Enhancing pedotransfer functions with environmental data for estimating bulk density and effective cation exchange capacity in a data-sparse situation

Soil bulk density (BD) and effective cation exchange capacity (ECEC) are among the most important soil properties required for crop growth and environmental management. This study aimed to explore the combination of soil and environmental data in developing pedotransfer functions (PTFs) for BD and ECEC. Multiple linear regression (MLR) and random forest model (RFM) were employed in developing PTFs using three different data sets: soil data (PTF-1), environmental data (PTF-2) and the combination of soil and environmental data (PTF-3). In developing the PTFs, three depth increments were also considered: all depth, topsoil (< 0.40 m) and subsoil (> 0.40 m). Results showed that PTF-3 (R-2; 0.29-0.69) outperformed both PTF-1 (R-2; 0.11-0.18) and PTF-2 (R-2; 0.22-0.59) in BD estimation. However, for ECEC estimation, PTF-3 (R-2; 0.61-0.86) performed comparably as PTF-1 (R-2; 0.58-0.76) with both PTFs out-performing PTF-2 (R-2; 0.30-0.71). Also, grouping of data into different soil depth increments improves the estimation of BD with PTFs (especially PTF-2 and PTF-3) performing better at subsoils than topsoils. Generally, the most important predictors of BD are sand, silt, elevation, rainfall, temperature for estimation at topsoil while EVI, elevation, temperature and clay are the most important BD predictors in the subsoil. Also, clay, sand, pH, rainfall and SOC are the most important predictors of ECEC in the topsoil while pH, sand, clay, temperature and rainfall are the most important predictors of ECEC in the subsoil. Findings are important for overcoming the challenges of building national soil databases for large-scale modelling in most data-sparse countries, especially in the sub-Saharan Africa (SSA).