Soil Moisture Meters, The Pros and Cons, and the Nuts and Bolts behind how they work.

This is a post that I've wanted to do for some time but haven't really had the time to do it.  But thanks to a fellow colleagues twitter post about considering purchasing a meter I found the motivation to write this.

First off I'll address the basic pros and cons of using the meters themselves, and then I'll bore you all to tears over the science and physics behind the process the meter uses to give you the data (soil moisture percentage).

The TDR 300 display and controls.

Here at my course we use the Spectrum Technologies TDR 300 FieldScout.  This meter comes with a carry case, several sets of rods (probes) of varying lengths, a data cable (to upload or clear the data) and some software for your computer.  The meter itself consists of a handle, the sensor probes, and the display.  When viewed from a users position there are 4 buttons below the display.  

On/off, Mode, Delete/Clr Avg, and Read.  The on/off is self explanatory.  The mode button controls what data is given to you by each reading.  The only one you should be really concerned with is the VWC mode, VWC meaning Volumetric Water Content. NOTE:  If you somehow get your modes switched around it will change the data you receive and you will either under or over water based upon which mode you end up in.  It is ALWAYS very wise to Check Your Mode.  This model will log 3,250 measurements without GPS (1,350 with GPS) and carry a continuous running average of the total number of readings.  Which can be uploaded to software that will keep track of the data and trends, as well as you can get mapping software to relate each reading to if you so desire (if you have the optional GPS.  If you choose to leave every reading on the meter at some point you'll have to download or press delete (a ton of times) to clear every entry.  Lastly there is the read button, which as the name implies, takes a reading when pressed and released.  Now for the rods, they are field replaceable and are made from stainless steel for durability.  They come in lengths of 1.5", 3.0", 4.8", or 8.0".  However the meter uses the metric equivalent lengths for measurement and not the standard units which may throw you a little when is asks for the length in cm.  Which ever set of rods you order you need to set the rod length in the meter or you'll get the incorrect readings.  I'm pretty sure this has something to do with the meter factoring in a certain amount of resistance for a certain length.  The rod length you get is most easily based on how deep you wish to measure the soil moisture content (maximum root zone depth).  For greens measuring purposes I'd say you'd need either the 1.5", 3.0" or maybe 4.8" rods.  Taking into account how your root zone and therefore moisture levels may change throughout the year.  

If you don't have the same model as mine these tips, tricks, things to pay attention to, and do's and don'ts can still come in handy for you:

• Rods should come to you straight (unbent) periodically depending on firmness or treatment of the meter you should observe to make sure that they are fairly straight.  Distance between rods affects to some extent the time it takes the electrical pulse to travel, the meter is programed for a constant distance if this changes it can skew your data but hardly any at all unless the rods are severely bent. (Differing resistance)
• Rods should be inserted until they reach the larger shoulders at their top ends.  If you don't fully insert the rods you'll be measuring the moisture content of the air, and the soil causing a discrepancy (albeit small)  The air being present where there should be soil changes the time required for the pulse to be measured.  (Differing resistance)
• Try to avoid inserting the rods at weird angles or allowing the rod end of the meter to be hit or heavy objects to rest on it.  Use the case when not in use and either hang by the case handles or place the case so that the rods face upwards.  Never leave resting on the rods (they cost approx $58 to replace a pair)
• Don't over-tighten the rods, doing so well strip the sockets and you more than likely just trashed a $1k meter
• Keep meter dry when in use, transporting, and storing.  This will keep the batteries and sensitive electronics from being damaged or corroding and ruining the meter.
• Make sure a responsible person is tasked with using the meter, this cuts down on some worrying and should help keep it functional for quite a while longer.  

Another tip, as you determine how far you can dry down (lower VWC %) don't fixate on a certain low reading on one green being acceptable for all greens.  Remember that a golf course as a whole is a micro climate in its surrounding area, as well as the fact that every hole has its own micro climate you could extrapolate that right on down to each green being a micro climate.  Or as one might say, whats good for the goose may not be good for the gander.  Likewise, determine a "safe" (i.e. cautious) low VWC % for each green and record it.   Assuming that your greens are consistent in mix and throughout their profiles is not the correct approach to take either.  There are differences between areas on the same green, let alone one the opposite end of the course.

