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Portable moisture meters transitioned from bulky research instruments to essential golf course maintenance tools only about 20 years ago, but since then they have become one of the superintendent’s most valuable assets. Instead of relying on the old pocketknife test or visual cues to gauge soil moisture levels, turf managers now have a reliable and objective way to assess volumetric water content (VWC). Despite the widespread adoption of the moisture meter, the USGA Green Section still fields plenty of questions about how they work, how to use them properly and what factors affect the accuracy of the readings. We also recently developed and released the USGA Moisture Meter, which means that our team learned a lot about moisture meter technology and sampling strategies. Below, we answer common questions about this indispensable tool and provide some best practices for how to use them.

How Is Percent VWC Calculated?

Truly measuring VWC in a laboratory setting is a lengthy, destructive process that involves removing soil samples and weighing, drying and reweighing them. Moisture meters offer a much faster, less-disruptive alternative; however, they do not directly measure soil moisture. Moisture meter tines create an electromagnetic field that allows the tool to indirectly measure the amount of water present by using various equations to convert the behavior of electrical signals into the percent-VWC reading that we are all familiar with. Moisture meters determine VWC by leveraging the electrical properties of water, which has a much greater ability to store and conduct electrical energy than other parts of the soil like organic matter, air and minerals.

Moisture meters estimate VWC using various techniques, and to better understand how they work, we need to briefly delve into the science behind these techniques. Superintendents are most likely to encounter meters that use resistance or dielectric permittivity, though several other techniques like neutron probes exist and are used primarily in research. Between resistance and dielectric permittivity sensors, the latter are most common on golf courses, are far more reliable, and include those based on time-domain reflectometry (TDR), capacitance, frequency-domain reflectometry (FDR), and complex dielectric with intersection (CDX) techniques. These methods measure the dielectric permittivity of soil, an electromagnetic property related to how well something stores a charge, which correlates closely with the amount of water present and yields an estimate of VWC after complex processing. TDR sensors essentially measure the travel time of the reflection of a signal produced by sensor tines as it moves through the soil. The more water that’s present within that soil, the greater the attenuation of the signal, slowing its travel time. Capacitance sensors emit a signal and measure how the soil’s ability to store a charge alters the frequency of the signal. FDR sensors are capacitance sensors that emit signals of multiple frequencies and determine the frequency where the charge flows most efficiently. CDX sensors are similar to FDR sensors but use wider- and higher-frequency signals in an effort to minimize the effects of soil texture and electrical conductivity (salinity) on measurements. The next time you hear terms like “CDX” and “TDR” associated with portable moisture meters you will know why.

VWC readings are influenced by soil texture, salinity, organic matter content and other site-specific conditions. The extent to which these and other factors influence VWC readings is largely a function of the signal frequency, processing and calibration equations each meter uses. With all the variables involved, superintendents shouldn’t get caught up in comparing VWC readings with other courses.

"With all the variables involved, superintendents shouldn’t get caught up in comparing VWC readings with other courses."

How, When and Where Should I Measure VWC?

Getting the most out of moisture meter data is about understanding relative differences and trends in VWC readings and how different VWC percentages correspond to turf health and playability characteristics. By taking and recording many measurements over time and observing how greens perform at various VWC readings, it becomes possible to develop accurate thresholds for irrigation. These thresholds will likely change throughout the year and there may be slightly different watering thresholds for different greens on your course. Watering thresholds will also be different from course to course because of differences in rootzone material, irrigation setup, growing environments, management programs, grass types, the moisture meter being used and countless other factors.

