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

Authors

John G. B. Leenaars, ISRICFollow
Lieven Claessens, ICRISAT
Gerard B. M. Heuvelink, Wageningen University & Research
Tom Hengl, ISRIC
Maria Ruiperez Gonzalez, ISRIC
Lenny G. J. van Bussel, Wageningen University & Research
Nicolas Guilpart, AgroParisTech
Haishun Yang, University of Nebraska-LincolnFollow
Kenneth G. Cassman, University of Nebraska-LincolnFollow

Document Type

Article

Date of this Version

2018

Citation

Geoderma 324 (2018) 18–36

Comments

© 2018 The Authors.

This is an open access article

https://doi.org/10.1016/j.geoderma.2018.02.046

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 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 georeferenced 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 74mm 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.