Many growers have real and critical needs to measure the elements. At the end of the day, the bottom line is usually profit, but increased production and quality, labor savings and survival itself all figure into how to best measure and manage wind, humidity, temperatures, solar radiation, rainfall and soil moisture.

Individual soil moisture measurement devices come in many shapes, sizes and packages. They are marketed as stand-alone solutions to moisture measurement. But moisture measurement is usually included as only one of many sensors in a typical weather station. Weather stations are typically multi-purpose instrument systems - able to measure and integrate many phenomena concurrently.

Moisture Sensors

The user of a dedicated moisture sensor must ask three important questions:

• How accurate and repeatable must be the gathered data?
  
• What "real" cost must I be willing to incur in obtaining and processing the data?
  
• How will the moisture data be used?
  

The accuracy and repeatability of basic moisture sensors depends on how well they are installed, used and maintained. A more personalized, labor-intensive involvement means a greater per measurement real cost. Basic lower-cost moisture sensors are often used by agricultural soil moisture consultants, civil engineers, small growers and low-margin ag producers. Basic devices can be grouped into two categories, Conductivity Devices and Tensiometers.

Conductivity Devices

For conductivity-based instruments (i.e., capacitive and resistive devices) the user can expect measurement accuracies of up to 75 to 80 percent without site calibration and accuracies of 90 to 95 percent with site calibration. Gypsum blocks and conductivity cells are typically used conductivity devices. They are subject to degradation from corrosion and dissolution (gypsum blocks dissolve with high moisture levels). This means high maintenance and annual replacement of the cells, probes or blocks.

The cost of maintenance and replacement includes the low-cost sensor and the labor to reinstall and recalibrate each site. These sensors are also not particularly accurate over time unless they are frequently inspected and maintained. They are "point" sensors, so the location of the sensor is critical to ensure that measurement is representative of the area.

Profiling with many point sensors also requires digging, boring and burying the sensors at various points in the vertical profile. This site disturbance may have an effect on the representative quality of the data as compared to the undisturbed region at the same site and settlement of the disturbed sensor location can change soil porosity over time and further complicate site calibration considerations.

Some conductivity sensors measure moisture content based on the behavior of electromagnetic waves as they pass through the soil. Because of the shape of this instrument (a loop device) soil moisture is measured in a circular, donut pattern and not at just one point. The volume of soil used as a sample is larger (about 20 liters) and the measurements are less likely influenced by variations in the soil or moisture.

This type of conductivity device works without delay and measures moisture content immediately. The probe can also be connected to a data logging device and a computer up to one kilometer away. This type of probe also does not require maintenance or calibration and measurement results are not affected by chemical composition of the soil or fertilizers. In order to install this type of moisture sensor, however, the soil must be disturbed.

Tensiometers

Tensiometers are also inexpensive moisture measurement tools. The accuracy of a well-maintained tensiometer is in the 90 to 95 percent range. They use a porous ceramic cup at the end of a long tube which is inserted in the soil. The length of the tube determines the measurement point in the soil column. Longer or shorter tubes, or the installation of the same length to varying depths in the soil column, help determine a moisture profile.

The tensiometer is also a point sensor because the ceramic cup at the tip of the tube is the contact point with the soil. The measurement is based on a soil tension effect that occurs when the tube is partially filled with water and sealed at the top with a tension measurement device. As the soil drys it absorbs water through the porous ceramic cup and creates a suction (partial vacuum) inside the tube that is proportionate to the change in soil water content.

As soil water increases, the ambient soil water pressure increases and forces water back into the ceramic cup, thereby reducing the suction or applying a positive pressure to the sensor. Tensiometers are not affected by salinity or other soil conductivity factors. They can be reliably accurate if properly maintained.

Tensiometers need ongoing maintenance because the ceramic cups become clogged with silt. In high clay or very fine silt soils tensiometers require regular inspection and maintenance. Even if a tensiometer is clogged, it will still provide a measurement; therefore, the user may not suspect that the device is clogged. A tensiometer must never lose the head of water in the tube and if it dries out, it must be cleaned and reprimed.

