Hairpin Resonator-Based Microwave Sensor for Detection of Nitrogenous Fertilizers in Soil and Water

Overapplication of nitrogenous fertilizers causes a detrimental impact on the environment, particularly on soil and water. As an accurate yet cost-effective solution, this work presents a hairpin-based microstrip sensor capable of detecting three nitrogen-based nutrients: urea, ammonium sulfate, and aqueous ammonia from both water and soil. The hairpin pattern is chosen from four distinct designs based on its enhanced sensitivity of 93.18 MHz/RPU in detecting materials with dielectric constants ranging from 3 to 25. To facilitate sample acquisition, an acrylic enclosure is integrated onto the FR4 substrate of the 6-GHz resonating sensor. Measurement results indicate that, compared to the unloaded state, the presence of water or dry soil causes an 83.6% and 5% change in operating frequency, respectively. The proposed compact and reusable sensor offers a high sensitivity of 3, 1.63, and 0.7 MHz per ppm (mg/L), along with low detection limits of 32.5, 16.2, and 80 mg/L, in sensing ammonia, ammonium sulfate, and urea, respectively, from water. Similarly, in soil, for the same compounds, sensitivity varies from 1.12 to 1.73 MHz per ppm, and the limit of detection (LOD) is between 5.72 and 16.41 (mg/kg). Such a multipurpose miniature sensor with a $3.635 \times 4$ -cm footprint is highly suitable for many applications related to environmental security.