Soil moisture monitoring – a selection guide

Correct irrigation management gives better crops, using fewer inputs, which increases profitability. Soil moisture sensors help with irrigation decisions. They are useful tools to understand what is happening in the root zone of your crop.

Soil moisture sensors only measure a tiny area of an irrigation shift and should not be the only tool used in an irrigator’s tool box. Correct irrigation should be a mix of evaporation based scheduling, soil moisture monitoring and grower observation.

This page is a guide to selecting an appropriate soil moisture sensor for your farm.

Sheduling Irrigation

To be used effectively, soil moisture sensors must be used in an irrigation shift that delivers water evenly, be installed correctly and placed in an area which is representative of the crop being grown. As soil moisture sensors only measure a tiny area of an irrigation shift they should not be the only tool used to make irrigation decisions. Correct irrigation should be a mix of evaporation based scheduling, soil moisture monitoring and grower observation.

Sensor types

There are basically two groups of sensors, water potential types such as tensiometers and various forms of granular matrix sensors and soil moisture sensors that give a percentage or relative content of soil moisture.

Water potential probes

These sensors measure how hard it is to remove water from the soil, providing the best indication of available water for plants. Soil type and water content influence the suction pressure required to remove water from the soil, but a monitored sensor, which is recorded and graphed will show the sharp fall that indicates water has become hard for a plant to access.

Two considerations should be made when choosing a water potential sensor.

  • Do they read accurately in the desired range for the crop in which they are being used, and
  • Do they react quickly enough to be useful for the crop being monitored?

The two most common types of water potential sensors are tensiometers and granular matrix sensors such as gypsum block and water mark sensors.  Tensiometers are the most reactive water potential sensor but also require the most care and maintenance. By selecting the appropriate tensiometer tip (two types are available, one is used in sands and the other for clays and loam), they react quickly to changes in water status.

Maintenance of tensiometers includes checking for bubbles and refilling the fluid used to create the vacuum within the tensiometers. Fluid drawn in and out of the tensiometers porous tip, depending on water availability, creates a suction pressure representing the suction force required for a plant to obtain water from the soil.  Measurements can be done by manually reading a vacuum gauge or automatically using a logging pressure transducer.

Granular matrix sensors pass a current across a porous media, with the electrical resistance changing proportionally to the amount of water drawn in and out of the media.

They are generally a low cost, low maintenance sensor.  Once installed they often last many years without intervention.

The reactivity of granular matrix sensors to changes in water status is the biggest limitation to their use.  Accuracy is somewhat poor and can vary greatly - between 10-25% of the actual measurement.

Most granular matrix sensors have low accuracy at low tension (0‑10 kPa). This is an issue if the soil type being measured has limited plant available water and the crop is a water sensitive crop, such as vegetables on the coarse WA sands and heavier clays.

Depending on the porous material and the construction of the sensor, the water seems to move in and out of these sensors slower than with tensiometers.  There tends to be a lag in the sensor wetting and drying in response to the soil.  The lag tends to be greater as the soil dries as opposed to re-wetting and may therefore lead to an underestimation of plant stress on the drying cycle.

Water content probes (soil dialectric)

These probes measure the water content of a soil using the time or frequency of a pulse travelling between or returning to electrodes. The most common types are capacitance and time or frequency domain. Most probes are accurate within two to three percent of the actual soil moisture.

Capacitance probes

Capacitance probes generally measure several depths at intervals of 10 to 20cm and come in lengths from 40 to 180cm.  Multiple depth measurement produces useful information on water movement through the soil profile and relative moisture content of the soil at different depths.

The limitation with most capacitance probes is they measure only a very small area of soil outside the access tube or wall of the probe.  Correct installation must maximise soil contact and ensure water is not allowed to move preferentially down the outside of the probe. If this occurs, the measurements will not reflect the situation in the undisturbed soil away from the probe.  The method of measurement means these probes are also affected by salts in the soil that increase the movement of the pulse waves giving inaccurate readings at high electrical conductivity.

