By: Michael Cook and Justin Scheiner, Ph.D. – Texas A&M AgriLife Extension Service
As grape growers, we know how important water is for profitable production. Water is involved in pretty much every physiological activity inside a grapevine. We also realize that water is a precious resource and no matter how you slice it, it costs us money. It is not only expensive to apply, but the indirect costs of over or under watering can lead to negative impacts on our bottom line.
Most climate models predict future growing seasons will be warmer, which will increase water use by grapevines and vegetation in vineyards. This may be good for our friends in Canada, “eh”, but for growers and grapevines in more arid and hot areas on the West Coast as well as in the warm continental and humid climates east of the Rockies, it would spell trouble. These trends mean that it’s increasingly important for commercial producers to adopt data driven technology to monitor water in the soil and in the vine to help guide our decisions. Long gone are the days when the small producer could say there are no sensors in their price range.
Thanks to advances in technology and manufacturing, there are sensors of all shapes and prices available to today’s grape grower. Having accurate, intuitive data about water use can save a grower serious money. Soil moisture sensors can be very useful in determining when to start irrigating and when to stop. Those that dry farm often irrigate the first few years of vineyard establishment and soil sensors can even be useful in these more temporary situations. Here we’ll briefly discuss soil sensor types and how even the smallest grower can benefit by adopting this technology as part of their irrigation management plan. In a future article, we’ll discuss monitoring vine water status directly and weather-based modelling for irrigation scheduling.
Soil-Based Approaches
Getting the most out of soil sensors requires some basic knowledge of soil water dynamics. A thorough treatment of these principles is beyond the scope of this article, but we’ll highlight a couple of key concepts here. The foundation for using soil-based water availability measurements, which are ultimately an indirect method of monitoring vine water status, is that we want to maintain some level (e.g., 50-80%) of plant available water (PAW) to vines throughout the growing season. Plant available water is the amount of the water in the soil that vines can actually take up versus the total amount present. While it can be represented as a percentage, we can also express PAW as a volumetric measurement (e.g., inches per acre). Once the soil begins to dry out and the moisture content decreases below a threshold, typically 40-50% of PAW, vines experience drought stress. The specific minimum depletion level that a grower may allow, often called the management or maximum allowable depletion (MAD), may be fine-tuned where a water deficit is allowed to persist over key phenological stages. However, once the soil moisture level approaches 50% most growers will opt to turn the irrigation on.
Plant available water for a given site or vineyard block can be easily estimated based on soil type, texture, and depth (Table 1). These soil qualities can be investigated by consulting USDA Natural Resources Conservation Service soil maps, collecting soil samples for soil texture and chemical analysis and by simply digging soil pits with an excavator to observe soil horizons, potential rooting depth for new vineyards, and actual rooting depth for mature vineyards.
Although we still need to get out and scout our vineyard blocks for visual cues throughout the season, soil sensors provide actual data about what is happening in the plant’s root zone. These sensors can be categorized by their function, including those that measure soil tension (qualitative) and those that measure soil moisture content or volume (quantitative).
There are a number of things to consider when choosing a soil sensor. One is determining the number of sensors to utilize as many can only provide a single point of measurement. Even when using multi-depth sensors, vineyard sites or individual blocks with higher soil variability and with greater topographical changes will require more sensors to accurately determine irrigation needs. The tradeoff for this is higher data resolution leading to more informed decision making. The viticulturist should also consider installing soil sensors at multiple depths in the soil profile so that water infiltration through the soil can be monitored (Image 1). This is also useful for determining if deep soil moisture is available during periods when irrigation may not be routinely applied such as near bud break. With so many soil sensor options available, cost, convenience and accuracy should be weighed to identify the best option.
No matter which soil sensor you choose, pay particular attention to the placement, installation, and maintenance protocol as each soil sensor type has different requirements. No soil sensor will provide accurate data if it is installed and maintained improperly. Sensors must maintain constant contact with the soil and should be located under the trellis in an area that would normally be wetted by the irrigation system but not immediately underneath an emitter. Make sure to flag soil sensors in the field, secure any cabling, and map locations for future reference as they can easily become lost or damaged.
Qualitative-Based Soil Sensors
Qualitative-based soil sensors, also called water potential sensors, matric potential sensors, or water suction sensors, measure how tightly moisture is bound to soil particles. These measurements are described in tension units (often as centibars, or cbars). Data can reflect suction or negative pressure as with a tensiometer or electrical conductance/resistance as with electrical resistance blocks. Water potential readings are higher (less negative) in fully saturated soil and become more negative as soil begins to dry due to the increase in soil water tension. In other words, there is still water in the soil, but the plant can’t use it because the soil won’t give it up. Soil tension sensors are generally soil-specific and can be inaccurate if the correct model, calibration and installation instructions are not followed.
These sensors are useful to indicate when to irrigate, but it typically requires some time to understand roughly how much water to apply to bring soil moisture up to an appropriate level.
Quantitative-Based Soil Sensors
Quantitative-based soil sensors, also called volumetric water content (VWC) sensors are the most used sensors in commercial vineyards. Based on a calibration, they measure how much water in liquid form is present relative to the amount of soil (i.e. volumetric). Measurements are generally in units like percent volume of water to volume of soil or cubic inches of water per cubic inches of soil. For example, a sensor giving a reading of 25% volumetric water content is the same as 0.25 cubic inches of water per cubic inch of soil. When compared to the water content at field capacity, VWC deficits can be used to drive irrigation scheduling decisions. Although the information provided by VWC sensors can be highly valuable, it does not provide any insight into other pertinent information such as water adherence to the soil or ease of extractability by the vine.
Two classes of quantitative-based soil sensors include Neutron probes and dielectric sensors. Neutron probes are primarily used by researchers and irrigation specialists, requiring bulky and expensive equipment and licensing. Dielectric sensors include Time Domain Reflectometry (TDR) and Frequency Domain Reflectometry (FDR), often referred to as capacitance sensors (Image 2). Of all the soil sensors currently on the market, FDR sensors are the newest and fastest developing area of technology. They can be cost effective, accurate, reliable, low maintenance, and easily integrated into other sensor-based modules like weather stations (Table 2). As with all sensor types, consult the manufacturers recommended installation and maintenance protocol for proper placement, density and depth per block.
Consider incorporating soil sensors in your water management plan to improve your confidence in irrigation decisions.
Table 1. Plant available water based on soil texture. (Adapted from Goldammer, T. 2018)
Table 2. Major soil sensor types suitable for vineyard use.
1based off 2025 MSRP for the sensor itself. $ low cost, $$ moderate cost, $$$ high cost.
2maintenance requirement rated from 1-3, with 1 being the lowest maintenance to 3 being highest maintenance.
Image 1. A single soil sensor monitoring moisture placed in the topsoil and another installed in the sub-soil at the same location.
Image 2. A capacitance type sensor being installed in a vineyard.
