Improved prediction of soil exchangeable sodium percentage (ESP) using wavelet

Abstract

In Australia, the soil is considered sodic when the top meter has an exchangeable sodium percentage (ESP) greater than six (6%). Sodicity is a natural feature of Queensland, particularly the case in northeast Queensland where sugarcane is grown. To ameliorate sodicity effectively, a digital soil mapping (DSM) approach with digital data (e.g. elevation, gamma-ray [γ-ray] spectrometry, and soil apparent electrical conductivity [(ECa)]) could be used. However, when a relationship between digital and soil data does not exist, wavelet transformation could be employed. Firstly, we aim to see if digital data, alone or in combination, can be used to predict topsoil (0–0.3 m) ESP using a linear mixed model (LMM). Secondly, we decompose the digital data using wavelet transform into components (i.e. detail [D] and approximate [A]) at different scales (i.e. 10, 20, 40, and 80 m). We then aim to develop an LMM to predict topsoil ESP using D or A or in combination. Results show that prediction of topsoil ESP by LMM using raw digital data either alone (Lin’s = 0.27) or in combination (0.37) was poor (Lin’s < 0.65). The prediction of topsoil ESP by LMM using D components alone was unsatisfactory at small scales, however, moderate agreement (0.65 < Lin’s < 0.8) was achieved at scales of 80 m (0.66). In combination with D and A, a moderate agreement was achieved at 40 m (0.71) and a good agreement at 80 m (0.85). When considering the decomposed digital data individually, and at the same scale, the prediction of topsoil ESP using decomposed ECa (0.56) was better than γ-ray (0.38). Interestingly, the decomposed elevation cannot be used individually to predict ESP. However, adding decomposed elevation to γ-ray (0.58) and ECa (0.67), the prediction of ESP was enhanced. The best prediction of ESP was achieved using all decomposed data at 80 m (0.85). However, further work in validation would be required to confirm topsoil ESP was non-sodic (<5 %) and where it was deemed to be strongly sodic (>20 %) as these were not measured. Nevertheless, the DSM offers the real prospect of applying the six-easy-steps ameliorant management guidelines (Schroeder et al., 2006) to apply suitable rates of gypsum, including 2 t/ha for sodic (ESP; 5–10 %), 4 t/ha for moderately (10–15 %) and 6 t/ha for strongly (>15 %) sodic topsoil.