Soil Moisture Based Irrigation Scheduling

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Soil Moisture Based Irrigation Scheduling

An analysis of soil moisture readings is a good way to determine when and how much to irrigate no matter what the crop or irrigation method. Irrigation scheduling through soil moisture analysis improves efficiency and can lead to improved quality, yield, and profits.

Soil moisture monitoring provides detailed information about the changing soil moisture status, indicating when crops are at risk for stress, when to irrigate, and when to stop. Soil moisture data reveals the wetting front as water moves down through the root zone. By irrigating according to soil moisture readings, it’s possible to apply water when it’s needed and where it’s needed, in the area where roots are established or desired.

It is easy to install soil moisture sensors at the depths appropriate for the particular root systems by boring access holes into the soil and inserting sensors. Some sensors are read at a gauge attached to the probe while others can be connected to a reader or data recording device that displays the moisture value. Sensors can also be adapted to automatically controlled systems by adding switching devices that will allow watering only when the soil moisture is sufficiently depleted.

Fewer Inputs, Better Performance

Managing root zone moisture not only helps use water efficiently, it helps make the best use of other agricultural inputs. When decisions are made based on information gathered through monitoring, nutrients remain in the root zone until absorbed by the plants instead of being pushed out through excessive irrigation. When over-irrigation occurs, nutrients can leach from the root zone and migrate to the groundwater.

Monitoring with soil moisture sensors also allows growers to get the best performance from low-application rate irrigation systems such as drip, microsprinklers, and Low Energy Precision Application center pivot technology. Low-volume irrigation continuously supplies water to replace soil moisture with frequent, slow applications through emitters, drippers, micro-sprinklers, or sprays. When the application rate of the irrigation system is very low, it can be difficult to compensate for scheduling errors. High-application rate systems like high-pressure sprinklers or flood irrigation, can quickly apply large quantities of water to correct diminished soil moisture in the root zone. Soil moisture monitoring ensures a deficit situation is avoided.

Root Zone Management

Microirrigation can lead to poor soil water distribution because it generally wets a small surface area. Distribution of the water into the crop root zone depends on the soil and its ability to transmit water laterally, which can be problematic in lighter soils. The number and placement of emission devices must match the ability of the soil to move water throughout the root system. Roots will not grow into dry soil, so it’s important to monitor soil moisture in the root zone as well as the desired root zone. In climates where seasonal rainfall causes extensive root development of tree crops, like citrus in Florida or California, it is important to ensure adequate irrigation of the outer root system during dry periods. Soil moisture monitoring can take the guess work out of this process.

Monitoring can also help protect crops from salinity. In areas where salinity is a factor, the outward and downward wetting front pushes salts away from the effective root zone. If this process is not continued, salts can move back into the root zone and damage crops.

Sensor Placement

Soil moisture measurement makes it easier to manage many irrigation issues. To be effective, sensors must be placed in active root zones in locations that will accurately represent how the field or area is wetted by the system. It helps to keep in mind that with a drip or trickle system, the emitter wets a very small surface area, with the subsurface moisture forming a “wetted onion” as the soil moves the water downward and outward. With microsprinklers or sprays, a much larger surface area is wetted. In tree crops, sensors should be located at or near the drip line where the active roots are found. In row and vine crops, sensors should be placed in the rows between plants.

Thoughtful interpretation of soil moisture readings from representative locations of the field will tell both when and how long to run the irrigation system, so that the maximum benefits can be achieved, yielding the greatest profit from the crop.

Tom Penning is President of the Irrometer Co., Inc. in Riverside, CA, a manufacturer of soil moisture measurement equipment.

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