Capillary Soil Nutrient Profiling Device: Pre-processing Free Approach for Rapid Soil Nutrient Assessment

The development of reliable point-of-source devices for soil nutrient profiling holds the key to unlocking maximum agricultural output while promoting sustainable practices with minimal environmental impact. The dynamic nature of the soil, its testing protocols, and multistep pre-processing of samples results in time-dependent responses from the sensors increasing the testing time and cost requires additional peripheral equipment. Thus, portability along with precision gets affected simultaneously. Moreover, signal processing, data generation, and acquisition also compromise the soil nutrient assessment. In this work, a standalone device was developed with an alternate soil nutrient quantification protocol for nitrate and potassium, leveraging the capillary forces in the cellulose substrate owed to porous architecture and inter-cellulose fiber voids to eliminate conventional protocols like extraction, centrifugation, and filtration (to eliminate matrix effects) to achieve single-step soil nutrient quantification. Additionally, the use of external 24-bit analog-to-digital conversion (ADC), a quick 2-point calibration smartphone was employed to increase the resolution of the measurements and accuracy of the nutrient measurements. Compared to traditional soil testing methods, the proposed system demonstrated a detection limit and quantization limit of 0.1 mM, with a linear response range of 0.5-21 mM for potassium and 0.2-1.4 mM for nitrate. Precision tests across 15 reuse cycles showed average variability below ±5%, confirming the reliability and repeatability of the sensor. The proposed approach can have broader implications such as the development of portable, low-cost, processing-free, and reliable soil nutrient sensors for in-field applications.