Deciding on When to Irrigate

Every crop has a threshold for how dry the root zone environment can get before yield and quality are compromised. Decreasing soil moisture past that threshold will inhibit plant transpiration, photosynthetic and metabolic processes, leading to decreased crop yield. In the other hand, excessive soil moisture maintained for extended durations can also be detrimental to plant growth as it decreases roots access to oxygen and carbon dioxide. In addition to significant yield losses, suboptimal soil moisture conditions may also trigger disease outbreaks when both excess and lack of soil water occur. Therefore, maintaining moist and well aerated conditions in the root zone for the majority of time is essential for achieving maximum yield and quality.  

Integrating Soil Moisture Sensors to Management Decisions

The use of soil moisture sensors associated with field observations (e.g., checking soil moisture with a probe/sampler) usually presents the best results for determining when to irrigate. In general, berry and vegetable crops such as strawberries, raspberries, celery and cabbage are more sensitive to water stress than agronomic crops such as cotton, corn wheat and alfalfa.


hortau sensors


Among all types of soil moisture sensors, tensiometers (pictured here on the left, Irrometer, and on the right, Hortau ) are the most accurate and suitable for the range of soil moisture observed in berry and vegetable fields. Click here for a more detailed description of soil moisture sensors including types, features and crop suitability. 


The table below summarizes suggested soil moisture thresholds for starting irrigation (along with installation depths) for some popular berry and vegetable crops grown in Ventura County.

threshold table




*Centibar and kiloPascal are interchangeable units: 1 cbar = 1 kPa.

**Two depths per monitoring location is recommended; the shallow sensor is installed where most of the active roots are, and the deeper one towards the bottom of the root zone. 

The irrigation threshold for starting the irrigation is applied to the shallow sensor's readings, while the deeper sensor is used to assess how much of that water reached the bottom of the root zone. The deeper sensor’s readings should always be lower than the shallow one. Think of the moisture content around the bottom of the root zone as a 'reserve' the crop can access when shallower soil water is limited. A crop is a lot more susceptible to water deficit conditions when the soil moisture at the bottom of the root zone is low.

Accordingly, ideal soil moisture levels should range from approximately 0 to 5 centibars (after irrigation) and the values suggested in the table above, when the irrigation should start. The values in the table above were determined based on the best available information and practical knowledge, and they are meant to guide irrigation decisions for optimal yields.

Although it is often difficult to maintain soil moisture within the suggested limits, maximizing the time a crop experiences ideal soil moisture will contribute to improved yield and quality. Also, keep in mind that the root depth of a crop can vary significantly among fields, depending on irrigation patterns (roots tend to accumulate where water and nutrients are) and soil physical properties. The installation depth of the sensors needs to be adjusted according to the specific conditions of the field. 


ideal sms graph
This graph illustrates ideal soil moisture content of a strawberry field where irrigation was guided with tensiometers.






The following two graphs illustrate typical cases of irrigation management not guided by soil moisture sensors, resulting in depleted soil moisture content that often exceeded the irrigation threshold by orders of magnitude.

non ideal sms graph1

non ideal sms graph2



The light blue band (0 to 10 kPa) represents the ideal range of soil moisture a strawberry crop should experience most of the time.

It's important to emphasize that irrigating on a fixed schedule (e.g., every 4 days) usually leads to inadequate soil moisture content (too wet or too dry) due to irregular changes in weather patterns.



A replicated study conducted in a commercial celery field located in the Oxnard Plain, Fall of 2017, quantified a 9% marketable yield decline (graph below) when the irrigation threshold went from 20 to 50 centibars (T-20 and T-50 represented in the x-axis). All treatments were irrigated based on the ET method and received the same amount of water.

swp celery yield











The successful use of soil moisture sensors is strictly dependent on the following factors:

  • Choosing the appropriate sensor: among all types of soil moisture sensors in the market, tensiometers are the most suitable for berry and vegetable crop production for several reasons, including higher accuracy in the moisture range these crops thrive (0-40 centibars) and lack of salinity interference in the sensor’s readings. In addition, the thresholds discussed above can be used with tensiometers in any soil type, while volumetric water content sensors need to have the threshold calibrated with changes in soil texture.
  • Proper installation: the goal of the installation is that the sensor is placed in the right location, with minimal disturbance of the soil and plants around it. The ceramic tip needs to have good contact with the soil, and placed strategically in the plant row so it can represent most of the root environment around it.
  • Location: blocks with different cultivars, soil type and planting dates should have their own set of sensors. Usually two depths per location is ideal.
  • Maintenance: tensiometers may require regular maintenance, especially if readings get up to 90 centibars, when it 'breaks tension' and water needs to be refilled. In addition, farm equipment and field workers can touch the sensors either breaking them or causing them to cease contact of the ceramic tip with the soil. 
  • Data interpretation and utilization: the process of guiding irrigation with soil moisture sensors needs to take in consideration the representativeness of the readings. Training farm staff and focusing on the details described above will increase likelihood of success.  


It is important to note that soil moisture sensors should not be used to determine when to stop the irrigation. While plant roots extract water uniformly from the soil, water added through irrigation generally follows a non-uniform pattern in the soil, especially with drip tape and micro-sprinkler systems. The best method to determined when to stop the water (or how long to irrigate) is the ET-based method.


Note: I have not seen anyone successfully using volumetric water content sensors to guide irrigation scheduling of berry and vegetable crops, including myself. Therefore, the discussions in this page are focused on tensiometers, which present numerous advantages over the other sensors types. Click here for a detailed description of soil moisture sensors types and features.


Here are a few options of tensiometers that we have had good experience with:

Useful resources:

Assessment of soil water tension thresholds for optimum irrigation scheduling yield and quality_Biscaro

Matric potential based irrigation management of field grown strawberry_ Effects on yield and water use efficiency_Caron