Deciphering Soil Moisture Sensor Data: Troubleshooting Unexpected Readings

"Why are my soil moisture readings not aligning with my expectations? Is my sensor faulty? Is my readout device malfunctioning?"

These are common inquiries we receive from both seasoned cultivation managers and novice gardeners alike. It's crucial to understand that discrepancies in sensor readings are a normal part of the learning process, regardless of experience level.

Remember, effective irrigation relies on a delicate balance of science, experience, and sensor technology. A soil moisture sensor is merely one variable in the equation, and its efficacy hinges on proper installation and utilization. No sensor can replace the judgment of a skilled irrigator.

Often, unexpected sensor readings lead to unwarranted accusations of faulty equipment or improper calibration. However, in most cases, the root cause lies in a lack of familiarity with sensor behavior and/or a misunderstanding of fundamental irrigation scheduling principles.

This article aims to provide a comprehensive guide to troubleshooting your moisture monitoring system, regardless of brand, and achieving more accurate readings.

Quickly Testing Your Soil Moisture Sensor

Anomalous moisture values, such as consistently high or low readings, or a lack of change over extended periods, can raise concerns. The initial step in troubleshooting is to verify the functionality of your sensor and readout device. For instance, a reading of 10% moisture in rockwool 24 hours post-irrigation warrants investigation.

To test your sensor, you'll need a container of water and your readout device. Begin by recording a reading in the air. Then, gradually immerse the sensor in the water, pausing for 60 seconds between increments, and record subsequent readings. The values should transition from a minimum (air reading) to a maximum (fully submerged) value.

For example, the APAS T1 moisture sensor (originally developed by DurUntash Lab LLC, San Diego, CA) in rockwool exhibits a range of approximately 0% to 100%.

The APAS T1 moisture sensor (originally developed by DurUntash Lab LLC, San Diego, CA) in rockwool exhibits a range of approximately 0% to 100%.

Following this test, insert the sensor into a fully saturated substrate (e.g., rockwool, coco coir) without a plant. Experiment with various installation methods (e.g., vertical, horizontal) compatible with your sensor and substrate. If the water readings were accurate, but the substrate readings are not, consider the following potential issues.

Potential Causes of Unexpected Readings

• Perched Water Table:

  • A "perched water table" occurs when water accumulates at the bottom of a pot or rockwool cube, leading to prolonged saturation and capillary action.

  • This results in consistently high moisture readings and can damage plant roots due to oxygen deprivation.

  • In outdoor soil, this occurs when water rests on a dense, impermeable layer.

  • Solutions include relocating the sensor, creating a soil mound, and avoiding over-watering in containers or rockwool.

  • Remember, the primary goal of sensor-assisted irrigation is to prevent this condition.

• Loose Sensor Installation:

  • Particularly common in coco coir, a loosely installed sensor may not accurately reflect substrate moisture.

  • Ensure the sensor is fully inserted or buried, preferably horizontally, to minimize the impact of perched water.

  • Secure the sensor cable to prevent movement during measurements.

• Air Gaps in Sensor Installation: 

  • Air gaps can cause erratic reading fluctuations as water fills and drains from these spaces.

  • Preferential water flow towards air gaps can also skew readings.

  • Proper installation requires experience and may involve repositioning the sensor for optimal results.

• Insufficient Sensor Deployment: 

  • Relying on a single sensor in an irrigation zone can lead to inaccurate assessments.

  • Investing in multiple sensors, like the SUMERIT irrigation node with dual sensor ports, provides a more comprehensive overview.

  • Alternatively, combine sensor readings with evapotranspiration data for a more holistic approach.

• Measurement Uncertainty:

  • Soil moisture measurements inherently carry a degree of uncertainty.

  • The APAS T1 sensor, for example, has a tolerance of ±5%.

  • Averaging readings from multiple sensors can mitigate this uncertainty.

  • Be mindful of potential error sources and consult relevant resources for further information.

• Improper Sensor Placement:

  • Sensors should be placed in representative or critical locations, such as pots with high water demand.

  • Avoid waterlogged areas and maintain consistent sensor placement throughout the growing period.

  • Spot measurements with a single sensor are not recommended.

• Focus on Moisture Trends:

  • For irrigation scheduling, prioritize moisture trends and ranges over absolute values.

  • Observe how readings change over time to determine optimal irrigation intervals.

• Vertical Installation in Rockwool: 

  • Vertical sensor insertion is generally preferred in rockwool due to its moisture profile.

  • Horizontal installation at the cube's center can be used if vertical readings are unsatisfactory.

• Inadequate Container Size:

  • Sensors may not function optimally in containers or rockwool cubes that are too small.

  • Small containers require frequent irrigation, potentially negating the need for sensors.

• Shared Sensor Grounds:

  • Installing multiple sensors close together with a shared ground can lead to unreliable readings.

  • This is due to potential ground loops and interference between sensors.

  • This is more pronounced at higher moisture levels.

• Cable Noise: 

  • Sensor cables can act as extensions of the sensing element, introducing noise.

  • Minimize cable contact with the substrate and utilize ferrite cores to reduce interference.

  • This is more pronounced at higher moisture levels.

• Unnecessary Substrate-Specific Calibration: 

  • Sensors designed for soilless media should not require additional calibration.

  • Calibration may only marginally improve accuracy and can even introduce errors.

  • Focus on addressing more significant factors affecting measurement accuracy.

• Miscellaneous Factors:

  • Sensor proximity to pot edges, insufficient moisture distribution post-irrigation, and reuse of rockwool can also affect readings.

Sensor Optimization for Rockwool

The APAS T1 moisture sensor is specifically optimized for soilless media, particularly rockwool, eliminating the need for additional calibration. Its design and scaling ensure accurate and reliable readings in this environment.

The APAS T1 moisture sensor (originally developed by DurUntash Lab LLC, San Diego, CA) is optimized for accurate measurements in rockwool.

By understanding these potential issues and implementing appropriate troubleshooting steps, you can ensure accurate and reliable soil moisture sensor readings, leading to more effective irrigation management.