Mapping rootable depth and root zone plant-available water holding capacity of the soil of sub-Saharan Africa

Leenaars, J G B and Claessens, L and Heuvelink, G B M and Hengl, T and Ruiperez González, M and van Bussel, L G J and Guilpart, N and Yang, H and Cassman, K G (2018) Mapping rootable depth and root zone plant-available water holding capacity of the soil of sub-Saharan Africa. Geoderma (TSI), 324. pp. 18-36. ISSN 00167061

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In rainfed crop production, root zone plant-available water holding capacity (RZ-PAWHC) of the soil has a large influence on crop growth and the yield response to management inputs such as improved seeds and fertilisers. However, data are lacking for this parameter in sub-Saharan Africa (SSA). This study produced the first spatially explicit, coherent and complete maps of the rootable depth and RZ-PAWHC of soil in SSA. We compiled geo-referenced data from 28,000 soil profiles from SSA, which were used as input for digital soil mapping (DSM) techniques to produce soil property maps of SSA. Based on these soil properties, we developed and parameterised (pedotransfer) functions, rules and criteria to evaluate soil water retention at field capacity and wilting point, the soil fine earth fraction from coarse fragments content and, for maize, the soil rootability (relative to threshold values) and rootable depth. Maps of these secondary soil properties were derived using the primary soil property maps as input for the evaluation rules and the results were aggregated over the rootable depth to obtain a map of RZ-PAWHC, with a spatial resolution of 1 km2. The mean RZ-PAWHC for SSA is 74 mm and the associated average root zone depth is 96 cm. Pearson correlation between the two is 0.95. RZ-PAWHC proves most limited by the rootable depth but is also highly sensitive to the definition of field capacity. The total soil volume of SSA potentially rootable by maize is reduced by one third (over 10,500 km3) due to soil conditions restricting root zone depth. Of these, 4800 km3 are due to limited depth of aeration, which is the factor most severely limiting in terms of extent (km2), and 2500 km3 due to sodicity which is most severely limiting in terms of degree (depth in cm). Depth of soil to bedrock reduces the rootable soil volume by 2500 km3, aluminium toxicity by 600 km3, porosity by 120 km3 and alkalinity by 20 km3. The accuracy of the map of rootable depth and thus of RZ-PAWHC could not be validated quantitatively due to absent data on rootability and rootable depth but is limited by the accuracy of the primary soil property maps. The methodological framework is robust and has been operationalised such that the maps can easily be updated as additional data become available.

Item Type: Article
Divisions: Research Program : East & Southern Africa
CRP: CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS)
Uncontrolled Keywords: Root zone depth, Rootability, Soil water, Digital soil mapping, Soil data, Sub Saharan Africa, Maize, Crop yields, Rainfed crop production
Subjects: Others > Digital Soil Mapping
Others > Rainfed Agriculture
Others > Soil
Others > Soil Science
Others > Water Resources
Others > Sub-Saharan Africa
Depositing User: Mr Ramesh K
Date Deposited: 05 Apr 2018 10:30
Last Modified: 08 Jun 2018 10:12
Official URL:
Funders: Open Access funded by Bill & Melinda Gates Foundation
Acknowledgement: This research was a collaborative effort, coordinated by ISRIC - World Soil Information (, of the Africa Soil Information Service project (AfSIS; and the Global Yield Gap and water productivity Atlas project (GYGA; Funding to support this work was provided by the Bill and Melinda Gates Foundation (BMGF) (Investment ID: OPP1082520) which made the AfSIS and GYGA projects possible. The GYGA project acknowledges support from the CGIAR research program on Climate Change, Agriculture and Food Security (CCAFS).
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