Thanks for reading this three-part series about using soil moisture sensors to inform irrigation management in vineyards. Part I focused on selecting locations to install soil moisture sensors. Part II took a look at depth of sensor placement and proximity to drip emitters and vines. Here, in Part III, we will cover common types of soil moisture sensors and tips for installation.
Common Types of Soil Moisture Sensors and Tips for Installation
There are many types of soil moisture sensors marketed for use in vineyards. The purpose here is not to determine which sensor type is best, but to provide an overview of common sensor types and how they work. Three common methods used to measure soil moisture in vineyards include matric potential, time domain reflectometry, time domain transmissometry, and capacitance measurements. Regardless of the soil moisture sensor used, the depth of placement and the proximity to vines and drip emitters must be determined prior to installation. How you install the sensors is likely more important than the type of sensor chosen. An online tutorial is available below to provide tips on installation with respect to soil depths and proximity to drip emitters.
View a tutorial on selecting sensor installation depots and proximity to emitters HERE.
Meet the author, Fritz Westover
Fritz is the technical program manager with the Vineyard Team. He has been working hands-on in the viticulture industry since 1999 and joined the Vineyard Team in 2013. Fritz obtained his BS in Horticulture from Penn State University and completed his MS in Plant Pathology at Penn State in 2003. He worked internationally as an assistant winemaker in the Tuscany region of Italy in 2004 and then held the position of Viticulture Research – Extension Associate for Virginia Tech Cooperative Extension from 2005 – 2007. From 2007 – 2013 Fritz served as Viticulture Extension Program Specialist for the Texas A&M AgriLife Extension Service where he worked statewide, developing viticulture education and research programs.
Matric Potential Sensors
Matric potential is a measure of how tightly water is held to the soil, which corresponds to water available to roots. Matric potential is also referred to as soil water potential, water tension, or soil water suction. Some devices, such as tensiometers, read soil water suction directly. Others, such as electrical resistance devices, indicate suction indirectly. These qualitative measures of soil moisture are useful to indicate when to apply water, but are limited in their ability to indicate how much water to apply. Using these sensors, it is possible over a season or two to determine soil tension (centibar) levels at field capacity, wilting point, and maximum depletion point. Matric potential sensors are commonly installed into the soil at multiple depths.
View a tutorial on trips for installing matric potential sensors HERE.
Capacitance sensors measure volumetric or relative water content using dielectric permittivity of soil. Excitation is placed in soil by the sensor and the frequency of that wave is affected by dielectric permittivity, which is affected by the water content. The greater the soil water content, the smaller the frequency. Volumetric water content can be used to help decide when to irrigate and how much water to apply. This type of sensor measures a relatively small volume of soil, so installation is particularly important. Capacitance sensors can be purchased as individual sensors or built into a probe or profiler to measure soil moisture at multiple depths.
View a tutorial on tips for installing individual capacitance sensors HERE.
View a tutorial on tips for installing capacitance probe sensors HERE.
This Coffee Shop article originally published by the the Vineyard Team, a nonprofit organization dedicated to promoting sustainable winegrowing since 1994. More information about the Vineyard Team can be accessed from their website: www.vineyardteam.org. The original document can be accessed by clicking HERE. 2014 All Rights Reserved.
Time Domain Reflectometry (TDR) and Transmissometry (TDT) Sensors
Similar to capacitance sensors, Time Domain Reflectometry (TDR) and Transmissometry (TDT) sensors measure volumetric water content (if calibrated) or relative water content (if not calibrated) using high-frequency electromagnetic waves. The waves are passed along two or three parallel probes, and the time required for the wave to pass through the soil or to be reflected back generates the di-electric constant, which relates to volumetric water content. Individual TDR/TDT sensors are larger than other types but measure water content of a larger volume of soil. Volumetric water content can be used to help decide when to irrigate and how much water to apply. These sensors are commonly installed into the soil profile at multiple depths.
View a tutorial on tips for installing individual TDT sensors HERE.
Pros and Cons of Soil Moisture Sensor Types and How to Choose
For all types of sensors, the installation process is absolutely critical in the ability to obtain accurate and usable soil moisture data. Poorly installed sensors may give erroneous data, leading to mistakes with irrigation amounts and frequency. Using a moisture sensor attached to a data logger (or telemetry to computer/tablet) is the most useful in limited water situations. Continuous measurements can identify soil moisture behavior that might not be evident with point measurements. Instead of watering to a set schedule, you water to need.