Tensiometer Tips for Vegetables

by Henry G. Taber Department of Horticulture Iowa State University   Updated: June 2006

1. How do they work?

The key to the tensiometer is the sensitive gauge.  You should handle them very carefully and be sure not to drop or bump the gauge.  Be sure you store properly during the winter so that they never freeze.  The tiny water droplets left in the gauge after a season’s use will ruin the sensitive measurement when frozen.

As the soil dries out, water is sucked out through the porous ceramic tip, creating a partial vacuum inside the tube that is read on the vacuum gauge.  As the soil continues to dry out, the soil suction withdraws water from the tensiometer, increasing the gauge reading.

When the soil is irrigated or rainfall occurs, soil suction is reduced and water is drawn back into the tensiometer by vacuum.  This reduces the vacuum and the gauge reading is lowered.

2. What do the numbers mean?

The gauge on the left indicates a drier condition (higher reading) than the gauge on the right (a low reading, less than 10 cbars).

The reading shows the relative wetness of the soil.  The higher the reading the drier the soil.  The numbers from 0 to 100 are called centibars.  One hundred centibars equals 1 bar, which is equivalent to 1 atmosphere.  A tensiometer can operate effectively within a range of 0 to 80 centibars.

A zero reading indicates a saturated soil in which plant roots will suffer from lack of oxygen.  Zero to 5 is too wet for most crops.  The range from 10 to 25 represents ideal water and aeration conditions.  As reading go higher than 25, water deficiency may occur for sensitive plants having shallow root systems, such as plants growing on coarse-textured soils.

Twenty-four hours is long enough to obtain a reading after installation.  If it is a brand new tensiometer, and under favorable soil conditions, a correct reading may be obtained in 30 minutes.  You can improve the response time of an old ceramic cup tensiometer by lightly sanding the surface. 

Always read the tensiometer at the same time of day.  If possible, an early morning reading is best because plants and soils have reached a condition near equilibrium.  Water movement in plants and soil at that time has almost stopped.

A minimum of three readings should be taken between irrigations.  Take readings frequently enough so that change from one reading to the next is not greater than 10 to 15 centibars.  With trickle irrigation systems, daily readings are necessary.

The crop and soil texture will be the main determining factors.  There should be at least one, preferably two, locations for each change of crop and field or soil texture. 

The depth to place the tensiometer and how many are needed at each location depends on the crop-rooting pattern.  Only one tensiometer will be needed for plants rooting less than 15 inches.    They should be placed between plants in a row and in the active root zone.  The shallow-depth tensiometer should be placed at about 6 inches deep for shallow rooted crops (peppers, lettuce, onions) and at 8 to 12 inches for deep-rooted crops (tomatoes, melons, sweet corn).  The second tensiometer should be installed about 12 inches deeper than the first one.

Depending on the length (6 inches to 3 feet) and the company source, tensiometer cost may run from $55 to $75 each.  Two major manufacturers are:  Irrometer Company, PO Box 2424, Riverside, CA 92516, Tel. (909) 689-1701 ( http://www.irrometer.com); and Soil Moisture Equipment Corp., Santa Barbara, CA Tel. (805) 964-3525 (http://www.soilmoisture.com)

This depends on the crop, soil type, and irrigation method.  With overhead irrigation, do not irrigate when readings are in the 0 to 10 range as the soil is too wet and plant roots may suffer a lack of oxygen.  In most field conditions, irrigation is not needed in the 10 to 25 range; but do not delay irrigation much after readings reach 75 to 80 as the soil has become quite dry. To evaluate a wider soil moisture range for overhead irrigation consider using Watermarks. They are effective in the range of 20 to 200 cbars. Watermarks are similar to tensiometers and maintenance free but require a hand-held meter to take the readings. Listed below is a chart to begin irrigation when using overhead sprinklers on a fine textured soil type. 

Crop	Reading
Tomatoes	60-70
Melons, carrots	50-60
Lettuce	40-50
Strawberries	25-35

 

With drip or trickle irrigation, the objective is to maintain readings within the 10 to 25 range by controlling the amount of water applied.  This may mean daily application or more frequent for sandy soils as you are only wetting a portion of the root zone.  Thus, for trickle irrigation allow only 20 to 30% soil water depletion from the field capacity level.  Tensiometers readings will rise about 12 to 18 cbars.

Soil Type	Field Capacity, cbars
Sand, loamy sand	7 -12
Loam, silt loam	12 - 20
Clay loam	20 - 35

 

Begin irrigation when the shallow tensiometer records about 12 to 18 cbars higher than the field capacity set point for the soil type listed in the table.  This value being equivalent to about 25% depletion of available water. Thus, for a loam the trigger point to begin irrigation would be about 30. The deep tensiometer should record about 10 cbars between irrigations.  If the deep tensiometer drops to zero you have applied too much water.  Conversely, if it continues to rise between irrigations you have not applied enough water.

9. How many gallons should I apply?

This answer depends on the crop rooting depth and water holding capacity of your soil type. You can obtain this information from your local Farm Service Agency (the old Soil Conservation Service agency that maintains the county soil survey maps). For example, a Clarion soilt loam has a water holding capacity of 2.4 inches per foot. Consider the following:

Pepper crop wetted volume - Clarion loam soil type (holds 2.4 inch available water/foot)

Rooting depth = 1.0 feet
Bed spacing = 6 feet (equivalent to 35 rows per acre) which gives 7315 linear feet per acre
Wetted radius of bed = 16 inches (or 32 inches width)
Thus, 2.67 feet X 7315 linear feet = 0.45 acres of plastic or wetted portion.

Now, rooting depth available water = 1-foot X 2.4 inches water/foot = 2.4 inches water/foot/acre
1-acre inch = 27,000 gallons
27,000 gals X 2.4 inches = 64,800 glas for full water capacity of the soil profile per acre
We only have 0.45 acres under plastic that is wetted.
Thus, 0.45 X 64,800 gals = 29,030 gals for full soil capacity
If we turn on the pump when the tensiometer reaches 25-30 cbars (25% depletion), we would need to apply
29,030 X 25% = 7,258 gals or about 7,300 gals to apply.

Yes.  It is best to record the readings on charts provided by the manufacturer of the tensiometer.  The chart lines will show the wetting/drying of the soil and give you an advance indication of what you can expect in a few days.  This will help you plan for the next irrigation or to see if a previous irrigation failed to penetrate adequately to the root zone. 

An instrument out of water or leaking will remain at zero.  Two or more days of successive zero readings are a sign of malfunction.  Unscrew the cap with stopper and add more water to the reservoir.  A yellow pencil helps in the flow of water down the tube to the ceramic cup.