Perhaps the reason why irrigation practices often fall short of optimum is that nearly all the action occurs in the soil and out of our view.
Determining when to irrigate and how much water to apply are not simple tasks. How can you assess whether irrigation practices are correct? This [article] provides information on soil moisture monitoring, a relatively simple and effective method for managing irrigation on alfalfa and irrigated pasture.
Monitoring soil moisture is easier and more practical than most other irrigation scheduling techniques, and it can help determine when to irrigate, if irrigations are too frequent or too infrequent and if the proper amount of water is being applied.
What’s the best strategy for irrigation scheduling?
The decision of when to irrigate is usually based on past experiences, weather-based information (crop evapotranspiration data) or soil-based measurements. Past experiences may not be applicable and are often not adjusted for annual changes in weather.
Scheduling irrigations based on crop evapotranspiration can be difficult because, unlike other crops with a single harvest per season, the multiple harvests of alfalfa and pasture confound the process. Irrigation water cannot be applied too close to a cutting, and fields obviously cannot be irrigated while the crop is curing. Therefore, there is typically a 6- to even 20-day period during which fields cannot be irrigated. This can make irrigation scheduling using weather-based information problematic.
In addition, it may be difficult to obtain accurate evapotranspiration data for some locations, and even when data are available, the task of keeping track of data for individual fields can be time-consuming. Because of the difficulties and shortcomings of these methods, soil-based irrigation scheduling may be the preferred technique.
Soil-based measurements may be far more practical and easy-to-use for alfalfa and pasture producers. Soil moisture content often goes unchecked. If soil moisture is monitored, it is usually only done using a shovel or soil auger. While better than nothing, using a shovel or auger is imprecise and is only useful for a gross evaluation of the soil moisture content in the upper foot (or less) of soil. Several inexpensive technologies may help growers monitor soil moisture.
Expressing soil moisture content
Soil moisture levels can be expressed in different ways, depending largely on the instrument used. Soil moisture content is often expressed as a percent (the weight of the water in the soil divided by the weight of oven-dried soil × 100). Other soil moisture monitoring devices use soil moisture tension to indicate soil moisture levels.
Soil moisture tension refers to how strongly water is held on soil particles; the higher the tension the more difficult it is for plant roots to extract water from the soil.
Therefore, low soil moisture tension indicates moist soil and high soil moisture tension indicates dry soil. Soil moisture tension is usually expressed in centibars. For some types of soil moisture resistance blocks, the instrument readings must be converted to soil moisture tension using appropriate calibration relationships.
Resistance blocks: A useful tool
More than six years of field studies in the Scott Valley (a high-elevation valley in northern California) demonstrated the usefulness of soil moisture monitoring in alfalfa and irrigated pasture fields.
These studies showed significant improvements in irrigation management were possible in many fields by monitoring soil moisture levels and adjusting irrigation practices accordingly. In some cases, yield losses were corrected through additional irrigations, and in other cases, overirrigation was prevented, producing improvements in water-use efficiency.
While resistance blocks do not directly measure volumetric soil moisture, in many cases resistance readings may be a better indication of plant-available water, since soil moisture tension is more directly related to the ability of a plant to obtain water from the soil. The simplicity of electrical resistance blocks provides a cost-effective technique for improving irrigation management for growers.
Ask questions about water management
• When do I start irrigating?
This is often a difficult decision. The soil profile is often filled from winter rains. But after the crop resumes growth in spring and the weather warms, you must decide when the soil moisture is depleted enough to require irrigation. Most irrigation systems do not have the capacity to “catch up” when the soil profile has been excessively depleted in spring.
Answer: The first irrigation should occur when soil moisture tension reaches the recommended value for your soil type (see Table 1*).
• Do I irrigate again before harvest?
Alfalfa is most sensitive to water stress after cutting when the plants start to regrow. Resistance block readings help assess whether the soil moisture content is sufficient to avoid water stress through the duration of the harvest period until irrigation can be resumed.
Answer: Track soil moisture to predict the rate of soil moisture depletion. By graphing the data and extending the line through the harvest period, you can anticipate soil moisture loss during harvest. If anticipated soil moisture levels fall well below the recommended values for your soil type, an irrigation or partial irrigation may be desirable before cutting.
