Tensiometers

Tensiometers monitor changes in soil water without disturbing the roots of growing plants. They need no calibration and they measure the actual availability of water in the soil. Tensiometers respond to irrigation and rainfall, giving an indication of how effective the added water has been.

Tensiometers are a good instrument to assist in irrigation management. They are designed to complement regular field inspections and not replace them. Combining the use of tensiometers with a drum to monitor evaporation provides added information at little cost.

How do Tensiometers Work?

A tensiometer is a hollow plastic tube with a porous ceramic tip at one end. The tube is filled with water and sealed with a cap. Some tensiometers have an attached vacuum gauge. Others have an electronic vacuum gauge that can be transferred from one tensiometer to another.

All tensiometers read in centibars (cb). As plants use water and the soil dries out, water is drawn out of the tensiometer through the ceramic tip. Because the tube is sealed, a vacuum is created which is measured on the gauge. Conversely, as the soil in contact with the ceramic tip becomes wet, water is sucked into the tube, reducing the vacuum which registers on the gauge.

How Many Sites?

Each site you monitor should be representative of the crop being irrigated, the soil type, the land slope and timing of irrigation.

One monitoring site to represent about four hectares should be sufficient, provided the soil, the crop and the irrigation are uniform. If it takes more than three days to irrigate the whole crop, you will need to monitor the soil water at a site representing early irrigation and at another site representing irrigation later in the cycle. More than one monitoring site is advantageous when the irrigation cycle is interrupted by rainfall to provide information which may modify where and when in the paddock to start irrigating again.

The site chosen should be midway between the irrigator and the limit of it's throw.

Avoid wet hollows or dry banks.

How Many Tensiometers at Each Site?

For all crops, each site should have two tensiometers. A shallow one is the main guide for deciding when to start irrigating. A deep one is used to make sure enough water is applied and to prevent over-irrigating.

Where are They Placed?

Install tensiometers shortly after emergence, between healthy, average sized plants. For potatoes this is in the centre of the mould, as shown. Install one tensiometer at a depth of 30cm and another at 60cm below the ground surface or top of the mould.

If the crops are growing on a shallow duplex soil, then a deep tensiometer may not be needed or else it should be placed immediately above the heavy clay subsoil.

Remember to mark clearly where the tensiometers are in the paddock. They are easy enough to see when plants are just emerging but tensiometers are easily lost under full plant cover. A white fibreglass electric fence pole is ideal.

How to Assemble the Tensiometer

Read and follow the manufacturer's instructions. Different brands have slightly different requirements but by following their instructions, most problems will be avoided. Some points to remember:
  • Do not touch the ceramic tip with your fingers or with greasy/oily rags. Oil will seal off the fine holes and stop the tensiometer working.
  • Hold the plastic part of the tip to screw it on, then put a plastic bag over the end to protect it on the way into the paddock.
  • If the components are not screwed tightly together, water will leak out from the joints and the tensiometer will fail.
  • Tensiometers require clean fresh water. Unless some algae growth inhibitor is added to the water, the water in the tube will become green or brown and readings may become inaccurate. A couple of drops of commercial algicide or laundry bleach should be used.
  • Soak tensiometers in a bucket of water overnight prior to installation.
  • Write the size (30or 60cm) on the face of the tensiometer gauge with a waterproof marking pen to enable easy identification when in the paddock.

How are They Put in the Ground?

The shallow (30cm) tensiometer can normally be pushed firmly but gently straight into the potato mould to the required depth.

The 60 cm deep tensiometer (and the shallow one if the soil is too tight) are installed using a T bar screw auger (as shown). This can be made from an old wood auger with an outside diameter slightly narrower than the tensiometer shaft (13/16 in or 21 mm).

If the hole is oversized, contact between the ceramic tip and the soil will be poor. Inaccurate high (dry) readings will be registered on the gauge. Then, after rain or irrigation, free water will flow down the hole and inaccurate (low) readings will register.

The correct depth can be marked on the auger to ensure the correct depth of installation.

After augering out the hole, pour some water down the hole. This will lubricate the sides and help the tip to make good contact with the soil.

