Standing Wave, or Amplitude Domain Reflectometry (ADR), uses an oscillator to generate an electromagnetic wave at a consistent frequency, which is transmitted through a central signal rod, using outer rods as an electrical shield. The electromagnetic wave is partially reflected by areas of the medium with different dielectric constants (water content), producing a measurable voltage standing wave. ADR measures volumetric soil water (VSW%) independently of all other soil variables, including density, texture, temperature and electrical conductivity. ADR does not require in-situ calibration to accurately measure Volumetric Soil Water (VSW%).
Environmental, agriculture & engineering applications requiring assessment of the changes of soil moisture in absolute mm and the exact volumetric soil moisture use ADR or TDR technologies. ADR sensors have been buried permanently in landfills are still functioning after 15+ years.
Time Domain Reflectometry (TDR) measures the time taken (in nanoseconds) for an electromagnetic pulse to propagate along a waveguide surrounded by soil. Time of travel, or velocity, of this pulse is effected by the dielectric constant (Ka) of the soil. Wetter soil with a higher dielectric constant, produces a slower velocity pulse. TDR measures volumetric soil water (VSW%) independently of all other soil variables, including density, texture, temperature and electrical conductivity. TDR does not require in-situ calibration to accurately measure VSW%.
Capacitance sensors measure the dielectric permittivity of a surrounding medium.
The configuration is either like the neutron probe where an access tube, made of PVC, is installed in the soil or buried probes connected to a data logger. In either down hole or buried configuration, a pair of electrodes form the plates of the capacitor with the soil in between these plates, acting as the dielectric. The oscillating electrical field is generated between the two plates and extends into the soil medium either through the wall of the PVC access tube or conformal coating of the buried probe. Changes in dielectric constant of the surrounding media are detected by changes in the operating frequency. The output of the sensor is the frequency response of the soil’s capacitance due to its soil moisture level.
Capacitance sensors come in many configurations and have many shapes. Due to the low cost and low power consumption capacitance sensors are common. The impact of temperature and conductivity on the measurement of volumetric soil moisture means they are suited to monitor relative changes of soil water content and require in-situ calibration for accurate measurement of volumetric soil water content (VSW%).
Capacitance sensors have a small volume of measurement and usually a limited life of several years in situ. Capacitance sensors are widely used for irrigation scheduling.
The force with which water is held in the soil by the soil particles, is referred to as soil suction, soil tension, or soil water potential. It indicates how tightly the water is bound in the soil, and how much energy must be exerted by plant roots to remove and use the water.
Jetfill tensiometers measure in the range 0-70 kPa. The tensiometer can measure very accurately small changes in soil water potential and because of the fast response these are immediate. The vacuum inside the tensiometer is measured by a vacuum transducer (ICTGT3-15), which gives a continuous analogue output signal. A resolution of 0.1 kPa is attained for this tensiometer transducer. Turf and vegetable crops are typically irrigated at 30kPa and cereal crops closer to 50 kPa. The basic components of a tensiometer include a porous ceramic cup, a plastic body tube, water reservoir, and a vacuum transducer. The ceramic cup is placed in good hydraulic contact with the soil and allows transfer of water into and out of the tensiometer body according to the tension in the soil. The vacuum inside the tensiometer body equilibrates with the soil water tension, and there is direct response with a vacuum transducer.
Solid ceramics necessarily have a wide range of pore sizes that can typically measure soil water potential of -100kPa and drier. Solid ceramics have a slower response time than tensiometers and so are more suited to general monitoring of soil suction within or under structures of buildings, roadways and mine sites.
Drainage volume and nutrient loss are important measurements for determining fertiliser and water use efficiency and for measuring environmental performance. Drainage is often measured in field laboratory settings using column lysimeters, or experimentally in the field using soil solution samplers (suction cups) or miniature lysimeters / drainage gauges. However, due to field variability such measurements require a large number of devices to be installed to obtain accurate field averages which are usually labour-intensive and do not usually allow real-time monitoring.
The GTLA GroundTruth Lysimeter System combines a very large repacked strip lysimeter with automated, real-time drainage measurement and water sampling. This allows accurate measurement of nutrient losses in the field, viewable in real-time. Each strip lysimeter is a transect, usually 10m long. The actual dimensions can be larger and are tailored to the site. The GTLA lysimeter can be installed across crop rows or plots, to integrate variation in the field. Once installed, the lysimeter is completely below-ground, unaffected by farm operations, and undamaged by cultivation.
ICTO2 Soil Oxygen Sensor
The ICTO2 soil oxygen sensor is used to continuously monitor soil oxygen concentration; which is crucial to the productivity of economic crops such as avocado, cotton, tomato and tobacco. Anaerobic soil conditions prevent uptake of water as the roots cannot respire due to excess water in the soil profile and daily water use rapidly declines with resultant significant crop yield loss.
There are two types of O2 in soil – soil pore O2 and dissolved O2 in soil solution. Soil pore O2 directly impacts upon plant health, and dissolved O2 upon soil microbial health. An equilibrium exists between these two, so simply measuring the bulk soil O2 is enough. The ICTO2 sensors are also used for monitoring of oxygen and water (MP406) in waste rock stockpiles to control acid mine drainage. The Teflon coated ICTO2 sensor has automatic temperature compensation via an in-built thermocouple circuit. The sensor is designed for long term in-situ measurements.
The THERM-SS is a high-quality thermistor embedded in a protective stainless-steel body which can be used in a wide range of applications, from soil monitoring in agriculture to industrial landfill, or mine tailing and concrete monitoring.