Soil moisture instrumentation:
Sensors & strategies for the 21st century

by Richard Mead

Irrigation technology has advanced tremendously in the last 50 years. Irrigation systems are more efficient than ever before. The concept of using soil moisture sensors to determine irrigation needs has also moved forward with literally dozens of widgets to plug into the soil. As with any consumer walking into a local supermarket, irrigation professionals today face an overwhelming number of choices when it comes to the variety of sensors used for making better irrigation decisions.

Historical benchmarks

For the last three decades there have been three basic sensors used for irrigation scheduling: gypsum blocks, tensiometers, and neutron probes. All three types of sensors continue to be extensively used today.

Gypsum blocks work on the simple principle of electrical resistance. The wetter the gypsum block (ergo the soil), the lower the electrical resistance. As the soil dries, resistance increases. Using a common AC ohmmeter, many sensors can be read manually by moving through the field, or they can be multiplexed and read on a continual basis. The numerical units traditionally associated with ohmmeter readings are either centibars (cb) or kilopascals (kPa) soil water tension.

Gypsum blocks are essentially the least expensive method of soil moisture measurement. However, they respond slowly to changes in soil water status, potentially suffer from hysteresis and have a rather narrow range of soil tension measurement. They are also known to breakdown due to soil chemistry such that fertilizers could dissolve around the block and accelerate deterioration.

Tensiometers have been around about as long as gypsum blocks. Their advantage is simplicity. The ceramic cup inserted into the soil establishes an equilibrium with the soil water tension. Once you obtain a soil tension reading (again in centibars or kilopascals), there is no need to calibrate. A tensiometer reads what the plant root senses in terms of how much energy is needed to absorb water. Tensiometers are not affected by dissolved salts from fertilizer.

Tensiometers do have some limitations. They do not read very well in drier soils (eg, past -70 centibars) and when experiencing these dry readings there is potential to break tension, thus increasing a maintenance component with owning them. Irrometer has several types of tensiometers (based on soil type).

The third and final benchmark device over the years is the neutron probe. Dozens of irrigation consulting companies throughout the country still use this device as their standard irrigation tool. Neutron probes need to be calibrated for each type of soil. Good water balance studies and irrigation scheduling strategies can be performed using the neutron attenuation technique.

Lugging around a neutron probe however is labor intensive. The varying sphere of influence emanating from the neutron source varies with soil water content. This makes near soil surface readings (less than 1 foot) difficult to accurately obtain. Continuous measurements are not practical and, the very nature that it is radioactive has some individuals hesitant to use it. It can be rather difficult and expensive to dispose of. The paper work involved with owning a neutron probe can be bureaucratic. Nevertheless, it is a solid piece of equipment requiring very little maintenance over the years. It could virtually last a lifetime.

Microchips + Dielectric constant = An explosion of new sensors

In the last decade, two things have happened in the soil moisture sensor industry. The first has been the price and power of computer chips. In a September 1997 Wired magazine article, it was stated that "...as the size of silicon chips shrinks to the microscopic, their cost shrink to the microscopic as well. The number of chips in objects other than computers is rising faster than computers." The other phenomenon has been the accelerated science and technology of measuring soil capacitance or its dielectric constant as an indication of soil moisture. Blending computer chip technology with the soil dielectric concept has permitted numerous companies the ability to design and manufacture inexpensive and fairly accurate soil moisture instrumentation.

There are two basic methods that directly or indirectly measure the soil's dielectric constant. These two methods are time domain reflectometry (TDR) and capacitance. TDR measures the propagating velocity of an electro-magnetic pulse traveling alone the sensor. Capacitance measures the resonance frequency of a circuit where the probe itself is used as a capacitor in the circuit. There are some companies creating dielectric hybrids of the two measuring techniques with Wave Velocity Technology being an example. To keep things simple for this discussion however, I will refer to all instrumentation using the dielectric constant concept as "dielectric."

Some "TDR" dielectric devices include Dynamax's Vadose, ESI's Gro Point, Streat Instrument's Aquaflex SE 200m and Campbell Scientific's CS615-L.

The Vadose system is distributed by Dynamax and can be multiplexed with up to 240 probes. It has been used successfully in fully automated irrigation systems. It consists of a three-prong device that can be inserted either horizontally or vertically in the soil.

ESI's Gro Point was just recently released. It also involves a three-prong device that can be multiplexed or read by a handheld meter. Campbell Scientific's CS615-L also includes three prongs and several of these devices can be hooked up to a Campbell Scientific weather station.

The Aquaflex SE 200 simultaneously measures soil conductivity, soil temperature in addition to soil moisture content. The rational here is to improve accuracy of readings.

The unique feature of the Aquaflex SE 200 is that it is normally installed horizontally. This is done to integrate the average soil moisture around the 10-foot long sensor cable. Some "capacitance" dielectric devices include Delta-T's Theta ML 1 probe, Netafim's Flori probe, the Gopher system, Troxler's Sentry 200 AP and the current Cadillac of systems, the Sentek Enviroscan. Delta-T's Theta ML 1 probe is distributed by Dynamax in the United States. It is a cylindrical device with 4 prongs approximately 2 inches long. Several University of California researchers are now using this device to measure soil moisture in profile pits in lieu of gravimetric soil sampling. The Theta ML 1 can be portable or installed permanently in the soil.