Firmly but gently push the tensiometer down to the base of the hole. Don't push too hard! The tips are strong but can crack under excessive pressure. Only experience teaches how hard is too hard but at $20 per tip, this is not a cheap lesson.

When to Take the Readings

Take tensiometer readings at the same time each day. Early in the morning is recommended so as to avoid any fluctuations caused by heating up of the tensiometer and the water inside.

If the tensiometer has been installed after overnight soaking, a reading can be taken 30 minutes after installation.

Check tensiometers and take readings at least twice a week in the early part of the growing season. During the main part of the growing season and when the tensiometer readings are above 40 cb, readings need to be taken every couple of days and daily on sandy soils.

When taking a reading, tap the face of the gauge lightly until the needle settles on a reading.

If more than 2 cm air gap is present at the top of the water column below the reservoir, then it should be refilled from the reservoir (Jetfill and irrometer) or topped up (Soilspec or Loktronic). After several refills the reservoir will need topping up with more water. The drier the soil the more often this refilling will be required but remember that water will be drawn back into the tensiometer after irrigation has wet the soil.

When visiting the site for a recording do not repeatedly stand too close to the tensiometers as this will compact the soil which can directly affect readings and also affect water infiltration into the soil and runoff. Do not tread on or damage plants near the tensiometer as readings will no longer be indicative of the rest of the crop. If plants adjacent to the tensiometer become diseased, and the rest of the crop isn't, it may be necessary to reinstall the tensiometers in a more representative site.

Recording

Write down the readings on the gauge for both the shallow and deep tensiometers plus the date, the amount of rainfall and/or irrigation if any has been applied and the date it was applied. Results can be kept in a table or plotted on a graph for better visual interpretation.

The record will allow for interpretation of readings and the modification of irrigation decisions as the season progresses. It will also indicate the success or otherwise of previous irrigations. Your records will lead to a fuller understanding of your soil and the use of water by the crop.

What do the Readings Mean?

The vacuum gauge readings show the relative suction energy (and thus wetness) of the soil A high reading on the gauge is caused by a dry soil which has a high suction.

Tensiometer gauges have a scale reading from 0 to 100 centibars (cb). A tensiometer can operate effectively within a range of 0 to 80 cb.

Reading
0 - 5 cbSaturated soil. Plants will suffer from a lack of oxygen in the root zone.
10 cbField capacity. After one or two days of draining a saturated soil, free water has drained away leaving a good balance between water filled and air filled pores in the soil.
10 - 25 cbIdeal soil water and aeration conditions for potatoes.
25 - 80 cbAs moisture is removed from the soil, the thickness of the water film surrounding soil particles becomes thinner and is held on with greater tension. Decreased availability of soil water to the plant at this stage results in evaporative forces drawing moisture from plant cells quicker than the soil can provide it, with consequent reduction in plant turgor. This water loss can be reduced to some extent by stomatal control, however, stomatal closure also restricts uptake of carbon dioxide necessary for photosynthesis. Consequently, when soil water is limiting, plant growth is not optimised due to reduced starch production (photosynthesis) and lowered cell metabolism (turgor).
80 - 100 cbExcessive quantities of air enter the tensiometer. Eventually the water column in the tube will be broken and the vacuum lost. The tensiometer will then show a zero reading, despite the soil being very dry. If this happens, remove and clean the tensiometer, resoak and reinstall.

When to Irrigate

Evaporation and the activity of surface roots normally cause surface soil to dry out more rapidly than soil deeper down the profile. This is reflected in the tensiometer gauge readings, with the shallow tensiometer readings rising more rapidly than the readings of the deeper tensiometer.

Readings from the 30 cm deep tensiometer are used to indicate when irrigation is necessary.

    clay and clay loam textured soils (krasnozems)
      irrigate at 50 cb
    fine sand textured and sandy duplex soils
      irrigate at 30-40 cb
    coarse sand textured soils (Panshanger sand)
      irrigate at 20-30 cb
Sandy soils are irrigated at lower suction because soil water suction increases rapidly due to of their low water-holding capacity.