VWC data is only as good as the methods used to gather it. Starting with a properly calibrated moisture meter, data should generally be collected from as many places on as many greens as is needed to make good watering decisions. Research has shown that taking three to four readings per 1,000 square feet of putting surface is the minimum number needed to get a representative value, but the optimal approach to taking these readings varies from course to course (Magro et al., 2022). Some courses are successful with sampling just a handful of key areas, while others with highly variable growing environments or extremely precise irrigation goals may take 50 measurements or more on every green. How to approach sampling – i.e., number of readings per green, number of greens tested, number of times per day – will depend on your agronomic and playability goals. Staff typically develop a feel for the greens after a while and will know which spots on which greens are first to become dry. Once these areas are near the threshold to initiate watering, it’s typically time to start checking more locations on more greens.

The approach to sampling putting green moisture varies widely. Some courses take readings in a grid pattern across the surface, while others focus their attention on areas of concern like high and low spots or hole location areas. Readings from both high and low spots can help identify areas that are not only trending too dry but also too wet, which is important because saturated conditions can be just as problematic for turf health and playability as excessively dry conditions. If an area tends to stay wet, it may be a sign to adjust sprinkler run times or it may indicate the need for targeted cultural practices. Developing a comprehensive understanding of wet, dry and “normal” areas on each green is invaluable for making adjustments that improve turf health and playability. Most courses set thresholds for the upper and lower limits of VWC that align with their overall objectives, then they manage water within those bounds.

It’s worth briefly mentioning here that portable moisture meters can also be a valuable tool for collecting VWC readings on tees, fairways and roughs to identify potential adjustments that can improve soil moisture uniformity. Increasing soil moisture uniformity can pay dividends in both water savings and improved turf health and playing conditions. The USGA Water Conservation Playbook has more information on this and many other water-related topics, making it an excellent resource for any course looking to use water more efficiently.

An important consideration is whether you are collecting moisture data to guide hand watering or overnight irrigation decisions. When hand watering, you’re looking for dry spots, whereas you’ll want a broader sample when gathering information for irrigation scheduling. Sampling in a grid pattern can help add consistency to measurements between different staff members. It’s also helpful to collect the readings at the same time of day so you’re comparing apples to apples against factors like overnight irrigation and daytime evapotranspiration. For example, many superintendents have a target VWC range during morning maintenance. This range would be understood to typically provide desired playing conditions and minimize the risk of relatively dry areas wilting later in the day.

Tine length should be selected based on the average root length of your greens and the areas of the soil profile that are the highest priority at the time. Signals from soil moisture sensors typically extend from the tines to about 1 inch horizontally and 0.5 inch vertically, creating a cylindrical sample slightly larger than what you might expect. It’s common for superintendents with annual bluegrass putting greens to use 1.5-inch tines and superintendents with healthy bentgrass or bermudagrass greens to use longer 3- or 5-inch tines. If you really want to go deep, 8-inch tines are available for some models, but they require special handling, are prone to damage, and can even require pilot holes.

Tracking VWC readings with pen and paper is helpful in the short term for individuals hand watering but this approach makes it time consuming and difficult to better understand things like variability between and within greens, problem spots, or the connection between soil moisture and putting green firmness. Different software options allow data from portable moisture meters to be displayed using maps or charts, which can streamline communication and analysis.

What Factors Affect Accuracy?

Before answering this question, it’s worth restating that soil moisture meters use an indirect radiofrequency measurement to calculate an estimate of VWC – they are not directly measuring the amount of water in the soil. That said, research has shown that moisture meters offer accurate and repeatable estimates that are closely correlated to values measured via gravimetric VWC – the disruptive method mentioned at the start (Evett et al., 2002; Tanriverdi et al., 2016; Topp & Davis, 1985; Quinones et al., 2003).

It is important to follow the manufacturer’s recommendations for calibration to get the best readings possible. Moisture meters typically come factory calibrated but may require occasional recalibration using known standards such as air or distilled water, but may sometimes require sending them back to the manufacturer. Measurement technique also matters. Avoid the temptation to “stick-and-move,” where the tines are inserted and the reading taken all while in motion. This approach might speed things up a little, but it can also change the depth of the measurements or introduce air gaps around the tines (known as “dry bias”), which can lead to inconsistent or inaccurate readings. There is also greater risk of lifting the turf around the tines and leaving noticeable marks on the greens.

