Proximal and remote sensor data fusion for 3D imaging of infertile and acidic soil

Abstract

Soil cation exchange capacity (CEC) and pH affect the condition of soil. To improve soil capability in sugarcane growing areas, Sugar Research Australia introduced the Six-Easy-Steps Nutrient Guidelines based on CEC and pH of topsoil (0–0.3 m). A three-dimensional digital soil mapping (DSM) framework has been used to predict CEC and pH by fitting equal-area splines to four depth intervals (i.e., topsoil, subsurface [0.3–0.6 m], shallow [0.6–0.9 m], and deep subsoil [0.9–1.2 m]) to resample soil data at 0.01 m increments. A single quantile regression forest (QRF) was calibrated to model the relationship between spline-fitted soil data and individual digital data. These included proximal soil sensing (PSS) data such as electromagnetic (EM) induction and gamma-ray (γ-ray) spectrometry, remote sensing (RS) Sentinel-2 imagery, a light detection and ranging (LiDAR) based digital elevation model (DEM), and soil depth. Various data fusion methods and minimum calibration size have been evaluated, including concatenation and model averaging approaches, namely, simple averaging (SA), Bates-Granger averaging (BGA), Granger-Ramanathan averaging (GRA), and bias-corrected eigenvector averaging (BC-EA). In all cases, an independent validation was used to assess prediction agreement (Lin's concordance correlation coefficient—LCCC) and accuracy (ratio of performance to deviation—RPD). For CEC, γ-ray (LCCC = 0.82) was the best, with EM (0.78) and Sentinel-2 (0.77) producing similar agreement, whereas DEM (0.64) had worst performance. For pH, EM, γ-ray, and Sentinel-2 were similar (0.69, 0.73, and 0.77, respectively), and DEM poor (0.48). Optimum results were achieved when PSS, Sentinel-2, and DEM were fused using GRA; CEC agreement (0.88) and accuracy (RPD = 2.14) were strong, while for pH, concatenation had good agreement (0.79) and accuracy (1.59). Neither agreement nor accuracy varied among sample size, with a minimum of 30 (CEC) and 80 (pH) sites necessary (0.4 and 1.1 sampling sites ha−1, respectively). The final DSM for topsoil CEC and pH, were useful for lime application; the northern fields required 2.5 t ha−1 of lime, whereas the southern fields required variable rates (4 and 5 t ha−1).