Fact Sheet FS1306
Most orchards in New Jersey are primarily rain-fed and may need supplemental irrigation during prolonged dry periods. Remember, sizing fruit to their full potential (size and market price) requires irrigation as a supplemental water source when rainfall is inadequate. Growers who do not currently have irrigation systems are strongly encouraged to install a diversion system to support a drip or sprinkler irrigation system. Using a tensiometer is a low-cost and effective way to determine if your orchard needs irrigation during critical growth stages.
Orchard Tree and Fruit Development
Fruit trees require sufficient water for production of wood structure, potential fruitfulness, sizing of the fruit, and cooling the canopy by evapotranspiration during the warm days of summer. In peaches, irrigation is critical for fruit sizing, gaining up to 70% of its final volume during the last 30 days on the tree. Inadequate irrigation during this growth stage results in smaller fruits. High-density, young apple orchards need frequent irrigation due to their shallow and smaller root systems, and are particularly at risk of suffering drought stress. Achieving high yields in the third to fourth year is critical for economic sustainability of these capital-intensive orchard systems. Dry soils during the growing season may significantly reduce canopy development and delay achieving full cropping. Also, incidences of bitter pit have been associated with dry spells and growers failing to regularly irrigate apple orchard blocks during fruit development.
Methods for Scheduling Irrigation
There are three approaches to irrigation scheduling: 1) water balance estimates, 2) measuring plant responses, and 3) measuring soil moisture. Water balance, or water budget estimates are calculated using weather data to estimate crop water use by evapotranspiration, rainfall, previously applied irrigation, and stored soil moisture. The grower can then add water whenever the available water in the soil goes below a predetermined threshold, say 50% of total available water capacity. Measuring various plant characteristics (e.g. stem water potential, sap flow, canopy temperature) can estimate the degree of water stress in the plant at that instant, which, combined with knowledge of the plants' water stress response, can be used to make irrigation decisions. Soil moisture measurements using various instruments (e.g. tensiometers, electrical resistance blocks, neutron probes) can tell us either how much water is in the soil (soil water content) or how hard the soil is pulling the water. Tensiometers measure the latter and provide a method that is inexpensive, easy to install and operate, does not require electrical power in the orchard, and can indicate imminent need for irrigation before plants experience damaging water stress.
Using a Tensiometer in Orchard Production
Using a tensiometer provides an accurate and relatively easy way to measure soil moisture when making irrigation decisions. The first step before you begin to irrigate is to assess the existing soil water status, specifically under the tree.
Working Principle of the Tensiometer
Figure 1. Tensiometer (Courtesy, Irrometer®).
Tensiometers consist of a transparent plastic or plexiglass tube with a porous ceramic tip (Figure 1). It is connected to a vacuum pressure gauge on the top. Tensiometers work by measuring the tension of the water in the soil. The greater the tension, the less water is available for the plant to access and utilize. The simplest tensiometers do not require a power source or any electronics so they can easily be placed anywhere in the field. When the tensiometer is inserted into the soil, water inside the plastic tube will get absorbed into the soil through the porous ceramic tip. The amount absorbed depends on the dryness of the soil. The "suction" created lowers the water column and will create a partial vacuum (negative pressure) in the tube, which is measured by the pressure gauge. The tip of the tensiometer mimics the root of a plant, which in the case of a peach tree could be 6" to 24" below the soil surface, depending on tree age and soil type. Note that sandy soils dry much faster so roots tend to grow deeper compared to heavy soils. Tensiometers designed especially for soils that have low water tension (i.e. coarse-textured soils) are available.
If the soil is uniform, fewer stations of tensiometers are required, even for a large orchard block. For a block with varied soil types, stations should be placed in each distinctly different section. For each station, using 2–3 soil tensiometers at multiple depths (e.g. 1, 2, and 3 feet), can give a better understanding of how hard trees are working to pull water from the soil. It also helps in estimating how far water has reached after a precipitation event or an irrigation.
