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Soil Moisture Sensor Calibration

Calibrating Sensors

All sensors measuring the dielectric constant require calibration to obtain volumetric soil moisture. Most manufacturers provide a universal “factory calibration”, others provide a calibration(s) based on soil texture (normally sand, silt or clay) and nearly all recommend field calibration.

HydroServices measurements have shown that all sensors should be field calibrated to accurately measure volumetric soil moisture V%. At a trial site in Canterbury a number of different sensors are installed in a silt loam soil, the “best” soil texture for dielectric type sensors. Each sensor has had the “factory” calibration applied (universal or silt loam calibration). Aquaflex is the only sensor to approximate the gravimetric and the neutron probe soil moisture measurements (Figure 1). The sensors have very different responses. Soil moisture measured by the 5TM and Acclima sensors dry to V% close to wilting point for this soil type - likely under a well irrigated pasture. Aquaflex is less sensitive and does not dry to the same degree as the 5TM and Acclima sensors.

Field calibration is required to ensure accurate soil moisture readings.

Field calibration is not as simple as applying a correction factor. Soil moisture (V%) must be measured at different soil moisture content to develop a relationship – an equation to correct the sensor measurements. Figure 2 shows a “factory” calibrated 5TM sensor and the soil moisture following field calibration – it is not a simple offset or correction factor.

Field calibration has an important outcome for available soil moisture. Once field calibrated the readily available soil moisture increases from about 12mm (factory calibration) to 19mm (field calibration) in the 0-20cm depth for this soil. The accurate measure of available water has a significant effect on:

  • Return interval for irrigation; i.e. 7mm is the about 2 days crop water use;
  • Overseer drainage and N leaching; i.e. higher water holding capacity results in lower N leaching.
Figure 1. Comparison of soil moisture sensors in a silt loam soil, Canterbury, New Zealand

Irrigation Management Contacts

Phil Neill
Manager, Irrigation Management
M: 027 457 9863
P: +64 3 341 0970
phil@hydroservices.co.nz

Mark Fitzgibbon
Mid/South Canterbury Area Manager
M: 027 457 0413
P: +64 3 341 0970
mark@hydroservices.co.nz
Job Quantock
North Canterbury Area Manager
M: 027 534 3886
P: +64 3 341 0970
job@hydroservices.co.nz

Melanie Smith
North Island Area Manager
21 Ruahapia Road, RD 2
Hastings 4172
M: 027 444 7349
P: +64 6 873 4045
melanie@hydroservices.co.nz
SCHEDULING ORDER FORM

Please note emailed reports require rainfall and irrigation records emailed to This email address is being protected from spambots. You need JavaScript enabled to view it. and broadband is an advantage.

TERMS AND CONDITIONS: Payment must be made by the 20th of the month following the date of the invoice, unless prior arrangement for extended credit has been made. Interest of 18% will be charged monthly on overdue accounts. Debt collection will be taken at the discretion of HydroServices Limited after 60 days. This cost will be added to your account. Any instructions received by HydroServices from the client for the supply of our services, shall constitute acceptance of the temrs and conditions contained above.
SCHEDULING ORDER FORM

Please note emailed reports require rainfall and irrigation records emailed to This email address is being protected from spambots. You need JavaScript enabled to view it. and broadband is an advantage.

TERMS AND CONDITIONS: Payment must be made by the 20th of the month following the date of the invoice, unless prior arrangement for extended credit has been made. Interest of 18% will be charged monthly on overdue accounts. Debt collection will be taken at the discretion of HydroServices Limited after 60 days. This cost will be added to your account. Any instructions received by HydroServices from the client for the supply of our services, shall constitute acceptance of the temrs and conditions contained above.