Time and frequency domain probes

True time domain reflectometry (TDR) probes are very accurate but require quite complex and expensive measurement equipment.  A similar, less expensive alternative are probes that measure using water content reflectometry (WCR) and time domain transmissometry (TDT).

This type of probe generally consists of two or three metal prongs between 5 and 30cm long that are pushed into the side of a soil pit to measure the undisturbed soil.  The measurement extends to about 3 to 6cm around the probe giving a larger volume of soil measured (0.3L to 8L).

With correct installation into undisturbed soil and the larger volume of soil being measured data from these probes will be more representative of the whole area compared to capacitance probes. They are also less affected by salts in the soil.

Accurate estimation of water availability with both time/frequency and capacitance will only be achieved by calibration with soil tension measurements. If calibration is not done, estimation of water availability relies on interpretation of the change in curve produced by taking regular measurements and graphing them.

Choosing a probe for your farm

Choosing a soil moisture monitoring system can be a difficult decision. Systems that deliver data to a website or your local computer are readily available and are a better option than a manually read probe.

The following questions may help assess the suitability of a system for your farm.

  • Are you more concerned with available water (water potential) or the movement of water in the soil?
  • Do the sensors react well in the soil type and range of soil water in which the crops are being grown?
  • Is accuracy important? How sensitive is the crop being monitored? Will a delay in identifying the lower level of soil moisture and stress point result in yield loss?
  • Are you prepared to maintain sensors (e.g. check for air in tensiometers)?
  • Are the graphs or values easily understood and is support available to interpret the data from the system?
  • Is the system adaptable?  If you change your mind about the type of sensor you want, will your logger take different probes?
  • Does the information log automatically to a computer system or does it have to be read manually?
  • If the system is web based, is the site reliable so you can depend on data being available when needed.
A guide to water potential sensor selection according to crop and soil type

Soil Types

Water potential/tension sensors

Tensiometer

Granular matrix

Gypsum block

 

 

 

Coarse sand

Yes

No

No

Sandy loam, loam, loamy clay

Yes

Yes

Yes

Heavy clay

Yes

Yes

Yes

Suitable Crops

 

 

 

Vegetables and strawberries

Yes

No

No

Perennial fruit and table grapes

Yes

Yes

No

Pasture

Yes

Yes

yes

Wine grapes

 No

No

Yes

Maintenance required

Moderate

Low or none

Low or none

Approximate cost of soil water potential sensor types ($)

Granular matrix

50‑400

Gypsum block

80‑150

Manual tensiometers

100‑400

Logging tensiometers

400‑700

Auto refill tensiometers

1000‑1500

Many farmers may have purchased and previously used manual tensiometers.  Many of these can be retrofitted with pressure transducers and can be logged using commercial computer packages.

If soil moisture or capacitance sensors are preferred, the shape and pattern of data measured is most important. Base the purchase of these types of probes on the following criteria:

  • Is the probe likely to be affected by salinity in the soil and does the probe still work in these conditions?
  • Repeatability: will the reading be the same if soil moisture has not changed so graphs will be clear and simple?
  • What volume of soil is the probe measuring? Is a larger area desirable?
  • Some soil moisture probes also measure temperature and electrical conductivity; would these be useful to track?
  • Installation will affect measurement. What assistance is there to ensure proper installation such as an instruction manual, video or demonstration?
  • How robust are the probes? Are they likely to be damaged and can they be easily be repaired?

Summary

Correct soil moisture probe selection will ensure that the information delivered will be useful to your irrigation management.  Unless using a system that measures both soil moisture content and plant available water, crop water sensitivity and soil type should guide your purchase.

Remember that soil monitoring is just one tool to assist irrigation scheduling. Other steps to deliver the best irrigation outcome for your crop will include using evaporation or evapotranspiration as a reference, knowing your soil type and crop and using good irrigation design to delivering water evenly.

Contact information

Rohan Prince
+61 (0)8 9368 3210
Page last updated: Thursday, 7 September 2017 - 10:24am