Remember, if the tensiometer is located on the side of the paddock which is irrigated first, by the time the irrigator has been moved from one side of the paddock to the other, the last run to be irrigated will be much drier than the 50 cb. Consequently it may be desirable to start your first irrigation of the season at readings 5-10 cb below those recommended above.

How Much Water to Apply

As a rule of thumb, apply 1 mm of water to reduce the tensiometer reading by 1 cb. For example, when the reading on the 30 cm deep tensiometer reads 40 cb apply 40 mm of water to recharge the root zone. One millimetre of irrigation equates to 1 litre per square metre or 10,000 litres per hectare.

By using this figure as a base from which to start, you can refine the amount of water applied at each irrigation by using the readings from the deep tensiometer. If the reading on the deep tensiometer falls below 10 cb after an irrigation, too much water has been applied and the soil is saturated. However, if after an irrigation the reading on either tensiometer does not fall to 10 cb, not enough water has been applied.

In most soils, allow about 24 hours after an irrigation for water to penetrate to the depth of the ceramic cup on the deep tensiometer before checking the readings. In sandy soils water can penetrate more rapidly.

Experience has shown that applying 50mm of irrigation at once is often too much in one irrigation and results in surface water runoff, soil erosion, and incomplete soil water recharge in the potato mould. A better option is to apply less at each irrigation but to irrigate more frequently.

End of Season Retrieval

If your tensiometer is treated properly it will give years of trouble free service. When irrigation has ceased and you no longer require the tensiometer, follow these steps.
  • Remove tensiometer slowly by turning the whole unit in a clockwise direction (to prevent unscrewing of the tip) and then pulling up and out.
  • Disassemble unit by unscrewing all components and rinsing with plenty of fresh water.
  • Clean tip of soil by holding it under a running cold tap and very lightly scrubbing with a fine brush.
  • Soak the ceramic tip in household vinegar overnight to remove any salts which have crystallised. Wash clean in water.
  • Allow to dry and store in a clean cupboard at room temperature. Preferably store with the tip wrapped in a plastic bag to prevent greasy hands contacting the tip.

Interpreting Tensiometer Readings

The following graph is an example of how to interpret tensiometer readings.

Graph plotting tension and days.


A Crop using water rapidly from the entire root zone as shown by the rapid changes in the readings from both tensiometers. Reading on 30cm deep tensiometer (50 cb) indicated that irrigation was needed.

B 50mm of irrigation water applied which brought both tensiometers back to zero. 60cm deep tensiometer remained on zero for 24 hours and both tensiometers had readings below 10 cb for several days indicating too much water had been applied and resulted in saturated soil conditions in the root zone.

C Crop using water steadily from the entire root zone and 10mm of rainfall resulted in 10 cb drop in upper tensiometer but no change in the deep tensiometer reading.

D Steady water use by crop from the entire root zone with soil becoming drier than recommended irrigation start-up reading.
    Twenty millimetres of irrigation applied which reduced the 30cm deep tensiometer reading by 20 cb but had no effect on the 60 cm deep tensiometer, ie. not enough water had been applied to wet the entire root zone to optimum water content.
E Soil continuing to dry out. Another irrigation of 30 mm applied which rewetted the entire root zone and returned both tensiometers to 10 cb.

Trouble Shooting

Problems are not uncommon with tensiometers. Do not cast aside the instrument if they arise but check the list below and contact the supplier or an adviser for assistance.

Problem: Gauge Always Reads Zero
  • The tensiometer may be leaking from a loose connection. Remove the tensiometer, check that O-rings aren't missing, firmly tighten all joints and reinstall. If tensiometer continues to read zero, the tip may be cracked and will require replacing.
  • The soil is saturated from irrigation, rainfall or poor drainage.
  • The tensiometer has no water or has lost suction due to low water level. Refill the tensiometer.
  • The vacuum gauge is faulty. Remove the gauge, rinse with clean water and suck it. If the needle does not move the gauge needs to be replaced.
  • Soil has become too dry and the tip has lost contact with the soil. Refill the tensiometer and irrigate the soil.
Problem: The Tensiometer Does Not Seem to Record True Soil Water Content

This is the most common complaint. Usually the complaint is unfounded due to the fact that actual soil water content is very different from what was thought to exist.
  • To check, use a soil auger to take samples about 15cm away from the tensiometer and at the exact depth of the ceramic tip. Visual inspection or preferably, determination of the actual soil water content, will usually demonstrate that the tensiometer readings are accurate.
  • There may be poor contact between the ceramic tip and the soil because of improper installation. Reinstall correctly.
  • The tip may be touching a stone with an air pocket around it. Reinstall at another site.
  • The vacuum gauge needle may be sticking due to rusting because water has entered under the glass. Repair or replace the gauge.

