Design and fabrication of an automated soil-water micro-sampling system

Abstract

Sustainable management of soil nutrients is a challenge for food production to meet the nutritional demands of a growing population of humans, which has surpassed eight billion. Informed management decisions toward maintaining suitable availability of plant macronutrients in soils without excess fertilizer inputs is limited by the ability to collect and analyze water chemistry in small sample volumes extracted from intact soils over time. Additionally, the semi-arid climate and increasingly more frequent meteorological drought conditions in soil systems like the agricultural regions of the Northern Great Plains limit the practicality of conventional soil-water collection and analysis techniques due to the small amounts of water available in the shallow vadose zone during the growing season. In this work, I present progress toward a solution at the intersection of automation, microfabrication, and environmental monitoring systems. The Microfluidic Environmental Solute Analysis (MESA) system has the potential to allow multiple deployments providing enhanced spatial and temporal resolution compared to conventional soil-water collection techniques in measurements of soil water solutes critical to understanding the soil chemistry that supports agricultural production. Using only 100 microliters of water extracted from the soil, the MESA system provides onboard, real-time electrical conductivity analysis (future work will include temperature, pH, and nitrate sensing). The electrical conductivity (EC) sensor uses single-frequency electrochemical impedance spectroscopy (EIS) to measure the bulk fluid resistance within the measurement chamber of the MSM. Calibration of the MSM of EC ranging from 100 - 6440 microseconds cm -1 has shown that the cell constant is 9.530 cm -1, although this parameter is sensor and package dependent. In-situ conductivity measurements in engineered soil columns have revealed that the sand tested has an intrinsic conductivity of approximately 380 microseconds cm -1. The maintenance-free system is intended to be buried in the soil and provide automatic measurements throughout the Montana growing season without being disturbed. The deployment of the MESA system can provide researchers with new data that may enhance our understanding of biogeochemical cycling in dry-land agricultural settings.