In order to clarify the concept of soil as used in this report the following definition is adapted: A soil is a three-dimensional body occupying the uppermost part of the earth's crust and having properties differing from the underlying rook material as a result of the interactions between climate, living organisms (including human activity), parent material and relief over periods of time and which is distinguished from other “soils” in terms of differences in internal characteristics and/or in terms of the gradient, slope-complexity, micro-topography, stoniness and rockiness of its surface (Brinkman and Smyth, 1973).
The following soil surveys were carried out in or around the Kainji Lake Basin:
| Pullan and de Leeuw (1964) | - | East Bank (reconnaissance) |
| Klinkenberg (1965) | - | West Bank (reconnaissance) |
| Klinkenberg and Hildebrand (1964) | - | Town site Mew Bussa (detailed) |
| Valette (1967) | - | HE Branch Niger (reconnaissance) |
| Klinkenberg (1973) | - | Papiri, Gafara (detailed) |
The underlying geology associated with the various landscape elements are the main key to the soil mapping legend of these surveys. Indeed the rock types influence to a large extent soil characteristics and properties (Table 1).
Table 1
Data of epipedons of upland soils derived from different rocks (mean values)
| Rock type | No. of observations | Depth cm | % clay | pH | % C | Total ppm | P. CEC | Total bases | meq/Ha | 100 g K | soil Ca + Mg |
| Sandstone | (3) | 0–18 | 5.5 | 6.0 | 0.63 | 90 | 2.9 | 1.8 | 0.04 | 0.09 | 1.9 |
| Gneiss | (4) | 0–11 | 7.0 | 5.9 | 0.60 | 130 | 5.3 | 2.8 | 0.06 | 0.15 | 2.6 |
| Metasediment | (7) | 0–14 | 9.7 | 6.7 | 0.76 | 153 | 7.3 | 4.3 | 0.24 | 0.15 | 4.0 |
| Granite | (1) | 0–18 | 4.0 | 6.6 | 0.48 | 76 | 3.6 | 9.5 | 0.42 | 0.20 | 8.9 |
Source: Klinkenberg, 1965
These trends are confirmed by other data available (Klinkenberg et al.1964) In addition, the following changes were recorded with depth in most soils before impoundment: 1) general decrease in pH; 2) decrease in percentage organic carbon; 3) decrease in total phosphorous.
The clay mineralogy is dominated by the presence of 1:1 lattice clay, mainly kaolinite (60–90%) with illite (10–30% and montmorillonite (5–20%) as occur in most soils.
The heavy mineral content of the sand practices is very low (1–2%) and its composition reflects sharply the nature of the surrounding Basement-Complex rooks and the Nupe Formation (Adegoke et al. 1970). The light mineral composition is characterised by the presence of quarts, K-felspar and anorthite and albite (NEDECO et al. 1961).
On the basis of previous surveys in the area a number of soil associations are recognised in the draw-down areas which comprise the following soil series:
| Soil Association | Parent Rook | Soil Series |
| Foge | Metasediments | Amboshidi, Sangunu, Sangwabe, Makoshi, Koshi, Mako, Kokoli, Wallau, Ushaba, Mahuta |
| Kainji | Gneisses | Suteku, Agwarra, Rofia, Kocerka |
| Menai | Granites | Menai, Katai, Tai |
| Shagunu | Sandstones | Kulfo, Saipa |
| Silt stones Conglomerates | Kutukpachi, Kuru, Mokowa | |
| Wara | Alluvium | Mospa, Spa |
| Anna, Bansura, Inwa |
In all associations rockoutcrops and lithosols (10 cm deep) may occur. These soils have been described in the various reports, especially those by Pullan et al. (1964) and Klinkenberg (1965).
