- Author: Ben Faber
Irrigation timing can be determined more precisely using a tensiometer. These water-filled tubes with a pressure gauge accurately reflect the amount of energy a plant needs to extract water from the soil. The pressure gauge measures tension values in centibar units (cbars). For citrus, when the gauge reads 30 centibars, it is a good time to irrigate. Tensiometers must be placed in the root zone between the emitter and the tree trunk. Having two tensiometers next to each other can be helpful in deciding when to turn the system on and off. For example, a tensiometer at a depth of 1 foot would indicate when to turn the water on, and a second at 3 feet would indicate when to turn the water off. Prevent tensiometers from being damaged during harvesting and other grove operations by placing a plastic milk crate or some other structure over each device.
Other devices can also be used to measure soil moisture. Gypsum blocks are very effective; the part in the ground is inexpensive but the reading device costs about $250, so a relatively large acreage is required to spread out the cost of the system. Portable meters rely on an electrical current carried by water in the soil. Even cheap $10 meters can give a rough estimate of the soil water content, but they are not very effective in rocky ground, because their sensitive tips break easily.
Soil-based methods monitor an aspect of soil moisture that, depending on the method, requires a correlation to plant water use. Some methods are well understood and inexpensive, others are expensive, inaccurate, inappropriate, or not well researched. Some methods allow multiple site readings, while others require a device to be left in place. Some measure soil water directly (e.g., oven-drying), and others measure another parameter, such as electrical conductance. Some methods are affected by salts or soil iron content, and others have limited value in the desired soil moisture range. Some, like tensiometers and gypsum blocks, give a reading from a porous material that comes to equilibrium with soil moisture, while many others use the soil directly as the measured medium—an important distinction, since discontinuities in the soil caused by rocks or gopher holes can affect readings. Also, some of the older techniques have been improved. For example, gravimetric oven-drying can now be done by microwave, considerably speeding up the process, and tensiometers and gypsum blocks can now be found with digital readouts and connections to data loggers that make data easier to manage. Many types of monitoring devices are available; the table below describes their characteristics.
As with any tool, the value of these devices increases with use and familiarity. Even though several are stationary devices, by placing them in representative positions in the grove, they can accurately reflect the entire grove. Some types of device can be stationary or portable, depending on the model.
Method |
Cost |
Ease of use |
Accuracy |
Reliability |
Salt-affected |
Stationary |
Gypsum block |
L |
H |
H |
H |
L |
YES |
Tensiometer |
L |
M |
H |
M |
L |
YES |
Portable tensiometer |
M |
M |
H |
M |
L |
NO |
Solid-state tensiometer |
M |
H |
H |
H |
L |
YES |
Time domain reflectometer |
H |
M |
H |
H |
M |
BOTH |
Neutron probe |
H |
L |
H |
H |
L |
YES |
Feel (soil probe) |
L |
H |
H |
H |
L |
NO |
Gravimetric (oven) |
L |
M |
H |
H |
L |
NO |
Conductance |
L |
H |
M |
M |
H |
BOTH |
Capacitance |
M |
H |
M |
H |
M |
BOTH |
H – high, M – medium, L - low
- Author: Ben Faber
In order to properly irrigate any crop, you need to know how much water you are putting onto the crop. A grower with a small acreage recently asked how to irrigate avocados, and I said amongst other things that it is not only necessary to know how much water the emitters are spraying, but where that water is going. Each manufacturer rates output at a certain flow at a certain pressure. So at 20 psi a rated emitter will put out something like 8, 9, 10, 14 gpm, whatever. If pressure changes, the output changes and some emitters respond to pressure change more than others.
As elevation and distance from the inflow valve changes, pressure changes. That can be corrected by using pressure compensation in-line or at the emitter or both. So even if pressure changes, there will be a known output if the manufacturer has done right by the product. Then it is up to the grower to make sure that clogging, filter cleaning and other maintenance practices are followed.
So now the grower is left with where the water is going. When an orchard is about 8 years old in a conventional planting, the roots of all the trees are starting to get entangled amongst themselves. It becomes one big root system. So you may not think that it matters what pattern the water takes. It all gets to the trees, right? No. Because most microsprinklers as far as I know put out a pattern that puts less water in the pattern than in other parts of the pattern. Typically the water decreases with distance from the microspinkler. Lots of water near the emitter that also leaches accumulated salts from the profile, but with distance, the amount decreases and leaching decreases. The wetted pattern might look like a nice circle, but salt will accumulate with depth in that pattern because there is not sufficient water arriving at that point to drive the water below the root zone. Rain makes up for this faulty wetted pattern by leaching those accumulated salts from those underirrigated areas. A grower could compensate for lack of rain by running the system for a much longer time to make sure those poorly leached areas get a good wetting. But in a drought and with high water costs growers hesitate to put on more than they think might be necessary.
