Notes on Controlled Biodegradation of Capacitive Soil Moisture Sensor

Abstract

  • The research focuses on a biodegradable soil moisture sensor for precision agriculture.
  • Enables real-time soil characterizations to optimize water and nutrient inputs in agriculture.
  • Designed to degrade after the growing season, removing the need for sensor retrieval.
  • Utilizes a capacitive structure and biodegradable materials to ensure low-cost and simple fabrication.

Introduction

  • In 2017, U.S. agricultural production expenditures reached $359.8 billion, with high inputs in chemicals, fertilizers, and labor.
  • Precision Agriculture (PA) technologies are essential for efficient resource management in heterogeneous soils.
  • Sensor technologies provide critical data to enhance irrigation efficiency, potentially saving billions of gallons of freshwater.
  • Current sensors are often cost-prohibitive and disrupt farm operations, making temporal and spatial variability measurement challenging.

Current Limitations of Sensors

  • Existing sensors require maintenance, retrieval before harvesting, and result in added costs.
  • Nondegradable sensors can hinder soil quality and crop growth due to toxicity.
  • Deployment of numerous sensors is impractical with current sensor costs and maintenance needs.

Additive Manufacturing (AM) Benefits

  • AM techniques such as inkjet and screen-printing allow for customization, reduced waste, and lower costs.
  • Printed sensors are lightweight and permit high-density deployment, necessary for accurate soil moisture assessment.

Fabrication of Biodegradable Sensors

  • The developed sensor includes biodegradable components such as:
    • Structural Substrate: Made of balsa wood, which degrades rapidly.
    • Printed Electrode: Zinc electrodes on PHBV film, a polymer that also degrades in soil.
    • Encapsulation: Beeswax and soy wax blend protects sensor and regulates moisture exposure, increasing lifespan.

Performance Characteristics

  • Capacitance Response: Linear increase observed as soil volumetric water content (VWC) increased from 0% to 72%.
  • Well-defined lifecycle with predictable degradation, ensuring distinction between functional and nonfunctional states.

Experimental Testing

  • Material Selection: The sensor's degradation was tested in soil to confirm environmental impact and ecotoxicity.
    • Mass loss of phbv film and other materials was monitored to evaluate degradation rates under various conditions.
  • Plant Growth Tests: Conducted with maize to observe potential phytotoxicity.
    • No significant growth impacts from sensor materials were observed after 60 days.

Results of Sensor Tests

  • Various encapsulant thicknesses produced valid responses in different moisture conditions, demonstrating versatility.
  • Increasing encapsulant thickness led to reduced sensitivity, and recalibration may be necessary for field applications.
  • The sensor's performance demonstrated an ability to function stably until degradation affects critical components.

Conclusions

  • The biodegradable sensor effectively meets the need for low-cost, maintenance-free agriculture monitoring.
  • Highlighted is the potential for reducing water usage and enhancing agricultural productivity through more efficient sensor deployment.
  • Future research will explore large-scale field testing to correlate laboratory findings with real-world conditions and integrate with RFID for wireless monitoring.
  • The $/sensor manufacturing cost is anticipated to be low due to bulk material purchase and printing, leading to potential widespread adoption in the agricultural sector.