Irrigation Scheduling and Tensiometer Tips

By Henry G. Taber Department of Horticulture Iowa State University
Updated: May 2010

Commercial Vegetables

Most commercial vegetables grown in Iowa require irrigation during the growing season for top quality and yields. A few exceptions might include sweet corn and pumpkins grown on the fine-textured silty clay loams, and the root crops (carrots, onions, and potatoes) grown on the peat soils of north central Iowa. Frequency of irrigation will depend on rainfall and soil type. The two basic questions growers have are: 1) when to turn on the irrigation pump (soil moisture too low), and 2) how long to run the pump (gallons to apply). To answer the first question – when to irrigate – we need to know the water lost from the rootzone. Water loss equals the evaporation from the soil surface and the transpiration from the crop leaf surface. The two phenomena together are called evapotranspiration (ET). Therefore, irrigation need = (ET) – rainfall.

Evapotranspiration is affected by: crop species, weather, and soil type. Crop species factors include rooting depth, planting density, shading of the ground, and mulching (plastic or organic). Effective crop rooting depth is most important in determining available water to the plant. 

Table 1. Effective rooting depth for selected vegetable crops.

Shallow (6-12") Moderate (18-24") Deep (>36")
Broccoli Cabbage Asparagus
Greens Cucumber Lima bean
Onion Muskmelon Watermelon (seeded)
Snap beans Eggplant  
Pepper Potato  
  Tomato  

Green coded means plastic mulch is generally used with this crop and the rooting depth is shallower. 

Major weather parameters influencing ET are temperature, light intensity, wind speed, and relative humidity. The agriculture weather stations located throughout Iowa automatically determine daily ET (for an actively growing alfalfa crop). Go to: http://mesonet.agron.iastate.edu/agclimate/index.phtml. However, to properly use the ET value you will need the crop coefficient (called the Kc) for your particular vegetable crop. These values are published in FAO-24 (see Allen, R.G. and Pruitt, W.O. 1991. FAO-24 reference and evapotranspiration factors. J. Irrigation and Drainage Engineering, 117: 750-773). Or, you can estimate the Kc by assuming a value of 1.0 and multiplying by the percent the ground is shaded by the developing crop. For instance, from late May to mid-June a transplanted tomato crop may cover 40% of the surface area and the Kc value would be 0.4. Thus, with the appropriate ET value and a simple rain gage you can calculate the irrigation need (see Taber, H.G. 2007.) View tomato irrigation scheduling for optimum production. The major soil types of sandy loams, loam, silt loam, and clay loam effect soil moisture availability to plants by their water-holding capacity. 

Table 2. Water holding capacity of major soil types at field capacity (after gravitational water drains through).

Soil Texture Inches/Feet
Sands 0.5-1.0
Sandy Loams 1.0-1.5
Loams 2.0-2.5
Silt Loams 2.4-2.6
Clay Loams 2.0-2.5

 

Overhead Irrigation Systems

 

Overhead irrigation systems wet the entire acre-surface area so irrigation models utilizing ET, soil moisture measurement, and water budget balance methods are effective. However, we will concentrate on irrigation scheduling of trickle systems where only a portion of the rootzone is wetted. Because only about ¼ to 1/3 of the rootzone is wetted trickle systems require a higher level of soil moisture monitoring than overhead irrigation. There is very little capacity for error. Soil moisture monitoring devices, like tensiometers, are excellent for this purpose.

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. 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.

Tensiometer

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 declines.

Be sure you store them properly during the winter so that they never freeze. The tiny water droplets left in the gauge after a season’s use will freeze and ruin the sensitive gauge.

What do the numbers mean?

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 (cbars). One hundred cbars equals 1 bar, which is equivalent to 1 atmosphere. A tensiometer can operate effectively within a range of 0 to 80 cbars.  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 readings go higher than 25, water deficiency may occur for sensitive plants having shallow root systems, such as plants growing on sandy, coarse-textured soils. 

Tensiometer Gauge

 

 

 

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

 

 

 

 

 

After placing, when can they be read?

Twenty-four hours is long enough to wait before taking a reading after installation. If it is a 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. 

When should readings be taken?

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 cbars. With trickle irrigation systems, daily readings may be necessary.  

How many do you need?

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. It should be placed between plants in a row and in the active root zone. For deep rooted crops two tensiometers are needed per location. The shallow-depth tensiometer should be placed ½ to 1/3 of the effective rooting zone (about 6 inches deep for peppers, lettuce, onions and at 8 to 12 inches for tomatoes). The second tensiometer should be installed about 12 inches deeper than the first one, near the bottom of the rooting zone. The deep tensiometer is an indicator of how well the rootzone has been wetted. For instance, a 0 reading after irrigating indicates too many gallons were applied. 

Approximate cost?