Tensiometer Values and Their Meaning
Depending on the growth stage of the crop, values of the tensiometer can be used to decide when to irrigate (Table 1). The higher the reading number, the greater stress the tree is undergoing (see below table). Typically, as a general rule of thumb, growers begin to irrigate at a reading of 30 Cbr (Centibars, or its equivalent unit kilopascals, kPa). However, that decision depends on the water holding capacity of the soil, in other words, the soil type within your orchard. Therefore, it is very important to know your soil profile for properly interpreting the tensiometer readings. For example, a plant may show moderate stress at 30 Cbr in a sandy soil, but the same level of stress may be experienced at 70 Cbr in a heavy clay soil. Initially you may need to calibrate the tensiometer readings for your soil and site, noting the tensiometer’s readings at:
- After irrigating the orchard to field capacity;
- At partial drying; and
- At full drying of the soil, corresponding the stress symptoms of the tree under which the tensiometer is located.
Conversely, newer tensiometers are already calibrated for different soil types, allowing knowledgeable growers to order the correct instrument specific to your soil type. Remember that the intention of supplemental irrigation is to keep the soil moisture content of your orchard up to field capacity, which depends on the water holding capacity of the soil, a property of the soil type. Coarse (sandy) soils have lower water holding capacity than fine textured soils (loamy or non-aggregated clay). However irrigation has to be supplied before the stress sets in, generally speaking at 50% moisture depletion. Expected tension (Cbr) values for different soil types are shown in Table 2.
| Reading (centibars) | Soil Water Status | Interpretation |
|---|---|---|
| 0–10 *Cbr | Saturated | Water should be drained. |
| 10–20 Cbr | Field capacity | Optimal condition. No Irrigation needed. |
| 30–70 Cbr | Limited | Mild to moderate stress. Irrigate depending on soil type. |
| >70 Cbr | Dry / Droughty | High to severe stress. Irrigate to field capacity. |
*Cbr (Centibars) is a unit of pressure measurement, equivalent to kilopascal (kPa).
1 bar = 14.5 psi, and 10 centibars = 0.1 bar. Latest tensiometer models allow you to choose from multiple units of pressure measurement.
| Soil type | Approx. centibars readings at 50% moisture depletion |
|---|---|
| Sand | 20 Cbr |
| Loamy sand | 30 Cbr |
| Sandy loam | 40 Cbr |
| Loam | 65 Cbr |
| Silt loam | 90 Cbr |
Tensiometer Installation and Maintenance
Figure 2. Using a vacuum pump to suck out air from the tensiometer before inserting it into the soil.
Although each brand of tensiometer will have its own maintenance specifications, some principles will apply to all of them. It is important to use distilled water to fill up the cylinder of the tensiometer. Water should be stored in the airtight container before transferring into the tube. A few drops of bleach per 4 to 5 gallons of distilled water should be added, which will prevent algal formation in the tube. Make sure to avoid creating bubbles during mixing. Prepping the instrument before installation is also very important, and the ceramic tip should be clean and uncracked. It will likely be recommended to soak the tip for several hours or days in clean water before installation. The photo below (Figure 2) illustrates the use of the vacuum pump to set up an Irrometer® brand tensiometer. This pump will set the vacuum inside the instrument to allow for it to work properly once it is installed in the ground. (This pump is not part of the tensiometer, and only one pump is needed regardless of how many tensiometers are purchased.) A hole the depth of the tensiometer should be dug, this should have a snug fit, particularly at the tip as this is the component that actually measures the soil water availability. A mud slurry can be poured into the hole immediately after installation to provide for better soil contact. All tensiometers should be removed from the orchard at the end of the growing season, cleaned, dried, and stored. They should never be allowed to freeze.
In summary, estimating the water stress is the first step towards precision irrigation of an orchard. Tensiometers can be a great tool in irrigation management, specifically during dry spells if installed, operated, and maintained appropriately. Movement of water through soil depends on soil types, and it is important to understand your orchard soil profile and to interpret tensiometer readings and adjust timing and duration of irrigation, accordingly. All proper cultural practices should be followed to prevent tree stress and achieve full crop potential.
References
July 2019
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