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Soil Moisture Tension Probe
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Introduction

The Weather-Pro Soil Moisture Tension Probe has been developed for the measurement and logging of the force with which moisture is held by the soil matrix. This force is often referred to as soil potential, soil tension and sometimes as soil suction. The measurement is indicative of the energy that a plant root system must expend to obtain moisture from the soil matrix. This probe also has an internal temperature sensor which can be used to log the soil temperature. The probe has been designed for connection to the Weather-Pro and Weather-Pro Plus Weather Stations. The Weather-Pro Soil Moisture Tension Probe is available in two ratings, one designed for working with lower tensions and the other for high tension applications. 

Theory of Operation

The soil tension probe has a porous ceramic cup, filled with water, which is positioned in the soil so as to allow water to enter and leave the probe body. This transfer of water across the porous ceramic cup generates a pressure which is measured by a gauge pressure sensor in the probe. The gauge pressure sensor is vented to the atmosphere by a breather tube in the probe electronics cable. A weather station at the surface records the soil moisture tension and temperature.   
 
 

Use of the Probe in Practice

Before use, the water reservoir of the soil moisture tension probe must be filled. Two filler tubes are provide for this task. Water is injected into one of these tubes until it flows out the other. The tubes are then sealed. An auger is used to drill a hole in the soil that has the same diameter as the tension probe. Soil taken from the bottom of the hole is mixed with water, to make a slurry which is poured back into the hole. The probe is then inserted into the hole. The slurry ensures good hydraulic contact between the ceramic cup and the soil. The probe is then then connected to the weather station.       

 

Specification

Tension Probe
Low Water Tension Probe
   Tension Range +300cm 
   Tension Accuracy ±0.5% full scale
   Tension Resolution ±0.2cm 
   Temperature effect 
   0-50oC
+1% full scale
   Temperature range 0 to 50oC
High Water Tension Probe
   Tension Range -600 to +900cm
   Tension Accuracy ±0.5% full scale
   Tension Resolution +0.5cm
   Temperature effect 
   0-50oC
+1% full scale
   Temperature Range 0 to 50oC
Temperature sensor Internal to probe
Temperature Accuracy ±0.3oC
Temperature Resolution ±0.1oC
Physical Dimensions
Probe Length inc. cup 310mm (12.2in)
Diameter 33mm (1.3in)
Weight

Choosing the best pressure range for the job

The Low Water Tension Probe is used in conditions where the change in water tension will be less than -300cm. If a change of -300cm to -600cm of water tension is expected, the High Water Tension Probe will be required.  
 
 

Part Numbers

Quote when ordering

Low Water Tension Probe (with Cup) SMTP5WITHCUP 
High Water Tension Probe (with Cup)  SMTP15WITHCUP 
Low Water Tension Probe (without Cup)  SMTP5NOCUP 
High Water Tension Probe (without Cup)  SMTP15NOCUP 
Ceramic Cup SMTPCUP
Auger (33mm diameter with 1m shaft)  SMTPDRILL1
Auger (33mm diameter with 2m shaft)  SMTPDRILL2
Using the soil moisture tension probe for monitoring irrigation

The amount of water stored in the soil matrix and available for plants varies with soil type. Soils with a fine texture hold more water than coarse textured soils and sandy soils will hold less water than clays and loams. The soil's capacity to hold water is defined by three terms:

  • Field Capacity
  • Permanent Wilting Point
  • Available Water Content

The Field Capicity is the amount of water remaining in the soil matrix 2 to 3 days after it has been saturated.
The Permanent Wilting Point is where the soil has become so dry that plant roots can not exctract any more water from the soil matrix.
The Available Water Content is the Field Capicity minus the Permanent Wilting Point. It represents the total range of soil moisture that could be available to the plant. Available Water Content is often given as a percentage with the Field Capicity of the soil being 100% and the Permanent Wilting Point being 0%.

A Soil Moisture Probe on a Weather-Pro and Weather-Pro Plus can be set to read in Percent Available Water (%AW). The Probe can be set for five soil types:

  • Loamy Sand
  • Fine Sandy Loam
  • Sandy Loam
  • Loam
  • Clay

Once set for the correct soil type, the Soil Moisture Probe can be used over the following %AW ranges:

Soil Type Max Reading Min Reading
Loamy Sand 100%AW 18%AW
Fine Sandy Loam 100%AW 25%AW
Sandy Loam 100%AW 32%AW
Loam 100%AW 45%AW
Clay 100%AW 73%AW