Depending on the length (6 inches to 3 feet) and the company source, tensiometer cost may run from $55 to $90 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).

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. Grower evaluations indicated a preference for the tensiometer primarily for ease of monitoring and removal in the fall (see Taber et al. 2001. Scheduling microirrigation with tensiometers or watermarks. Proc. Int’l. Irrigation Assoc., pp. 65-71). 

How should they be installed?

  • Fill the tensiometer with tap water and soak for 24 to 48 hours in a bucket of water.
  • Apply a strong vacuum with the hand vacuum pump to remove trapped air. The vacuum should be drawn up to 80 cbars, usually for 5 or 6 quick pulls. Do this with the ceramic tip below the water surface. There should be a few air bubbles emerging. Repeat this procedure until all air is removed from the instrument. You may have to soak for an additional 24 hours. Replace the cap with stopper, but do not over tighten. Be sure to transport to the field by protecting ceramic tip from air-drying either in the bucket of water or plastic sleeve tied around the tip.
  • Drive a half-inch conduit pipe, or use a soil sampling test tube, to within 1-inch of the depth that the tensiometer will be installed. Add a little water to the bottom of the hole to allow the soil to soak up and make firm contact between the ceramic tip and surrounding soil. Insert tensiometer into hole and gently push down the remaining 1- inch to make a firm soil-ceramic cup contact.
  • Where the tensiometer comes out of the ground, be sure you mound up soil to prevent a depression and water running down the side of the tube during an irrigation or rain.
  • Don’t let the gauge touch the soil. The rubber bottom of the gauge needs to expand and contract to allow an accurate reading. 

At what readings should I irrigate?

Irrigation Soil Probe

 

Use soil prove to make hole correct depth.    

 

 

 

                               

 Tensiometer Instructions

 

Add small amount of water to bottom of hole for good contact of ceramic cup to soil.

 

 

 

                                     

Tensiometer Instructions

 

 

Firm the soil around the base of the tensiometer so rainwater does not run down the side of the tube and distort your reading.

 

 

 

This depends on the crop and soil type. Do not irrigate when readings are in the 0 to 10 cbar range as the soil is too wet and plant roots may suffer a lack of oxygen.

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, allow only 20 to 30% soil water depletion from the field capacity level. Tensiometers readings will rise about 12 to 18 cbars.

Table 3. Tensiometer set points, in cbars, for soil moisture conditions on various soil textures.

Soil Texture Field Capacity1 25% Depletion2
Sandy Loam 5-10 10-15
Loams 10-15 22-30
Silt Loams 15-20 25-35
Clay Loams 25-40 40-50

1Soil saturated, no irrigation required.
2Of the available water capacity in the effective rootzone (see Table 2).

Begin irrigation when the shallow tensiometer records about 12 to 15 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 soil the trigger point to begin irrigation would be about 25 to 30 cbars. 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. 

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) or approximate from Tables 1 and 2 above. For example, a Clarion silt loam soil in central Iowa has a water holding capacity of 2.4 inches per foot (Table 2). Consider the following for a transplanted pepper crop: black plastic mulch, 6-foot row bed spacing center to center with twin-rows per bed at 18-inch apart and inrow spacing of 15-inch:

Pepper crop wetted volume - Clarion loam soil type (holds 2.4 inch available water/foot) Effective rooting depth = 1.0 feet (Table 1).

Bed spacing = 6 feet (equivalent to 35 rows per acre) which gives 7315 linear feet per acre (consider an acre to be 209 feet X 209 feet. Therefore, 209 feet divided by 6 ~ 35 rows at 209 foot long. Thus, 35 X 209 feet = 7315 linear feet).

Wetted radius of bed = 16 inches (or 32 inches width or 2.67 feet). Value determined by observation for particular soil type. For instance, a loamy sand in the Muscatine region has a wetted radius of 6 inches.

Thus, 2.67 feet X 7315 linear feet = 0.45 acres of plastic or wetted portion (an acre = 43,560 ft2 and 2.67 X 7315 = 19,531 ft2 divided by 43,560 ft2 /acre = 0.45 acres)

Now, rooting depth available water = 1-foot X 2.4 inches water/foot (from Table 2) = 2.4 inches water/foot/acre

1-acre inch = 27,000 gallons (conversion value, a given)

27,000 gals X 2.4 inches = 64,800 gals 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 gals (full soil capacity) X 25% (depletion of soil capacity in the effective rooting zone)= 7,258 gals or about 7,300 gals to apply.

Application can be accomplished by knowing the delivery rate in gal/hr of your system and setting the time clock, or use a flow meter valve to set the required number of gallons (Bermadon at: http://www.bermad.com/page.asp?product=79&pline=2).

Should I record readings?

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. 

How do I know when the tensiometer is not working?

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.

 

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