• Did I fill the profile?
Soil moisture sensors are very useful in assessing the moisture status at the lower end of the root zone, especially for a deep-rooted crop like alfalfa. The lower half of the root zone supplies moisture reserves to draw upon when needed and should not be excessively depleted.
Answer: If the soil profile is filled after irrigation, soil moisture readings at all depths should return to less than 10 centibars for a sandy soil and less than about 30 centibars for medium- to fine-textured soil. If the sensors do not respond after irrigation, the water did not penetrate to the depth for the sensor. Monitor soil moisture after each irrigation to determine how many irrigations are needed to refill the profile.
• Should I change my irrigation practices?
Soil moisture monitoring is helpful to verify current irrigation practices satisfy, but do not exceed, the needs of the crop. A graph of soil moisture readings over the season provides a sound basis for altering and fine-tuning irrigation practices.
Answer: Plot soil moisture readings on a graph. The lines on your graph should oscillate. The highest soil moisture tension reached should be the values where irrigation is recommended.
Then, following irrigations, the values should return to less than 10 for a sandy soil and less than about 30 for a medium- to fine-textured soil.
If values exceed the recommended range, the soil is becoming excessively dry between irrigations and the field should be irrigated more frequently or with more water per irrigation. If values are low (indicating adequate moisture), irrigation can be skipped or delayed until soil moisture sensors indicate irrigation is needed.
Using soil moisture data
Resistance blocks
Electrical resistance blocks (also called gypsum blocks or resistance blocks) are not new, but advances have improved their accuracy and ease of use.
Resistance blocks evaluate soil moisture tension by measuring the electrical resistance between two electrodes. The blocks take up and release moisture as the soil wets and dries. The higher the water content of the blocks the lower the electrical resistance. The electrical resistance is measured with a portable meter connected to wire leads coming from the moisture sensors.
Interpreting the resistance block readings
An important point to understand when using the sensors is that the lower the reading the higher the soil moisture content, and conversely, the higher the reading the lower the moisture content.
When the soil is saturated after a rainfall or irrigation (air spaces are mostly filled with water) the reading is low, typically less than 5 to 10 centibars. As evaporation from the soil surface and transpiration by the crop dry the soil, the moisture sensor readings gradually increase until they indicate need for an irrigation.
The centibar reading at which irrigation is necessary depends on soil type. Sandy soils retain far less water than soils with a high clay or organic matter content, so irrigation on sandy soils should occur more frequently and at a lower soil moisture tension value. Soil moisture sensors may not be useful for very sandy soils with extremely low water-holding capacity, as the sensors might not respond quickly enough to the rapid decline in soil moisture.
After the field is irrigated the centibar readings typically return to the teens or single-digit values. These wetting and drying cycles continue throughout the season as the crop is irrigated and then the soil dries with crop water use.
The key to proper irrigation management using soil moisture sensors is to monitor the sensors regularly, track the soil moisture level and irrigate when the centibar readings are in the desired range for your soil type. Irrigating when the soil moisture readings exceed the desired range may result in crop stress and yield loss. Irrigation before the readings reach the desired range may result in excessive irrigation, water wastage or runoff.
Using and interpreting a soil moisture graph
The best way to use the soil moisture measurements is to plot them on a graph. The plotted data present a picture of the soil moisture status and indicate how fast the soil is drying.
After a few points are plotted, you can estimate approximately how many days it will take for the soil to dry before irrigation is needed. Plot the data from each sensor with a different color line so that the various depths can be easily distinguished.
We prefer inverting the y-axis (the centibar readings) so that zero is at the top of the graph instead of the bottom. This simplifies interpretation. Using this orientation, the line on the graph drops as the soil dries and rises when the soil is wet. This pattern is more consistent with how one conceptually thinks of changes in soil moisture content.
Interpreting soil moisture data: Using soil data to guide irrigation decisions
Why monitor soil moisture? Soil moisture tension data can help “train” the manager or irrigator to make wise irrigation decisions. Combined with observations of the crop and other irrigation scheduling techniques (e.g., the “checkbook” method), soil moisture blocks can help you know what is happening in the soil and to ‘ground truth’ your decisions. It can help answer questions such as:
• When do I start irrigating?
• Is there enough deep moisture?
• Am I applying enough water?
