Why Soil Moisture Readings Differ in Sandy and Clay Soil

When two different brands of soil moisture sensors show 23% and 70%, the first reaction should not be "who is broken", but first confirm whether they measure the same definition, the same soil, the same density and the same reference method.

Soil texture alters the electromagnetic response of the sensor

The mineral composition, porosity, bulk density, salinity, and bound water state of sandy, loamy, and clay soils differ. For sensors that use dielectric properties to estimate moisture, these factors all alter the calibration curve, so uniform factory calibration can produce systemic deviations in certain soils.

Most Reliable Field Validation: Establish Local Sample Reference

The reference value can be established by using the drying method: take a representative soil sample, weigh it wet, and dry it to a constant weight according to the standard method to obtain the mass moisture content; if it needs to be compared with VWC, it also needs to be combined with the soil bulk density conversion. The sensor readings are then recorded under the same soil samples, the same compaction and temperature conditions to establish a correction relationship.

Control the variables when doing comparative experiments

• The two probes are placed in the same uniform soil sample as much as possible, but keep a sufficient distance to avoid mutual influence.
• Unify insertion depth and orientation to avoid significant voids near the probe.
• Mix the soil samples thoroughly and wait for the moisture to redistribute.
• Record the temperature and EC as high salinity may change the dielectric measurement.
• Do not directly compare the mass moisture content with the VWC.

When custom soil calibration is needed

Ordinary irrigation management is usually more concerned with the consistency of trends and thresholds; scientific research, soil moisture balance, and accurate irrigation models are more suitable for establishing calibration curves with local soil samples.

FAQ

Q1: Are the sand readings low and normal?

A1: May occur. Sandy soil has low water retention and different pore structure, and the same appearance of "very wet" does not equal high VWC.

Q2: The drying method gets 60%, why is the sensor only 40%?

A2: First confirm whether 60% is the mass moisture content or the volume moisture content. The denominators of the two indicators are different and cannot be directly compared.