Once salt damage has occurred, though, it is not going to go away. Eventually the tree will shed those leaves and the tree will become less productive. Therefore it is important to know how much of that water is not adequately leaching the salts that are naturally occurring in the irrigation water. So our grower followed a little trial that is very commonly done in the turf trade. He put out catch cans (tuna cans with lids removed) and looked at the spray pattern of the microspinklers. He ran the system for increasing amounts of time and was able to get an idea of how much water could infiltrate at one spot.
He could then place the emitters in a pattern that might more effectively leach salts. First of all he knew that he didn't want the spray pattern to hit the tree trunks to avoid crown rot. So he could then move the emitters further away from the trunks. Also the most efficient roots for water uptake are not right at the trunk. That's where coarser roots are and much of the water will bypass them. Where the fine roots are, are further out from the trunk. So he could then move them out a little further from the trunk. At this point there comes some overlapping of the emitter spray patterns so there will be increased leaching occurring where the overlap occurs.
You can probably see where this is going. The optimum distance for a relatively mature orchard is when the emitters are midway in between trunks and there is some overlap of spray patterns. It might be necessary to on higher output emitters to make sure there is some overlap within reason. This would make the pattern of output more or less even across the wetted area. It will never be perfectly even, though, because that is not how microspinklers were designed. The grower now has a better handle on where that water was going.
1) Spray pattern with distance from the emitter (top graph).
2) Salt accumulation with depth and distance in that spray pattern (bottom graph).
- Author: Jim Wolpert - UC Davis
Soil Moisture Sensors
Tensiometers Electrical Resistance Blocks Neutron Probes Di-Electric Sensors More Info
Jim Wolpert, University of California, Davis
Soil Moisture Content
The quantity of water in soil is called the soil moisture content. After rainfall or irrigation, some water drains from the soil by the force of gravity. The remaining water is held in the soil by a complex force known as surface tension and varies depending on the amount of sand, silt, and clay. Sands, with larger particles and smaller total surface area, will hold less water than clays, which have much smaller particles and larger total surface area. The drier the soil, the greater the surface tension, and the more energy it will take for a plant to extract water.
Vineyard managers often measure soil water content as a guide to determine their irrigation timings and amounts. There are several methods for monitoring soil water content. Correlating these methods with actual inches of moisture per foot of soil is very complicated (see Recommended Links) but at the very least can help a grower to identify patterns of water use, depth of irrigation, and soil water content trends over time.
Tensiometers
A tensiometer, as its name implies, is a device for measuring soil moisture tension. The design is a simple tube with a porous cup at the lower end and a vacuum gauge on top. The tube is filled with water, sealed airtight, and placed in soil. As soil dries, water is pulled from the porous cup into the soil, creating a vacuum and causing the gauge to move. As soil continues to dry, more water is pulled out and the suction increases. As soil re-wets after a rain or irrigation, water moves back into the cup and the suction decreases. Installing tensiometers in soil requires attention to detail to obtain accurate readings (see Recommended Links for installation downloads).
Tensiometers are usually placed as a pair with the shorter tube positioned in the middle of the rooting zone (e.g., 18 inches deep) and a longer tube positioned near the bottom of the rooting zone (3 to 4 feet deep). Growers can use the difference between the two tubes to monitor water usage and determine the effective depth of irrigation. At least two stations (two tubes per station) are recommended per field, or more depending on soil variability.
Tensiometers have the advantage of being inexpensive, and easy to install, maintain, and read. They are better in fine-textured soils where good contact can be made between the porous cup and the soil. They do not work well in coarse sands where good contact may not be possible. Because the gauges are aboveground, the units are prone to damage by vineyard equipment.
Electrical Resistance Blocks
Electrical resistance blocks are also known as gypsum blocks or soil moisture blocks. They are simple devices with two electrodes embedded in a block of gypsum or other similar material. When blocks are buried in soil, water moves into or out of the block, depending on the moisture of the soil, changing the resistance between the two electrodes. Like tensiometers, gypsum blocks are cheap and easy to install. They are usually installed in at least two stations per field, at two depths, and must be installed correctly to provide accurate readings. Some block designs perform better under wet soil conditions and some correct for soil temperature. The meter used to read the blocks can be moved from field to field, but is specific to the block design (i.e., it is not a simple ohm meter). The wires aboveground are much less prone to damage by equipment compared to tensiometers.
Neutron Probe
A neutron probe uses a radioactive source for measuring soil moisture. A tube, usually made of PVC or aluminum, is installed in soil to a depth of interest and the radioactive probe is lowered into soil to measure soil moisture at as many depths as desired. The probe emits fast neutrons that are slowed by water in the soil in a way that can be calibrated to the soil water content. The probe has a significant advantage, especially for perennial crops, because access tubes are easy to install and relatively permanent. Another advantage is the reading accounts for a spherical area about 10 inches in diameter, much greater than other methods. The major limitation to this method is the probe itself; it is expensive and the presence of a radioactive source triggers requirements for operators to be trained and licensed in handling, storage, and use. In some production regions, service providers are available, usually at a fixed cost per access tube for a growing season.
