The soils The study of the soils of the Delta is carried out on the basis of the textures recorded on the 169 sites sampled to assess vegetation and ecological parameters. Soil texture measurements carried out by Pierre Hiernaux estimate the mass contribution of clays (between 0 and 20 µ), loams (between 20 and 50 µ) and sands (between 50 µ and 2 mm) for five soil depth s : [0-6 cm], [6-12 cm], [12-25 cm], [25-50 cm], [50-100 cm]. These data have been simplified by grouping soil depths in two levels: (HOR1) from 0 to 25 cm and (HOR2) from 25 to 100 cm. The texture of each level is assessed by the arithmetic mean of the mass contribution of clays, silts and sands of each component layer . The grouping of the three top layers is justified by the fact that, on the one hand, the particle sizes of the first two layers (0-6 cm) and (6 - 12 cm) do not generally differ and that, on the other hand, the differences with the third layer (12 - 25 cm) are relatively unmarked. This grouping is also justified by the root system of most aquatic plants in the Delta, which generally does not exceed 25 cm in depth. The grouping of the two deeper levels is supported by the large similarity of texture in the two component layers. The soil texture of vegetation mosaics is assessed by averaging the clay, loam and sand contribution of soil supporting each plant association component, for HOR1 and HOR2. The texture triangle was defined by Pierre Hiernaux. It is derived from the texture triangle used by US soil scientists, but adapted to the very fine textures soils of the Delta. The soils of the Delta have been classified into 9 classes, according to their textures , and numbered from 0 to 9 in the databases: [0 - Sandy soil, denoted S], [1- Sandy-loamy, denoted SL], [2- Loamy-sandy, denoted LS], [3- Sandy-clayed, denoted SA], [4-Loamy-clayed-sandy, denoted LAS], [5- Clayed-loamy-sandy, denoted ALS], [6-Loamy, denoted L], [7- Clayed-sandy, denoted AS], [8 Loamy-clayed, denoted LA] and [9- Clayed, denoted A].
The soils of the Inner Niger Delta are mostly fine textured soils, heavily loaded with loams and clays. Two main categories dominate the soils of the alluvial basin: the LA – Loamy-clayed – and ALS – clayed-loamy-sandy - formations. LA-type soils like those of the “ burgu ” vegetation contain 45 to 50% of loam, more than 30% of clay and less than 20% of sand. ALS-type soils are also heavily loaded with loam (more than 45%) but contain less clay (about 20%) and more sand (25 to 35%). Sandy clay soils only come in third position with sand contributions exceeding 50% and rising up to 80%. They are mostly found on the “ togge ”, unflooded lands in the Delta. Table 3 shows the relationship between plant associations and soil types. Among the plant associations supporting deep flood, OP is related to the most clayed soils, followed by “ burgu ” BP and B. The link between these vegetation associations and fine textured soils is explained by the edaphic characteristics of the herbaceous plants that develop in basins or plains flooded by still waters and also by the filter function of the vegetation, since Oryza longistaminata , like Echinochloa stagnina, trap sediments. In contrast, VB, the deep Vetiveria savanna, grows where the stream current is stronger. Its erect tufts filter much less sediment and trap more sand. Table 3 : Plant associations per soil texture type (top/deep) and depth of flooding
Table 1 : Main types of Horizons
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Table 2
HORIZON 1 (0 - 25 cm) Soil Types Delta areas (%) ALS 38.9 LA 33.6 SA 12.2 A 6.4 LAS 4.6 SL 2.1 AS 2.0 LS 0.3
HORIZON 2 ( 25 - 100 cm) Soil Types Delta areas (%) LA 53.7 ALS 36.2 SA 7.4 LAS 1.8 AS 0.6 L 0.2
Soils profile A/LA LA/LA ALS/LA ALS/L LA/ALS AS/ALS ALS/ALS LAS/ALS SL/ALS LAS/AS LS/LAS SA/SA Hor1 /Hor 2 TB TC TD THY TS AG MB VH P PAS MB PAN VSP MB VOR MB PAM MB 2,80 – 4 m BP PAK coded 98 88 58 56 85 75 55 45 15 47 24 33 VB EOR R 1,5 – 2,80 m OP B 0,6 –1 ,5 m O 0,3 – 0,6 m ESP 0,1 – 0,3 m P ZB TA 0 – 0,1 m PAR not floodable TT Classe s Type s of horizons Symbol s Limits of classes 0 S andy S 60 < S ≤ 100 - 0 < L ≤ 15 - 0 < A ≤ 6 1 Sandy - loamy SL 60 < S ≤ 85 - 8 < L ≤ 30 - 0 < A ≤ 6 2 Loamy - sandy LS 30 < S ≤ 70 - 30 < L ≤ 60 - 0 < A ≤ 6 3 Sandy - clayed SA 57 < S ≤ 95 - 0 < L ≤ 30 - 6 < A ≤ 12 4 Loamy - clayed - sandy LAS 25 < S ≤ 76 - 30 < L ≤ 60 - 6 < A ≤ 12 5 Clayed - loamy - sandy ALS 12 < S ≤ 57 - 30 < L ≤ 60 - 12 < A ≤ 25 6 Loamy L 0 < S ≤ 40 - 60 < L ≤ 100 - 0 < A ≤ 40 7 Clayed - sandy AS 45 < S ≤ 90 - 0 < L ≤ 30 - 12 < A ≤ 25 8 Loam y - clayed LA 0 < S ≤ 75 - 0 < L ≤ 60 - 25< A ≤ 40 9 Clayed A 0 < S ≤ 40 - 0 < L ≤ 60 - 40 <A ≤ 60
Distribution of the main types of soils in the Delta : Maps and table 2