Frequency-dependent dielectric response of fertilized and moist sandy clay loam soils: insights from experiments and machine learning
摘要
The dielectric properties of sandy clay loam with moisture contents (MC) of distilled water (DW) and fertilizer solutions were examined, using a vector network analyzer (VNA) over 500 MHz–10 GHz frequency spectrum. The solutions of the different fertilizers namely, Urea, DAP, NPK, NP, K, and Organic fertilizer were mixed with the soil. The complex permittivity of fertiliser solutions was also measured over the same frequency range. It is found that at given MC in the soil the variation in dielectric loss εʹʹ follows the frequency dependence as εʹʹ1.4 GHz > εʹʹ5.35 GHz > εʹʹ9.5 GHz. The results show that the dielectric loss for fertilised soil is higher than that of moist soil using DW as a function of volumetric content. It is found feasible to detect presence of fertilizers in soil based on its dielectric properties. In addition, a CatBoost regression model was employed to predict the real and imaginary parts of the dielectric constant based on frequency, moisture content (MC), bulk density, and fertilizer type. The model established a good analytical performance and was further interpreted using SHAP analysis to identify key contributing features and validate its physical relevance.