Cameron Zuber0 commentsMarch 14, 2021Irrigation

Field Note: Irrigating and Deep Soil Moisture After Rainy Season

The rainy season this year in the San Joaquin Valley has been fairly dry and has probably got many growers thinking about how much water they should start applying. In orchards, one consideration to add to the list is the soil moisture at deeper depths. As an example, let us look at information from a wetter rainy season in 2018. In the beginning of that year, soil moisture measurements were collected every month at a research site in Parlier, CA. While not extensive, it does show something interesting. Overall, the soil moisture increased at all depth month to month (i.e., green line to orange then to grey). By March, the soil moisture above 1½ feet reached the maximum wetness the soil could hold, called the field capacity (e.g., red box in image). However, no matter how much it rained, the soil moisture at and below 2½ feet consistently stayed below the field capacity. The trees at this location were planted in 2009 with a rootzone well past a five-foot depth and were beginning to leaf-out by the middle of March. While trees typically transpire much of its water needs from the upper portion of the rootzone, a decent amount is still absorbed from the lower portion. At this site, the soil moisture at and below the 2½ feet may not be enough to supply the water needs of the trees even with appropriate moisture in the topsoil. When scheduling your first few irrigations realize that you may have different soil moisture at different depths. A few minutes with an auger and using the “feel” test or looking at sensor data can give an idea of the soil moisture profile across all depths. Think about the size of your tree’s rootzone, the soil types in your field, and how wet the

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David Doll1 CommentJuly 10, 2015Irrigation, Soil

Soil Moisture Sensors

Soil moisture sensors are great tools to aid in irrigation management. They provide feedback on the movement and depth of moisture within the soil, providing the ability to identify the proper duration of irrigation. Proper use relies on a thorough understanding of the soil characteristics of the orchard, which include soil type, water holding capacity, and salinity level. Sensors can be used to help schedule irrigation. Timing of irrigation usually occurs when moisture levels drop below certain trigger points at varying depths. These points are different for every soil and sensor type and require in-field calibration to help reduce unwanted plant stress. Calibration can occur by comparing sensors readings to plant stress responses (e.g. Pressure chamber readings) or to a “feel” test to determine how much water is still available to the plant. Several factors need to be considered when planning to install the sensors. Sensor locations should be placed to account for varying soil types of the orchard. If only a few locations are planned, the predominant soil types should be selected. If possible, sensors should be installed at varying depths to provide moisture levels in the middle, bottom edge, and below the active rootzone. A common 3 sensor installation pattern is 12-18″, 30-42″, and 48-60.” Work by the University of California has compared many soil moisture monitoring systems. Neutron probe data, dielectric, tensiometers, and electrical resistance blocks have all been found to respond to water applications similarly. Essentially, if sensors are properly installed and maintained, and time is taken to understand and interpret the data, they can provide similar information. The table below highlights some of the varying aspects of these systems. Each system has strengths and weaknesses. Please note that the sensors types are linked and when clicked will direct to further information. “Feel” Tensiometers Dielectric Sensors

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