ISRIC Report 2007/02 SOTER-based soil parameter estimates for Central Africa - DM of Congo, Burundi and Rwanda

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Niels H Batjes
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This report describes a harmonized set of soil parameter estimates for Central Africa, comprising Burundi and Rwanda (scale 1:1 million) and the Democratic Republic of the Congo (scale 1:2 million). The data set was derived from the Soil and Terrain Database for Central Africa (FAO et al. 2007) and profiles held in the ISRIC-WISE database, using standardized taxonomy-based pedotransfer (taxotransfer) procedures (Batjes et al. 2007).
The land surface of Central Africa, comprising some 2.4 million km2, has been characterized using 244 unique SOTER units, corresponding with 504 polygons. Each SOTER unit may consist of up to 6 soil components; each of these has been characterized by a representative profile. The main soil units mapped for the region have been characterized using 167 real profiles, selected by soil experts as being regionally representative for these units; soil analytical data were derived from soil survey reports.
These sources seldom hold all the physical and chemical attributes ideally required by SOTER. The primary database also includes 129 virtual profiles for which only the FAO classification is known; inherently, there are no measured data for these profiles. Gaps in the soil profile data have been filled using taxotransfer procedures
; these were developed using soil profiles from WISE (n= 5672) having similar FAO soil unit names as those mapped for the SOTERCAF region.
Parameter estimates are presented by soil unit for fixed depth intervals of 0.2 m to 1 m depth for: organic carbon, total nitrogen, C/N ratio, pH(H2O), CECsoil, CECclay,base saturation, effective CEC, aluminium saturation, CaCO3 content, gypsum content, exchangeable sodium percentage (ESP), electrical conductivity of saturated paste (ECe), bulk density, content of sand, silt and clay, content of coarse fragments (> 2 mm), and available water capacity (-33 kPa to -1.5 MPa). These attributes have been identified as being useful for agro-ecological zoning, land evaluation, crop growth simulation, modelling of soil carbon stocks and change, and analyses of global environmental change.
Soil parameter estimates are presented as summary files that can be linked to the SOTERCAF map using GIS, through the unique SOTER-unit code; applications should consider the full map unit composition and depth range. Individual parameter estimates should be seen as best estimates based on the current selection of soil profile data; each taxotransfer rule has been flagged to provide an indication of confidence in the derived data. The secondary data presented here are considered appropriate for exploratory studies at the continental scale (= 1:1 million). Correlation of soil analytical data should be done more rigorously when more detailed scientific work is considered.
Keywords: secondary data set,soil parameter estimates, Central Africa, Rwanda, Burundi, Democratic Republic of the Congo, environmental modelling, WISE database, SOTER database