Di-electric Sensors
Di-electric sensors measure the di-electric constant of soil, a characteristic that changes with changing soil moisture. A common method is called time domain reflectometry, or TDR. The theory behind how this method works is too complicated to be discussed here. The advantage of these types of systems is that they are designed to be left in place and provide continuous readings of soil moisture. The disadvantages are that the units are expensive and read soil moisture only a very small distance from the unit.
Conclusion
All measures of soil moisture suffer from the same limitation — the value of the information is dependent on the extent to which the soil where the measurements are taken reflects the rest of the field. Where soil variability is high, growers must exercise caution in relying too heavily on relatively few measurements.
Recommended Resources
Irrigation of Winegrapes, University of California
Soil moisture management, Irrometer
Soilmoisture Equipment Corporation
Irrigation Basics for Eastern Washington Vineyards, Washington State University
Reviewed by Ed Hellman, Texas AgriLife Extension and Eric Stafne, Mississippi State University
- Author: Ben Faber
California Small Farm Conference
Registration is now open for the California Small Farm Conference - the state's premier gathering for small-scale farmers and ranchers, farm employees, farmers' market managers, researchers, federal and state agriculture agencies, agriculture students, food policy advocates, consumers and others. The goal of the Conference is to promote the success and viability of small and family owned farming operations and farmers markets through short courses, field tours and workshops.
This year's California Small Farm Conference (CSFC), featuring opening plenary speaker, A.G. Kawamura, will be held at the San Diego Marriott Mission Valley in southern California on March 7 - 10, 2015.
Keep updated on all activities by liking us on Facebook and signing up for our E-Newsletter. We hope some of you will join UC Small Farm Program staff on the Agritourism Field Course or the Alternatives in Marketing Field Course.
Small Farm Blogs
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- Author: Ben Faber
Impacts of the recent drought conditions on Central Coast avocado production, and potential impacts of continued drought conditions
Avocados are the most salt and drought sensitive of our fruit tree crops. They are shallow rooted and are not able to exploit large volumes of soil and therefore are not capable of fully using stored rainfall. On the other hand, the avocado is highly dependent on rainfall for leaching accumulated salts resulting from irrigation water. In years with low rainfall, even well irrigated orchards will show salt damage. During flowering there can be extensive leaf drop due to the competition between flowers and leaves when there is salt/drought stress. In order to reduce leaf damage and retain leaves, an excess amount of water is required to leach salts out of the roots zone. The more salts in the water and the less rainfall, the greater leaching fraction. Drought stress often leads to diseases, such as black streak, bacterial canker, and blight (stem, leaf, and fruit). Defoliation leads to sunburned trees and fruit which can be severe economic losses.
Strategies to address drought conditions
-
Ensure that the irrigation system is at its greatest potential and is maintained. Avocados are grown on hillsides and pressure regulation is extremely important and is frequently neglected.
-
Significantly prune trees to reduce leaf area. Avocado can be a very large tree, and if half the canopy is removed, there can be as much as 1/3 reduction in water use. When trees are about 15 feet tall, removing half the canopy can reduce water use by one half.
-
In extreme drought conditions, the canopy can be reduced to just the skeleton branches which are white washed to prevent sunburn. Water use drops to zero, and then gradually as the tree leafs out, water can be slowly reapplied, but at significantly less amounts than with the full canopy. Stumping typically results in three years' worth of crop.
-
In orchards that have low producing areas, because of recurrent frost, high winds, shallow soils, disease, etc. the grower could decide to completely remove those trees, thereby saving water.
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White kaolin applied to leaves has been shown to reduce leaf temperatures and water loss. This can be used, but under the direction of the packing house, since if it is applied to fruit, it is very difficult to remove.
Impacts of the recent drought conditions on Central Coast citrus production, and potential impacts of continued drought conditions
Citrus is much less sensitive to salts and drought than avocado, partially because of its greater rooting depth. However, it is much more sensitive than deciduous fruit trees, resulting in smaller fruit and lower prices when drought cannot be addressed with adequate irrigation water. Drought also makes the trees more susceptible to leaf drop, and sunburned fruit.
Strategies to address drought conditions
-
The strategies for citrus are very similar to those for avocado. It is much more sensitive to pruning to reduce water use than avocado. Typically removing half the canopy results in half the water use. Because of thus greater control, citrus is rarely stumped.
-
By reducing canopy size, production can be maintained, often without loss of fruit size.
-
Kaolin clay can effectively reduced water use and can be applied soon after harvest without the problem of coating fruit making its removal difficult at the packing house.