. The relationships between H ÷ and Ca 2÷ are also expressed in percentage base saturation, as shown in Table III. It tends to decrease from profile to profile in going from the zone of low to the zone of high rainfall. Like the a m o u n t of exchangeable Ca 2÷, base saturation is also at its m a x i m u m in the surface layer of each profile and at a minim u m in one o f the subsurface horizons. Recycling of Ca :÷ by the vegetation adds t h a t element to the surface layers continually. A m o u n t s of free iron oxides in the surface layers range from 0.17 to 1.29% and those of aluminium oxides from 0.06 to 0.46% (Table III). Amounts in the deeper horizons are greater and range from 0.32 to 9.27% for iron oxides and from 0.10 to 1.96% for aluminium oxides. The maxima for both iron and aluminium oxides were in the deepest sampled layer of the Agege profile which was marked by c o m m o n concretions. Both forms of oxides tended to be the highest in the soils that were red, highest in clay, high in concretions, or some combination of two or more of the three. Contents o f available P ranged from 2.0 to 13.1 ppm in the surface layers and from 0.3 to 3.8 ppm in subsurface horizons, as shown in Table III. The m a x i m u m in the surface layer, like that of exchangeable Ca 2÷, is associated with the high level o f organic matter in the Etinan profile. Levels of available P are, on the whole, low.
Mineralogical properties The clay fraction {~ 2 pm) is predominantly made up of kaolinite in all the soils. Appreciable amounts of mica, vermiculite, quartz, vermiculite--smectite and mica--smectite interstratified materials are also present in the clay fractions in differing amounts {Table IV). The silt fraction is dominated by quartz (4.26A, 3.34A). Kaolinite, feldspar
TABLE IV M i n e r a l o g i c a l c o m p o s i t i o n o f t h e c l a y a n d silt f r a c t i o n s o f six s a n d s t o n e - d e r i v e d soils o f Nigeria Soil series name
Kaolinite
Interstratified material (mica-smectite)
Mica
Quartz
Feldspar
Gombe Dadin Kowa Danggappe
+++ +++ +++ +++ +++ +++
** ** ** *** *** *
** ** * ** * **
+++ +++ ++ ++ ++ +++
+ ++ *** * * **
Agege Alagba Etinan
N o t e : +++, 4 0 - - 8 0 % ; ++, 2 0 - - 4 0 % ; +, 1 0 - 2 0 % ; ***, 5 - - 1 0 % ; **, 3 - - 5 % ; *, 1 - - 3 % . T h e c o l u m n s f o r q u a r t z a n d f e l d s p a r w e r e a s s e s s m e n t s f r o m silt f r a c t i o n s .
344
(6.5A, 3.2A ) and mica also occur in differing amounts in the silt fraction. The silt fractions of the Gombe and Dadin Kowa profiles contain between 13 and 38% feldspar as shown in Table IV. The silt fractions of the sandstonederived soils from southern Nigeria contain less than 10% feldspar, as shown in Fig.4. Feldspar levels are thus appreciably lower in soils of the southern part of the country. This could reflect greater weathering under higher rainfall. The difference could also be related to aeolian additions of dust from the Sahara Desert to soils of northern Nigeria. More than likely, both greater weathering
CLAY
SILT
3
Ld MG* GLY, 25°C 0
GOMBE
co K*GLY,550°C
',""
~>-
,I,
MG* GLY, 25°C
bO Z
AGEGE
'~
W
Z
~D O
MG* GLY, 25°C
! I ~ '
co
J
I !
°~
< K* GLY,550°c
ALAGBA I
3 34
L
4.26
i
356
72 ,~ N G S T R
OM
72
S P A C I N G
Fig.4. X-ray diffractograms for silt and clay fractions of some sandstone-derived soils of Nigeria.
345
under higher rainfall and greater additions of feldspathic minerals in the drier part of the c o u n t r y have affected the present character of the soils. CLASSIFICATION OF PROFILES
Tentative classification of the sandstone-derived soil profiles in two systems is offered in Table V. The Gombe and Dadin Kowa profiles have been placed into the order of Alfisols in the 7th Approximation (Soil Survey Staff, 1967) and into the group of Nitosols in the FAO/UNESCO system (FAO, 1970). The Danggappe profile is placed into the order of Ultisols and the group of Ferralsols. The Agege profile is classified as an Oxisol and as a Ferralsol. The Alagba and Etinan profiles, both o f which are extremely acid and strongly leached, are classed as Oxisols, but the former is placed in the group of Acrisols and the latter in t h a t of Ferralsols in the FAO/UNESCO system. Placements are carried down to the subgroup and great soil group levels in Table V. TABLE V Tentative Classification of six sandstone-derived soils of Nigeria Local series name
7th Approximation
FAO/UNESCO
Gombe Dadin Kowa Danggappe Agege Alagba Etinan
Ochric Haploxeralfs Ultic Haploxeralfs Typic Paleustults Typic Eutrustox Tropeptic Haplorthox Psammentic Acrohumox
Rhodic Nitosols Dystric Nitosols Rhodic Ferralsols Plinthic Ferralsols Haplic Acrisols Xanthic Ferralsols
CONCLUSIONS
These preliminary studies of a selected group o f sandstone-derived soils in Nigeria suggest that some of their characteristics are related to rainfall or ecological zones and others to the parent materials. Still others seem largely independent o f both. Both pH and base saturation decrease with increasing rainfall. The highest values were found in soils of the low rainfall zone and lowest ones in the high rainfall zone. The d o m i n a n t clay mineral was kaolinite in all of the soils. Proportions of other minerals in the clay fraction had a slight tendency to be higher in the soils o f low rainfall zones, but this was n o t a consistent trend. On the other hand, the silt fraction did have a consistent tendency to be more feldspathic in soils of the drier regions. The sand fraction was a major one in all of the soils, regardless of rainfall zones, and a direct reflection o f the sandy parent materials.
346
ACKNOWLEDGEMENTS
The authors wish to express their gratitude to Dr. Klinkenberg and Mr. Valette of Ahmadu Bello University, Zaria, for their assistance during field trips to the northern states of Nigeria. The authors are grateful to the members of the field office division of the Institute of Agricultural Research and Training, University of Ife, Moor Plantation branch in Ibadan, for their assistance during field trips to Agege and Iperu. Thanks also go to Dr. A.U. Salami and Dr. D.O. Ataga for their assitance during field trips to the Mid-Western State of Nigeria. The authors are indebted to Professor J.B. Dixon of Texas A & M University for guiding the first author in the mineralogical analysis of the soils.
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