Some of their main properties are summarised in the following table:
Table 2
Summarized data on soil series
| Soil Series | Depth Indication | Texture Variation 100 cm | Drainage | Other |
| Amboshidi | moderate | scl/sl/cl | Moderately well | Mottling below 20 am and quarts gravel |
| Sangunu | deep | s/sl/ls | well | common pale yellow mottling and iron concretions below 36 cm |
| Sangwabe | very deep | s/scl | well | distint red mottling below 56cm gravel increasing with depth |
| Kokoli | very shallow | sl | well | very gravelly below 60 cm |
| Wallau | deep | ls/sl/scl | well | occassionally gravelly below 60 Cm |
| Ushaba | deep | ls/sl/scl | Moderately well | gravel increasing with depth red mottles common below 18 cm |
| Mahuta | very deep | s/scl | well | pale coloured topsoil over strongly mottled subsoil below 41 cm |
| Makoshi | moderately | sl/sc | Moderately well | very gravelly throughout, common red mottles |
| Koshi | deep | ls/sl | moderately well | gravelly throughout, common brown mottles below 56 cm; few iron concretions |
| Mako | deep | sl/s/sl | poorly | gravelly throughout and faint mottling |
| Agwarra | deep | s/ls/sl/scl | Moderately well | stone line present at 100 on variable iron concretions and mottles |
| Suteku | Moderate | ls/s/sl | Moderately well | common mottles below 20 en |
| Rofia | shallow/moderate | sl/ls/sl | Moderately well | gravelly throughout |
| Kocerka | Moderate | sl/scl/sl | moderately well | distinct mottling below 50 cm in grey matrix colour |
| Menai | very deep | s/ls/sl | well | many iron concretions below 50 cm and rook fragments |
| Katai | moderately | ls/scl | well | gravelly throughout, Many iron concretions, mottling below 61 cm |
| Tai | moderately | ls/scl | Moderately well | very gravelly increasing with depth abundant red mottling below 25 cm |
| Kulfo | very deep | s/scl | Somewhat excessively to well | homogeneous reddish brown to red |
| Saipa | shallow | gr/s | well | gravel within top 30 cm |
| Kutukipachi | very deep | s/scl | well | strongly Mottled below 60 cm |
| Mokowa | deep | sl/scl/ls | moderately well | mottled throughout |
| Kure | deep | s/sl/ls | well | much quarts gravel and iron concretions |
| Mospa | very deep | s/sl/ls | well | varying yellow Bottling throughout, few iron concretions |
| Spa | very deep | sl/c | very poorly | as “Mospa” |
| Auna | very deep | s | somewhat excessively | homogeneous very pale brown common Bottling below 50 cm |
| Bansura | deep | scl/sl/scl | moderately well | common iron concretions below 18 cm |
| Imra | very deep | s | somewhat excessively | pale brown, common mottling below 15 cm |
| Depth indication: | very shallow | : | 10–25 cm | ||
| shallow | : | 25–50 cm | |||
| moderately deep | : | 50–90 cm | |||
| deep | : | 90–120 cm | |||
| very deep | : | >120 cm | |||
| Texture indication: | s-ls | : | loamy sand ( < 7% day) | ||
| sl | : | sandy loan (>-20% day) | |||
| scl | : | sandy clay loam (20–35% day) | |||
| sc | : | sandy clay (> 35% day) | |||
| : | gravel | ||||
thus s/ls/sl = sand over loamy sand over sandy loam within 100 on depth.
Apart from socio-economic factors and water supply, soil use was formerly determined in the present draw-down area by soil depth, gravel, drainage and nutrient level. The latter is low to very low throughout, especially in N and P; K being more readily supplied by weathering of K-felspar. Commenting on the micronutrient content of West-African soils in general, Kang et al. (1972) indicate the low total content, especially in boron, molybdenum and sine. Sandy and alkali soils are particularly affected.
The considerable variety of soils occurring in the draw-down areas necessitates large scale mapping of these soils in development areas
It is anticipated however, that the development of the soils may subdue some of the differences between them, because of one common factor: seasonal impoundment. This causes:
A comparison between analyses data of flooded soils and their counterparts, excluding flooding, show the following changes in the former:
A change in clay mineralogy because of an alteration in environmental conditions was suggested by Siderius (1973) and is supported for the Kainji Lake soils by data from Asseez (1970).
Little changes in soils of the Volta Lake draw-down areas were reported by Amatakpor (1970), however his information covers only a relatively short time.
Apart from the reduced conditions caused by flooding, the dissolved and solid sediment load of the lake water will influence the toil composition.
In general there seems to be little variation in chemical composition of lake water during the season, indicating that the lake is vertically fairly well nixed, although the decree of mixing of inflow and lake water decreases with increasing inflow rate (Zimmermann et al., 1973).
The general composition of the lake water is given in Table 3
Table 3
Chemical composition of the lake water
| Dissolved oxygen | Oxygen concentration | Sulphide concentration | Cations (me/1) | Anions (me/1) | |||||||
| Ca | Mg | Na | K | KO3 | SO4 | NO3 | C1 | EC | |||
| 70% | 1–4 mg/1 | S 0.6 mg/1 | 0.2 | 0.2 | 0.01 | 0.04 | 0.55 | 0.05 | 0.05 | 0.05 | 45–60 |
Na and K concentrations have decreased but the Kg concentration has increased (Henderson, 1973).
