When to Irrigate

Every crop or plant 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.  

Using Soil Moisture Sensors

The use of soil moisture sensors associated with field observations (e.g., checking soil moisture with a probe) 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 and alfalfa. The table below summarizes suggested installation depths of sensors and soil moisture thresholds for starting irrigation for the main berry and vegetable crops grown in Ventura County.

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

**Two depths per monitoring location is recommended, where the shallow sensor is installed in where most of the active roots are and the deeper one towards the bottom. The sallow sensor’s readings should be used to apply the irrigation threshold, and the deeper sensor for assessing if the irrigations sufficiently reached the entire rootzone. The deeper sensor’s readings should always be lower than the shallow one.  

Therefore, ideal soil moisture levels should stay within a range between 0-3 centibars (after irrigation) and the values suggested in the table above, when irrigation should start.  Figure … illustrates ideal the soil moisture content of a strawberry field where irrigation was guided with tensiometers. Figure .. illustrates the soil moisture of another strawberry field where soil moisture sensor data was not taken in consideration for making irrigation decisions.

Ideally, this is how a soil moisture content looks like for a field where irrigation decisions were based on soil moisture sensors data and field observations.  

Intro to the graphs: blue range from 0 to 10 kPa for strawberries and 0 to 20 kPa for celery; 0 kPa represents soil saturation, usually after sufficient irrigation and/or rainfall. The higher the number in the Y axis, the drier the soil.

The graphs below illustrate typical cases of fields where soil moisture sensors data was not used to guide irrigation, resulting in soil moisture levels that often exceeded the soil moisture threshold by orders of magnitude.

Typical example of irrigation frequency being kept constant while plants are growing actively and the weather is warming up quickly, resulting in plant experiencing water stress for extended duration.

Show celery study data summary: treatments, yield graph, and bullet points summary. See inovagri presentation

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 these limits, maximizing the time a crop experiences ideal soil moisture conditions will contribute to improved yield and quality. Please be aware 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.

Key essential factors for the successful use of soil moisture sensors:

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 seem to be the most appropriate for berry and vegetable crops for several reasons, including its accuracy in the low tension range, where berries and vegetables operate, and the fact that soil and fertigation salinity does not interfere with the sensor’s readings. In addition, the thresholds discussed above can be used with tensiometers in any soil type, which isn’t the case for volumetric content sensors.

(picture of two tensiometers pointing out the ceramic tips, and showing different levels of data access: in-situ gauge reading, or through smartphone and computer apps)

• … 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 environment around it.
• Location: blocks with different cultivars, soil type and planting dates should have their own set of sensors. Usually two depths is ideal; understanding root depth is essential for representativeness of the data.
• Maintenance: often, tractors, sprayers, other farm equipment and even farm staff can touch the sensors either breaking them or causing them to cease contact of the ceramic tip with the soil. Also, when soil moisture levels get too low and reach 80-90 centibars, air can move through the ceramic tip (or ‘breaking tension’) and impair the sensor’s accuracy, requiring it to be reinstalled.
• Data interpretation: ma

The main goal while using soil moisture sensors is to make sure the readings represent what the crop experiences. Training farm staff and focusing on the details described above are usually the key to successful use of sensors. For more specific details, check the following article: >>>

It is important to note that soil moistures sensors should not be used to determine when the irrigation should stop. While plant roots extract water uniformly from the soil, water added through irrigation follows a non-uniform pattern in the soil and between plant rows, especially with drip tape and micro-sprinklers. The best way to determined when to stop the water, or in other words how long to irrigate, is through the use of the ET-based method.

Volumetric water content sensors: summary of how they work, unit, limitations, pros, experiences with these type of sensors,

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

• Hortau
• Irrometer
• Do it yourself