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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Soil _data.rar
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