In addition iron occurs but its concentration is variable (mean 0.01 mg/1). Highest value is measured during September (height of the white flood) indicating that the surrounding area is the main supplier of this constituent.
Nitrates and phosphates are low but variable while the silica concentration is very high. Data from Imebore (1970) indicate that the inflood of nutrients from tributaries of the former Niger river is considerably higher than the concentration of the main river itself. Also the total ionic concentration is higher for the white flood. The pH value ranges from 6.2–8.5, the black flood is slightly more alkaline than the white flood.
The lake water is considered very suitable for irrigation, the quality of the water being classified as C1-S1, low conductivity and SAR-ration (USDA, 1969).
The solid sediment load varies from 29–221 ppm according to the flood regime (Asseez, 1970). The white flood waters contain more colloidal particles than the black flood (NEDECO et al. 1961). However total amount of sediment is low and there is little danger for silting up of
| No. Sample | Location | Sampling depth cm | Description |
| 18.63 | South of Doro village at waterlevel | 0–15 | very dark greyish brown (2.5 Y 3/2) loam, weak fine subangular blocky structure; plastic, slightly sticky; many fine roots, |
| 18.64 | " | 15.35 | olive brown (2.5 Y 4/4) sandy clay; structure and consistence as above; common fine dominant yellowish brown (10 YR 5/6) mottles; fine Fe concretions increasing with depth, |
| at 35 cm blocked by hard plinthite. | |||
| 19.65 | 200 m SE of 18 | 0–20 | very dark grey (10 YR 3/1) sandy clay; moderate medium subangular blocky; friable; few fine distinct yellowish brown (10 YR 5/6) mottles; non sticky, slightly plastic, |
| 19.66 | " | 20–60 | dark greyish brown (2.5 Y 4/2) clay; very plastic, non sticky; few fine light olive brown (2.5 Y 5/6) mottles; few fine iron concretions increasing with depth, |
| 19.67 | " | 60–120 | light olive brown (2.5 Y 5/4) clay with common fine distinct yellowish brown (10 YR 5/8) black (2.5 Y 2/0) and grey (2.5 Y 6/0) mottles; many Fe concretions, some soft; firm, plastic, non sticky. |
Most of the coarser sediments will be deposited as bedload upon entrance in the Central Lake Basin. Finer particles (fine silt and clay) are expected to settle down in tranquil environment where conditions are suitable for their sedimentation. These requirements are satisfied in sheltered bays and inlets, where sedimentation is promoted by vegetation (grass) which reduces streamflow.
Sedimentation in these areas may be as much as 3 cm/season (Halstead, 1971). Normally however a rate of opposition of 0.5 cm/year is thought possible in such regions of the draw-down.
Some lake water samples, taken during the mission period, reveal the following:
| Sample site | date | depth | ppm | clay mineralogy |
| Papiri* | 13.9.1973 | 1 m | 48 | kaolinite dominant |
| Shagunu* | 13.9.1972 | 1 m | 80 | calcite ± 10% |
| Sam | 28.10.1973 | 5m | 20 | little quarts and felspar; very poorly crystalline chlorite (subject to weathering) |
The ppm data agree with existing information. In addition the report of the clay mineralogy is confirmed. The high degree of weathering indicates the old age of most of the soil material that was eroded and brought into the lake. Adegoke et al. (1970) report the heavy mineral composition of the sediment to consist of epidote, zircon, sillimanite, hornblende, staurolite and traces of kyanite.
In combination with visual tests relative to the turpidtty of the water it seems that the flood water with the highest sediment load does not reach the most southern part of the lake, the major part of the solid sediment load being deposited in the northern and central lake area.
Thus enrichment of the flooded area takes place mainly as a result of sedimentation. In addition the slight alkalinity of the flood water and the influx of bases promote the formation of 2:1 lattice day minerals in soils of the draw-down area. This causes an improvement in the base exchange properties of the soils in general.
Although improvement of all soils that are subject to flooding is occurring, initial differences between the soils, mainly caused by variations in parent material, will persist for a long time.
The sampling locations are indicated on Fig. 1. Representative of the Nupe Formation are sites Nos. 1, 4, 15 and 17; for the Metasediments Nos. 7, 8, 9, 14 and 16; for Gneiss Nos. 6, 10, 11 and 18, 19 and WAN II; for Alluvium Nos. 2, 3, 5, 12 and 13.
Soil data are summarized in Table 4.
