Distributable Screen-Printed Soil pH Sensor Shows Long Term Invariant Data Acquisition Across Soil Moisture, Compaction Levels and Soil Types

The global population is projected to surpass 10 billion by 2052, posing an urgent challenge for food production. As the agricultural sector strives to meet a 25% increase in food demand, precision agriculture emerges as a solution to improve sustainability and efficiency by optimizing resource allocation in the field. pH sensing lies at the center of this approach since soil acidity is critical in maintaining crop productivity, regulating soil nutrients, and maintaining a healthy soil biome. However, commercial automatic soil pH sensors for continuous measurements tend to be expensive and not long-lasting. At the same time, cheaper solid-state soil pH sensors tend to only work on solutions or not be reliable in the long term.
Here we present the advances and evaluations of a flexible, screen-printed, distributable soil pH sensor designed for long-term use in agricultural soil monitoring. The sensor is screen-printed on a PEN substrate and uses a carbon-Alizarin composite ink as the active sensing material in its working electrode. To protect the sensor and make it long-lasting, a PVB-NaCl membrane that covers the printed silver/silver chloride reference electrode was added, as well as a Nafion membrane for the working electrode.
Testing across a variety of gravimetric water contents demonstrated minimal error, with a deviation average of only 0.087 pH units, confirming its stability under different moisture conditions. Additionally, experiments under diverse soil compaction indicated that the sensor provides reliable readings and maintains effective contact with the soil regardless of the compaction conditions of its environment. In addition to the moisture and compaction tests, the sensor's performance was validated across three different types of soil and its long-term stability was verified over a week of continuous accurate pH measurements, without the need for recalibration or human intervention. The data collected across all experiments support the sensor’s suitability for its use in precision agriculture, particularly in environments where both moisture and soil conditions are highly variable.