Problem: Tensiometer Requires Frequent Refilling

  • Usually indicates under-irrigation and further irrigation is required.
Problem: Tensiometers Respond Slowly to Irrigations
  • May be due to slow rate of water infiltration due to soil type or soil compaction caused during installation. Remove and reinstall at another site taking care with installation.
  • Ceramic tip may be blocked. Clean and soak in household vinegar over night, wash and reinstall.
  • Gauge movement is excessively "sticky" due to minor damage. Tap the gauge lightly before taking a reading.
Problem: Reading Unchanged After Irrigation
  • Check that the irrigation system is working efficiently - check your rain gauge. Move the tensiometer to another position to confirm the reading.
  • Too little irrigation may be being applied.

What Tensiometers are Available?

The most commonly used tensiometers in Tasmania are the 'Jet Fill' which is manufactured by the Soil Moisture Equipment Corporation (USA) and the 'Irrometer" which is manufactured by the Irrometer Company (USA). These are available from Serve-Ag Pty Ltd and J.R. Stephenson Pty Ltd respectively.

'Soil Spec' tensiometers are available from Serve-Ag Pty Ltd. These feature inexpensive tubes with a rubber cap and a portable electronic monitor for reading the tubes in the field. With careful use these are convenient for monitoring large numbers of tensiometers and they provide a cheaper option when many tensiometers are to be used.

Some points to note on the operation of the 'Soil Spec' are:

  • Allow time for re-equilibration of the tensiometer following insertion of the needle, at least 3-4 minutes.
  • Maintain the air bubble at approximately the same size for each reading. This will probably mean reading the tensiometer every day and topping it up with water.
  • Replace the rubber stoppers regularly, ie at least once per season.
"Quickdraw" tensiometers are manufactured by Soil Moisture Equipment Corporation (USA) and are available from Serve-Ag Pty Ltd. These are portable fast response tensiometers.
Advantages of the 'Quickdraw' tensiometer are:
  • When several readings are taken, an indication of the average moisture status of the paddock is achieved rather than a single point value.
  • The operator is offered greater flexibility in the positioning of the tensiometer in regard to depth, proximity to plant and location (re: irrigation runs or soil type variations).
  • The same site is not revisited so that problems of soil compaction and plant damage do not occur.
  • Cost savings when large numbers of tensiometers are to be used.
Disadvantages of the 'Quickdraw' are:
  • Greater level of operator expertise required.
  • More time required to achieve optimum results due to
  • slow response time in clay soils
  • need to take multiple readings
  • need to check response time every day
  • Difficulty in inserting tensiometer in hard soils and below 40cm depth.
  • Higher initial cost when getting started in tensiometers.
Some operating tips for the 'Quickdraw' are:
  • Read the manual.
  • Check response time prior to going in the field.
  • Ensure the tip is clean and the gauge is reading 0 cb before inserting the probe. Use a squirt water bottle and a rag to wet and clean the tip and to zero the gauge.
  • A minimum of 3 readings should be taken at a site to be confident of the soil water status. It should be possible to achieve readings within a range of 8 cb.
  • Greater confidence in the gauge reading can be achieved by using the null knob to 'bracket' a reading. The null knob must move the dial at least 15cb.
  • To ensure tensiometer readings can be compared over time consistent placement is required in regard to depth, plant proximity, irrigation run proximity and soil type.
  • Do not twist the tip when cleaning it or inserting it in the soil. Rotation may result in loss of seal between the ceramic tip and the tensiometer body, or may break the tip in stony soils. If the tip need to be replaced do it in a clean environment at home so that dirt can't enter the probe.


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