Table 4
Data from epipedons (t) and B horizons (s) of soils in the draw-down area (450–455 feet) according to parent material
| pH | Org. C.% | % N | P2O5 avail. ppm | Exchangeable cations (Meg/100 g) | ||||||
| Na | K | Ca | Mg | Sum | ||||||
| Sandstone: | t | 6.0 | 0.43 | 0.04 | 32.73 | 0.05 | 0.10 | 1.41 | 1.36 | 2.92 |
| (4)* | s | 6.5 | 0.25 | 0.02 | 2.31 | 0.41 | 0.22 | 4.11 | 2–44 | 7.18 |
| Gneiss: | t | 6.4 | 0.29 | 0.13 | 13.65 | 0.07 | 0.19 | 4.65 | 2.15 | 7.06 |
| (3)* | s | 6.3 | 0.39 | 0.06 | trace | 0.06 | 0.24 | 3.54 | 3.37 | 7.21 |
| Metasediments | t | 5.7 | 0.54 | 0.05 | 13.60 | 0.06 | 0.08 | 2.79 | 1.74 | 4.67 |
| (5)* | s | 6.1 | 0.32 | 0.05 | 1.99 | 0.12 | 0.11 | 4.94 | 8.71 | |
| Alluvium | t | 5.6 | 0.70 | 0.06 | 8.40 | 0.07 | 0.09 | 1.91 | 1.00 | 3.00 |
| (5) | s | 5.9 | 0.17 | 0.02 | 6.98 | 0.13 | 0.18 | 3.04 | 1.41 | 4.76 |
The most remarkable changes are related to the pH value and the exchangeable cations; both values increase considerably with depth (see also 3.3). Compared with data from upland soils (Table 1) the base exchange is again the differentiating property.
Data from cultivated draw-down was obtained for Papiri (Klinkenberg, 1973). The differences between cultivated and non-cultivated draw-down are slight as yet but may increase due to management.
Of greater interest are data from cultivated draw-down but from different contours (Table 5).
Table 5
Chemical data of cultivated draw-down soil (Kutuk pachi Series) according to elevation
| Contour (ft) | PH | total N % | available P205 ppm | Exchangeable cations meq/100 g | ||
| K | Ca | Mg | ||||
| 464 | 5.6 | 0.06 | 13.86 | 0.13 | 2.53 | 0.98 |
| 467 | 5.9 | 0.04 | 19.56 | 0.07 | 2.23 | 0.54 |
| 472 | 6.1 | 0.03 | 16.56 | 0,07 | 1.91 | 0.45 |
| Upland | 5.6 | 0.03 | 15.06 | 0.06 | 1.13 | 0.55 |
These data indicate a suitable chemical environment for plantgrowth, apart from the low nutrient level in most soils, and comply well with information collected by Vine (1970) on Nigerian soils.
In addition to the chemical data the average textures of the control section (25–100 cm) are fine loamy to clayey. Most epipedons have a sandy loam texture. The increase in clay with depth combined with morphological observations (cutans) satisfies the definition of the argillic horizon, which is present in most soils. Apart from observations 14 and 17, the soils have ample rooting depth (> 90 cm deep). Structure is weakly to moderately subangular blocky. In clayey soils strong structural features may be observed, especially at drying (observation MAN II - Classified as Vertisol). Largest structural elements are coarse prisms determined by the cracks. The prisms break down into coarse angular blocky peds.
Red mottling is common in the (nib) soils of many draw-down soils. It is caused by the release and redistribution of iron in reduced milieu daring wet periods and its dehydration sad crystallization in the drier spells.
It is not considered as indication for bad drainage. Mottling caused by prolonged stagnation of groundwater was occasionally observed in the deep subsoil of some low lying soils. It is typified by yellowish colours in a grey-blue Matrix.
Few data are available relative to the classification of the soils according to the more Modern principles of soil classification, such as expressed in documentation by the FAO/Unesco and the USDA. However the following remarks can be Made relative to the soils in the drawdown areas. If sufficient soil depth is encountered, those occurring above the 475 feet contour are generally classified as Ferric Luvisols (FAO) or as Paleustalfs (USDA). Soils in intermediate position towards the lake classify as Plinthic Luvisols (FAO) or as Plinthustalfs, while soils under influence of the lake water in the lowest topographic positions are classified as Gleyic Luvisols with Plinthic Gleysols (FAO) or according to the USDA classification as Plinthaqualfs and/or Tropaqualfs. Occasional Vertisols are encountered which are classified as Chromic Vertisols (FAO) or as Chromusterts (USDA).