Another thing to note is that the "magic number" for each green will fluctuate daily based on weather.  Everything from the ET rate, cloudiness/clearness, temperature, wind speed, dew point and humidity all  need to be considered.  Especially when getting "ballsy" trying to get things to max firmness and quickness for a tournament be it a pro event, member event, or a full course sponsored event.  Also remember that nature can be fickle, err to the side of caution just in case things get too hot, dry, or windy so you don't lose grass and end up being the bad guy for having some extra crispy greens.

As for the Pros and Cons:

Pros:

• Ability to create coveted "firm and fast" conditions
• Ability to stretch water window by watering only areas that need it as they dry down
• Ability to conserve some water by using handwatering methods, and not using overhead irrigation from large rotors (inaccuracy of water placement)
• Deep and infrequent water regime can be established to avoid salinity issues with poor quality water, allowing for good quality flushes of the root zone.
• Just to name a few

Cons:

• Initial cost, training yourself, establishing a "magic number" for each green through experimentation.
• Training an individual to use the meter and interpret the data correctly and effectively
• Probably some toasted grass here and there
• Fast and Furious syndrome "Fast firm greens, furious golfers"
• Extra time spent setting up greens from handwatering

I think that I pretty much covered most of the key concepts that needed to be touched on for those considering or who have just purchased a moisture meter. 

More information for the Spectrum Meters FieldScout TDR300 can be found at: http://www.specmeters.com/soil-and-water/soil-moisture/fieldscout-tdr-meters/tdr300/

Now the science and physics lesson.  I'll give you the brief lay-persons explanation, and if you want to know more real science behind the meter there will be links to scientific papers explaining in detail what's going on every time you press that read button.

TDR isn't just a model designation, it is an acronym for Time Domain Reflectometer.  Time Domain Reflectometry is a process that in laypersons terms, measures the time it takes an electrical signal to be sent and received based upon the resistance of the media the signal is being passed through.  The precise definition is:

Time Domain Reflectometry: The soil bulk dielectric constant (Ka_b) is determined by measuring the time it takes for an electromagnetic pulse (wave) to propagate along a transmission line (TL) that is surrounded by the soil. Since the propagation velocity (v) is a function of Ka_b, the latter is therefore proportional to the square of the transit time (t, in seconds) down and back along the TL:

Ka_b = (c/v)² = ((c.t)/(2.L))²  (2)

where c is the velocity of electromagnetic waves in a vacuum (3•10⁸ m/s or 186,282 mile/s) and L is the length of the TL embedded in the soil (in m or ft)

Description: A TDR instrument (Figure 4) requires a device capable of producing a series of precisely timed electrical pulses with a wide range of high frequencies used by different devices (e.g., 0.02-3 GHz), which travel along a TL that is built with a coaxial cable and a probe. This high frequency provides a response less dependent on soil specific properties like texture, salinity or temperature. The TDR probe usually consists of 2-3 parallel metal rods that are inserted into the soil acting as waveguides in a similar way as an antenna used for television reception. At the same time, the TDR instrument uses a device for measuring and digitizing the energy (voltage) level of the TL at intervals down to around 100 picoseconds. When the electromagnetic pulse traveling along the TL finds a discontinuity (i.e., probe-waveguides surrounded by soil) part of the pulse is reflected. This produces a change in the energy level of the TL. Thereby the travel time (t) is determined by analyzing the digitized energy levels.  

Soil salinity or highly conductive heavy clay contents may affect TDR, since it contributes to attenuation of the reflected pulses. In other words, TDR is relatively insensitive to salinity as long as a useful pulse is reflected (i.e., as long as it can be analyzed). In soils with highly saline conditions, using epoxy-coated probe rods should solve the problem. However, this implies loss of sensitivity and change in calibration. It is interesting to notice that in addition to time of travel another characteristic of the pulse traveling through the soil (i.e., change in size or attenuation of the pulse) can be related to the soil electrical conductivity. Based on this some commercial devices incorporate the possibility of measuring water content and soil salinity simultaneously.

More information can be found at this link http://www.allianceforwaterefficiency.org/soil_moisture_sensor-intro.aspx 

The above information was taken from the same website and is the work of Rafael Munoz-Carpena so all credit for said work is his.

I hope that this post has proven insightful, helpful, and informative.  If you have any questions feel free to post on here, or shoot me a quick tweet on twitter @JoshfromCCTX.  Special thanks to @JohnKaminski for some thoughts and input on not getting focused on a number when you use these excellent tools for turfgrass maintenance.