Monitoring of Plant-Available Water with a Low-Profile Square Patch Loaded with a Circular-slot Planar Sensor based on Soil Texture-Dependent Moisture Characteristics

This work presents the design and application of a square patch sensor, modified with a centrally positioned circular slot, fabricated on Rogers RT/duroid substrate with relative permittivity (∈_r) of 2.2. The sensor demonstrates a reflection coefficient of -47.36 dB at an operating frequency of 2.4 GHz. This design is employed to investigate the dielectric properties of soil as a function of varying moisture content (M.C.) across different soil textures. A comprehensive characterization of sandy loam and clay loam soils is performed to capture the dielectric variations as M.C. changes, demonstrating the dynamic interactions between free and bound water molecules within the soil matrix. Experimental measurements are conducted by systematically adjusting the M.C. from 0% to 17% in both soil types. Results indicate that sandy loam soil consistently exhibits a higher ∈_r than clay loam soil at comparable moisture levels. Notably, at a moisture content of 17%, the ∈_r of sandy loam soil is observed to be 19% greater than that of clay loam. These findings highlight the significant impact of soil texture and water distribution on soil moisture characteristics, underscoring the role of free water content as a determining factor. A fourth-order polynomial equation is derived to accurately model the M.C. %, demonstrating a high degree of fit with a coefficient of determination R2=0.9605. Additionally, a linear relationship is observed between plant available water (PAW) and M.C., with notable variations between sandy loam and clay loam soils. This analysis provides valuable insights into the dependency of soil moisture retention and availability on soil texture, presenting a useful approach for soil moisture estimation and agricultural water management.