Netafim's Flori sensor is a stand-alone device used primarily in the ornamental and landscape markets. It either comes with a simple meter to take quantitative readings or can be used as an off/on switch for irrigation systems.

The Gopher system is manufactured in Australia. It is a portable soil moisture instrument inserted into 2-inch PVC vertical access tubes. It is approximately 44 inches long, and has a hand held meter associated with the probe.

Troxler's Sentry 200 AP was one of the first capacitance dielectric sensors in the US. It also is a portable device that uses 2-inch PVC vertical access tubes. Software also comes with the device.

Sentek's Enviroscan virtually dominates the Australian, New Zealand, and South African market. It is regarded in the irrigation scientific community as one of the most accurate capacitance sensors. The Enviroscan involves a permanent installation setup, such that sensors are housed in vertical PVC access tubes and readings are taken at intervals ranging from 1 minute to 1 week (normal is 1 hour). One Enviroscan datalogger can accompany 8 probes, each with 4 sensors. The individual sensors plug and play along the probe length and can be adjusted in 4-inch intervals. Probes are normally 1 meter (40 inches) long.

The Enviroscan comes with software that interpolates the readings from the datalogger. The software displays the dynamics of soil moisture through time. It depicts various soil moisture conditions such as saturation, drainage, field capacity, or plant stress. The software allows the user to color code and delineate these dynamic ranges such that blue would be above field capacity, green would be good growing conditions and 'red' would be a zone where plant stress occurs.

Recording data and installation techniques to ponder

There are other aspects of soil moisture instrumentation other than their variety and function. For instance, would you want periodic measurements (ie, once a week) or continuous logging? The main advantage of a periodic measurement involves keeping track of fewer data points and consequently not archiving reams of data. Usually, periodic measurements are done by hand out in the field. When the readings are taken, one can usually assess what is observed in the field with soil moisture readings. A possible disadvantage of a periodic measurement involves the potential to record readings when it's too late. This is painfully obvious if weather based models (eg, ETo and crop coefficients) are not used in conjunction with the soil moisture measurements.

With continuous monitoring however, you can visualize through time what goes on in the soil profile, possibly envisioning the root zone like fuel tank in a vehicle. Wouldn't you feel more comfortable driving your car if you could observe the level of the fuel gauge at any time instead of only one instant per trip? For example, I know of one grower who had his crop burned severely during a recent heat spell (greater than 100 degrees). He normally sends a technician to take periodic neutron probe readings once a week. The grower admitted that had he been watching soil moisture every day, especially in conjunction with weather models (ETo and crop coefficients), he would have foreseen the hot weather coming and adjusted accordingly by irrigating more frequently or applying more water per irrigation set. The continuous monitoring technique becomes particularly useful in sandier soils since the depletion of the soil reservoir occurs more rapidly.

Obviously sensor placement is critical and it is only logical to place the sensor where most of the roots are. Some growers like to place some of their sensors below the root zone to observe over irrigation or leaching. One should either place sensors in regions of the field thought to represent the average root zone or in areas that represent the drier, sandier spots. When these zones indicate low soil moisture readings, an irrigation is required. The danger with any point source measurement is the assumption that the point represents God's truth for the rest of the field. These assumptions sometime lead to drastic errors. Be prepared to move sensors around in order to fine-tune your field's representative soil moisture.

One final thing to ponder is the degree of soil disturbance upon sensor installation. The techniques for installing Sentek's Enviroscan and Troxler's Sentry 200 AP go to great lengths to insure the installation process does not disturb the soil.

On the other hand, many of the other sensor companies wager that after installation, soil will settle back to its natural bulk density. This all depends on how meticulous the placement is along with how wet the soil is during installation. The bottom-line of any soil moisture sensor installation process; make sure the sensor senses what the plant root zone is experiencing.

The Future

One unique way to integrate all of your field's soil moisture is to some how see it from afar. Satellite technology now offer 4-meter resolution images in infrared and multispecteral wavelengths. Airplanes (often known as "fixed wing'" offer higher resolution data (1 to 2 meter). However, none of the particular wavelengths mentioned see soil moisture per se, only the affect of soil moisture on vegetation.

EarthData Technologies out of Hagerstown, MD, is the one to watch. They offer a remote sensing system used with aerial photography that entails using a passive microwave radiometer. This sensor measures the dielectric permitivity of the earth, which radiates in the microwave range. According to Steve DeLoach, president of EarthData, "the depth of radiation from the soil is a function of the specific wavelength used, and we generally are able to measure moisture changes from between 1 and 3 meters."

Even soil moisture differences detected to 0.5 meters would tell us a lot about most crop-water relations. This emerging sensor technology could help determine where to place the next soil moisture widget in your field. In a not too distant future (a la The Jetsons), this technology could be combined with a satellite service that could automatically turn on your irrigation pump when your field's soil has hit a certain moisture threshold.

Richard Mead is currently the Business Analyst for United Agri Products - West, located in Fresno, California