Mapping Rooting Depth in Sub-Saharan Africa

Amongst all the soil properties, the rooting (or crop-rootable) depth is one of the most important for agriculture, especially rainfed crops, yet largely unavailable at large-scale. Lieven Claessens from IITA participated in the effort of Global Yield Gap and water productivity Atlas project to develop a new high-resolution (1 km²) map of rootable depth and root zone plant-available water holding capacity for sub-Saharan Africa. The paper has been published in Geoderma (V324).

Abstract
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 fertilizers. 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 parameterized (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 the 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). The depth of soil to bedrock reduces the rootable soil volume by 2500 km3, aluminum 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 operationalized such that the maps can easily be updated as additional data become available.

Leenaars, Johan GB, Lieven Claessens, Gerard BM Heuvelink, Tom Hengl, Maria Ruiperez González, Lenny GJ van Bussel, Nicolas Guilpart, Haishun Yang, and Kenneth G. Cassman. “Mapping rootable depth and root zone plant-available water holding capacity of the soil of sub-Saharan Africa.” Geoderma 324 (2018): 18-36. https://doi.org/10.1016/j.geoderma.2018.02.046