Managing nitrate is a central concert for precision agriculture, from delineating management zones, to optimizing nitrogen use efficiency through in-season applications, to minimizing leaching and greenhouse gas emissions. However, measurement methods for in-soil nitrate are limiting. State-of-the-art soil nitrate analysis requires taking soil or liquid samples to laboratories for chemical or spectrographic analysis. These methods are accurate, but costly, labor intensive, and cover limited geographic scope. Some colorimetric tests are available which give qualitative nitrogen information. Other researchers rely on measurements of applied nitrate, leaching, and uptake to calculate mass balance equations for larger areas, but errors in estimates of inputs or outflows lead to errors in nitrate concentration. Alternatively, NDVI or other vegetative indices can be used to estimate spatial variation of nitrate in plant material, but by the time nutrient deficiencies are evident in these indices, it may be too late to correct.
Quantitive soil nitrate sensors at high special resolution and multiple depts are needed to fully leverage precision agriculture technologies. Printed potentiometric ion-selective nitrate sensors could fill this need because they are small, low power, involve no moving parts, and are mass-producible. Printed nitrate sensors have been fabricated and characterized nitrate sensors in aqueous solutions. Sensors have been made from two types of materials: degradable and non-degradable. Non-degradable sensors are made from plastics, acrylics, and metals, and are designed to be robust for repeated insertion into soil, and have a long lifetime. Degradable sensors are made primarily from wood, paper, waxes, water-soluble plastics, and conductive carbon, and would be suitable for deployment at high special density in fields where electronic waste buildup is undesirable.
Both degradable and non-degradable sensors show sensitivity to nitrate near the theoretical limit governed by the Nernst equation, and are not significantly impacted by the presence of P2O5, K+, Mg2+, Cl−, NO2− and SO42−. The sensors are next characterized and calibrated in real soils of varying textures, water content, pH, electrical conductivity, cation exchange capacity, and organic matter content. Sensors are optimized for repeatability and stability over time.
