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How to calibrate soil moisture sensors


The calibration of soil moisture sensors is crucial for accurate and reliable soil moisture measurements. Many sensors are supplied with a generic calibration table. Following these steps it is possible to generate a calibration table specific for your sensor and soil, greatly enhancing your soil moisture data.


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Why calibrate a soil moisture sensor

Most soil moisture sensors are delivered with analogue output and a calibration table, or digital output (SDI-12 for example) with a % VWC output. These are typically derived from a generic soil calibration, which is likely to be different to the soil in your study area.

It’s also worth noting at this point that soil moisture sensors are likely to have different sensitivities, even within sensors of the same model. Individual calibration may be required in some situations.

How to calculate soil moisture content from a sample

  1. Collect soil samples from a representative area and depth. It’s important that you collect the type of soil you’ll be measuring in your study.
  2. Sieve out or manually remove any rocks, plant material or non-organic material from the samples.
  3. Dry the soil samples – The most efficient way to do this is in an oven. The Australian Department of Sustainable Natural Resources recommends a temperature of 105°C to 110°C.
    Soil samples can also be air dried on paper in a warm, dry room; however, this is likely to take days or even weeks.
  4. Place the dried soil into plastic containers that are large enough for the sensing area of your soil moisture sensor to be completely buried without touching the sides of the container. We recommend at least 2-3 cm of soil between the sensor and the closest edge of the container.
    You need 1 container for each calibration point you wish to use.
  5. Create a range of moisture in each sample by adding water, where the first container is kept dry and the final container is fully saturated. You can use the soil moisture sensor to check that each container is increasingly moist. Be sure to measure from the driest to the wettest, or make sure the sensor is fully dried between each container.
    It is important that the soil in each container is well mixed so that the moisture level is consistent.
  6. Measure and record the sensor output in each container, then take a sample from each container.
    Weigh these samples on an analytic balance and record the wet weight.
  7. Place each sample into an oven to dry. Once dried fully, record the dry weight of each sample.

Gravimetric Soil Water content

With both the wet and dry weights now known the Gravimetric Soil Water content can be calculated as follows:

Water Content = (wet weight – dry weight) / (dry weight * 100)

To convert Gravimetric Soil Water content to Volumetric Water Content, you need the bulk density of the soil. This can be sampled and measured using a SEC 0200 Soil Core Sampler for example. Bulk density is weight by area of soil.

VWC = (Gravimetric Soil Water) x (Bulk Density)

Important things to consider are setting any offsets or calibration curves as required for IoT installations