• Am I watering at the wrong time?
• Am I watering too much?
Match irrigation to crop needs
The principle behind irrigation management is to irrigate only enough to meet crop needs. Irrigation mistakes are easy to make, even for experienced growers. Soil moisture sensors improve the chances of making the right irrigation decisions.
Installation and management recommendations
Site selection
A critical part of sensor installation is selecting an appropriate site. Since irrigation of the whole field is based on the sensor readings, the sensors need to be in an area that represents the field.
Locate the sensors in an area having the soil type typical of the field, uniform crop growth and in an area that receives full uniform sprinkler or flood-irrigation coverage. If the sensors are placed in an area that is not representative, the results can be very misleading.
One sensor site per field is usually sufficient; however, if the field is variable, two sites are desirable. It may be helpful to install one of the sensors in a slightly sandier area of the field to use as an early indicator of when irrigation may be required.
Sensor placement
Proper sensor placement is critical for accurately representing soil moisture in the crop root zone. We recommend installing two or three sensors at each evaluation site. One sensor should be in the upper quarter of the root zone. Another should be toward the bottom of the root zone.
When three sensors are used in an alfalfa field, install them at 1 foot, 2 feet and 3.5 or 4 feet (depending on the depth of the soil). In cases where the rooting depth is more restricted, installations at 9 inches, 1.5 feet and 3 feet are recommended. Because of shallower rooting in irrigated pastures, the sensors should be installed at 6 inches, 1 foot and 2 feet. Three sensors present a more complete soil moisture picture and allow better evaluation of the depth reached with each irrigation.
The uppermost sensor indicates when to irrigate. The second sensor helps determine the depth of the last irrigation and is a check for the first sensor to make sure it is operating properly. The deepest sensor measures moisture reserves deep in the root zone. Maintain the deepest sensor within an acceptable range – not too dry (over 90 centibars) or not too wet (saturated conditions, readings less than 5 or 10 centibars).
Installation
Use a probe or coring device to create a hole slightly larger than the diameter of the sensor. The sensor must be in direct contact with the surrounding soil. To ensure contact, prepare a small slurry (mixture of the surrounding soil and water) to pour into the hole before seating the sensors. As the sensor is pushed into the hole, the slurry at the bottom squeezes up around the sensor to provide good contact between the soil and sensor.
The wire leads come up from the sensors to the soil surface. Carefully back-fill the hole so as not to damage the wires. A 4- to 6-inch length of 2-inch PVC with a screw-on cap works well and protects the wire leads from a swather or cattle. Dig a shallow (3- to 6-inch) trench for the wire leads coming from each sensor to the PVC housing. You can color code the wires to keep track of the depth of the sensors by tying a small piece of colored wire to each lead.
An alternative installation involves gluing a section of PVC pipe onto each sensor and feeding the wires through the pipe. This installation allows the sensors to be retrieved and reused. One-half-inch Class 315 PVC has an inside diameter that fits exactly over the top of the sensor collar. Use PVC to ABS cement to weld the sensor to the PVC. A cap can be fitted directly on the pipe to house the wire leads and the depth of the sensor engraved on the cap.
How often to take soil moisture readings
How often you take soil moisture readings depends on their intended use. Weekly readings should provide an overall picture of the seasonal soil moisture status of the field for evaluating current irrigation practices.
However, if the readings are used for irrigation scheduling, they should be taken at least twice a week, especially immediately before and after an irrigation. Commercial data logging devices are available that constantly record readings at predetermined intervals.
Conclusion
Irrigation management for alfalfa and irrigated pastures can be difficult; growers must schedule around harvests, making it problematic to use many irrigation-scheduling techniques. However, years of research and field experience have shown that soil moisture sensors are very useful in diagnosing needed changes and in fine-tuning irrigation practices. Relatively minor adjustments in irrigation practices could pay large dividends in increased yield or water savings. FG
*References and tables omitted but are available upon request at editor@progressivedairy.com
—From University of California – Davis Crop Management website
S. Orloff
University of California Cooperative Extension
University of California
sborloff@ucdavis.edu
B. Hanson
Department of Land, Air, and Water Resources
University of California
D. Putnam
Department of Agronomy and Range Science
University of California
dhputnam@ucdavis.edu