Soil salinity – measured via electrical conductivity (EC) and given in units of deciSiemens per meter (dS m-1) – can also affect moisture meter performance. USGA-funded research has shown that EC values greater than 5 dS m-1 can affect the accuracy of VWC readings (Leinauer et al., 2016). Certain moisture meters are less sensitive to salinity interference through a combination of signal frequency and the way those signals are interpreted by the meter’s calibration equations. Generally, lower frequency sensors are more susceptible to salinity effects. Special calibration is needed, especially for certain moisture meters, when soil has EC values greater than 5 dS m-1 if the absolute soil moisture value, rather than the relative difference in soil moisture, is most important to you.

Tine wear is an often-overlooked aspect of VWC readings. Most stainless steel tines should last for thousands of readings on sand-based rootzones, but they will eventually begin to wear. As the tines get shorter or thinner, the readings can change relative to a new set of tines. This can be problematic if there is more than one moisture meter in use on a course and the tines on each unit are in very different condition. Staff could be using the same numerical threshold for watering while using moisture meters that produce different readings for the same actual VWC. Like most tools subject to constant use, moisture meters require some maintenance to continue working properly – so keep the tines in mind.

What About Salinity and Temperature Measurements?

Most moisture meters also provide an estimate of soil salinity. This information is especially valuable for superintendents irrigating with high-salinity or recycled water sources. Using an app to record and track salinity data streamlines the decision process about when to flush salts from the rootzone. Benchmarking the EC value where you start to see turf issues, as well as measuring salinity directly after a leaching event, can help dial in the timing and amount of irrigation required for successful leaching.

Most portable moisture meters also have some form of surface or soil temperature sensor. Tracking soil temperature can be useful for timing certain fungicide or herbicide applications, as well as for predicting growth. Ensure the depth of temperature readings can practically inform management decisions. When collecting temperature measurements, it is important to allow the meter to equilibrate in the soil for the first few assessments.

What’s the Take-Home Message?

VWC is one of the easiest and most influential data points you can collect from a putting green. However, the data is only as good as the tools and methods used to gather it. Understanding how your moisture meters work, how to take care of them, and how to take accurate readings are all critically important for getting useful information. Once you have a good data collection process in place, you can improve water management and connect VWC with other data points like clipping volume, firmness, and organic matter content to optimize your overall putting green management program.

References

Evett, S.R., Ruthardt, B.B., Kottkamp, S.T., Howell, T.A., Schneider, A.D., & Tolk, J.A. (2002). Accuracy and precision of soil water measurements by neutron, capacitance, and TDR methods. In Proceedings of the 17th Water Conservation Soil Society Symposium, Thailand.

Leinauer, B., Serena, M., VanLeeuwen D., & Sevostianova, E. (2016). Accuracy of soil moisture meters in saline soils. USGA Green Section Record, 54(19), 1.

Magro, C., Macolino, S., Pornaro, C., McMillan, M., & Fidanza, M. (2022). Considerations with determining the minimum number of volumetric water content measurements for turfgrass root zones. Agronomy, 12(6), 1402.

Tanriverdi, C., Degirmenci, H., Gonen, E., & Boyaci, S. (2016). A comparison of the gravimetric and TDR methods in terms of determining the soil water content of the corn plant. Scientific Papers. Series A. Agronomy, Volume 59.

Topp, G.C., & Davis, J.L. (1985). Measurement of soil water content using time‐domain reflectrometry (TDR): a field evaluation. Soil Science Society of America Journal, 49(1), 19-24.

Quinones, H., Ruelle, P., & Nemeth, I. (2003). Comparison of three calibration procedures for TDR soil moisture sensors. Irrigation and Drainage: The journal of the International Commission on Irrigation and Drainage, 52(3), 203-217.