Tensiometers are usually read directly, so the data gathering process is labor-intensive and may be subject to data handling errors. Also, unless many devices exist in the region of interest, the data may not represent that specific area. Tensiometers respond slowly to changes in soil water which makes them unsuitable for monitoring soil water changes in real time. Depending on the application, this may or may not be an important factor.

If a high degree of accuracy is necessary, real-time measurements taken at frequent intervals, data recording and processing, and verifiable, repeatable information are required, other "high-end" moisture measurement instruments may be the best alternative. High-end products have a greater initial cost but the cost per unit of data over time can be less expensive than either conductivity devices or tensiometers.

Time Domain Reflectometry (TDR)

TDR moisture sensor instruments measure the dielectric constant of environmental soil versus the known dielectric standard of liquid water at 100 percent moisture content and dry sand, at 0.0 percent moisture content. Capabilities of these systems include an accuracy level of 97 percent or more in agricultural soils without the requirement of soil calibration.

Supplied as "point" spike probes, these devices can be used in soil depths of a few inches to about five feet with minimal soil disturbance. TDR probes also produce an integrated moisture value that represents a much larger sample region. They allow rapid, accurate measurements of soil water content under a variety of difficult or extreme conditions. Electronic microprocessors attached to the probes allow for digital readout, automatic data processing of multiple readings without soil calibration and the ability to process the signals from multiple probes using computers. Most microprocessor units capable of logging data are battery controlled and indicate both probe number and soil moisture readout.

Frequency Domain Reflectometry (FDR) and Neutron Probe

FDR is based on the change in frequency of signals due to differences in the capacitance of homogeneous or bulk materials having the same characteristics or consistencies (like wheat, sand, corn, etc.). After the material is calibrated to a standard, the probe readings will be very accurate.

The difficulty of using FDR for soil moisture is that most soils are not homogeneous. Many calibrations at multiple probe locations are usually necessary, which makes for a time consuming and expensive process. In addition, each site calibration is usually unique to the instrument and probe used. Therefore, if a probe fails, servicing and replacement must be customized to that location, only adding to the time and cost of servicing.

Neutron probes are another example of high-end soil moisture measuring devices. Although their accuracy is very high, the expense of installing, maintaining and supervising these instruments (because a radioactive source is used) makes them virtually prohibitive in cost for most users.

Weather Stations

Weather stations can be judged on three criteria:

1. Sensor capability (diversity and accuracy);

2. Practicality (e.g., portability, ease of assembly/operation, durability); and

3. Data logging ability.

Many weather stations include seven basic sensors: relative humidity, air temperature, grass temperature, soil temperature, windrun, rainfall and solar radiation. Other sensors, like soil moisture, water level and lightning strike shock are also available depending on user need. Some weather stations are specialized and are concerned with the measurement of soil moisture content and the moisture content stress level of the plant. These instruments measure real evapotranspiration (Etr), which is the measurement of the exact volume of water vapor released by a particular plant or crop.

The soil moisture requirements in this kind of instrument depend on determining the amount of water that is consumed by a particular crop and then deciding whether the plants are under stress when consuming that amount. Soil moisture measurement, as well as wind speed, are used in this instance since wind (along with sun) increases Etr, which, of course, affect the plant's demand for water. The importance of these moisture measuring weather stations is to provide accurate information for just-in-time (JIT) irrigation - irrigating just enough, just at the right time. Greater irrigation accuracy has the potential of producing healthier plants and using the optimal amount of water.

Many weather station systems can also store data in digital form in solid state data loggers, from which the data may be retrieved periodically. Since the data logger has a remote readout and control capabilities via an RS232 interface, this data can be immediately accessed at a central office by radio, cellular phone or satellite system. Continuous measurement of data by the sensors feeding the data logger increases reliability.

Many options are also available, like measuring and storing data within a user-specified range, the ability to compress data in a directly readable form and sensor calibration corrections which can be made in the data logger. Most weather station/data logger systems are provided with software to make them user-friendly, so that non-technical personnel can install and operate them. Digital systems also offer superior interfacing capabilities to sensors and are able to transmit frequency-coded signals over long distances without degradation.