The SCP1 Soil Water Content and Potential Meter is the only instrument available to scientists for the simultaneous measurement of soil water content and soil water potential.
The SCP1 can support up to 5 sensors which are pre-calibrated or can be individually calibrated for a specific soil.
Data is logged to an 8GB MicroSD card in user configurable increments. Data can be downloaded as a csv file for further data analysis and manipulation.
The SCP1 also features wireless communications, data can be sent remotely up to 250m from a nearby field site or laboratory.
How it Works
The SCP1 Soil Water Content and Potential Meter can support MP306, MP406, EC-5 or 10HS Soil Water Content Sensors. These sensors are traditionally used in science to measure the volumetric water content (VWC) of soils or other media. With a soil water characteristic curve (SWCC), also known as a moisture retention curve or water release curve, a mathematical relationship exists between VWC and soil water potential (SWP). Once the relationship between VWC and SWP is known, this equation can be entered into the SCP instrument as a script with one channel in the instrument configured to measure VWC and a second channel configured to measure SWP. In this way, a single sensor can measure both VWC and SWP simultaneously in the same location in the soil profile.
General textbook equations are available that conveys the mathematical relationship between VWC and SWP for a particular soil textural type, such as sand, loam or clay. These textbook equations can be used in the SCP1 if the soil textural type is already known. However, there will need to be interpretation of the results as the majority of these equations were generated with various soils from the United States and will most likely be inaccurate for other locations. A soil specific calibration by the user can easily be entered as a script into the SCP1.
Calibration for your specific soil is relatively straight forward, particularly if your lab already has commonly available soil equipment such as a balance, drying oven, pressure plate extractor or WP4C Dewpoint Potentiameter. For more information on calibration click here or contact a scientist at ICT International.
Soil water potential can be derived from a soil water content sensor using ICT International’s SCP1 Soil Water Content and Potential Meter. A mathematical relationship between soil water potential and soil water content for a particular soil type is derived using established laboratory techniques. The equation derived from this relationship can be entered into the SCP1 Soil Water Content and Potential Meter as a script. Soil water content sensors are installed in the field, such as the MP406 or MP306. The SCP1 Soil Water Content and Potential Meter then outputs data as both water content and water potential. This article provides a background to this technique, why a user will want to know soil water potential versus soil water content, and expands on the methodology to simultaneously measure soil water content and water potential with the SCP1 Soil Water Content and Potential Meter.
Soil water content is a well-established and wide-spread measurement. Soil water content is measured with ICT International’s MP406 or MP306 volumetric water content sensors. These sensors are commonly used by scientific researchers, irrigators, agriculturalists and education institutions around the world. However, only measuring soil water content limits the amount of information available from the soil environment.
The major limitation of soil water content data is the reliance on soil physical properties. That is, depending on the type of soil being measured, for example in a sandy or a clay soil, the same water content value carries a vastly different meaning. For instance, a 20% volumetric water content measurement on a sandy soil indicates a moist to saturated soil. A value of 20% recorded on a clay soil, on the other hand, indicates a near dry soil. Therefore, a user needs to have an intimate knowledge of the soil physical properties where they are measuring water content in order to gain meaningful information.
The reason why a value of 20% carries a different meaning for a sandy soil and a clay soil is related to the texture, structure and porosity. It more useful to have a tool which can measure soil water independently of these variables. What is needed is a universal technique that works irrespective of soil physical characteristics. Fortunately, the measurement of soil water potential is the universal technique.
The measurement of soil water potential is the measurement of the amount of energy available in the soil to do work. Or, to put it more intuitively, it is the amount of energy required for a plant to perform work in order to extract moisture from soil. Soil water potential is expressed by a number of different units. In soil science the most common units are kilopascals (kPa), megapascals(MPa), bar or centibar. Table 1 outlines the equivalency between these units of measurements.
Using kPa, the more negative the number then the greater amount of work a plant needs to do in order to extract moisture from soil. In agriculture, it is assumed that -33 kPa is field capacity (the optimal moisture potential for plants) and permanent wilting point is -1500 kPa (the point of plant mortality). Importantly, these values of field capacity and wilting point are assumed to be the same for all soil types (field capacity and wilting point does vary depending on the plant species under consideration, but this is a story for another day).
In order to derive soil water potential data from soil water content data there needs to be a mathematical or statistical relationship between these two variables. Fortunately, there exists a relationship between soil water potential and soil water content. Once this relationship is determined, it is a simple matter of algebra to derive soil water potential from a soil water content value.
The relationship between soil water potential and soil water content is known by a few different titles including: moisture release curve; water characteristic curve; water retention curve; and capillary pressure-saturation relation, among others. This article will not describe this method in detail. However, the moisture release curve is a well-established technique in soil science and detailed methodology can be found in Dane and Hopmans (2002). Also, you can find more information by contacting ICT International.
Figure 1 is an example relationship between soil water content and soil water potential derived from a pressure plate extractor. Curve A is a theoretical curve from a sandy soil and Curve B is a theoretical curve from a clay soil.
The mathematical relationships for these curves are:
Curve A Sandy Soil: Soil Water Potential = 5343 x Soil Water Content-1.852
Curve B Clay Soil: Soil Water Potential = 6 x107 x Soil Water Content-4.228
Note that these are demonstration curves only and the values presented above can vary depending on your soil type. The important point from Figure 1 is a strong mathematical relationship can be derived between soil water potential and soil water content. Once a reliable equation has been derived, the relevant parameters can be entered as a script into the SCP1 Soil Water Content and Potential Meter.
ICT International’s SCP1 Soil Water Content and Potential Meter has a 5-sensor capacity with 5 virtual channels. For now, let’s assume there are 5 x MP406 Volumetric Water Content sensors, installed at a depth of 10cm, at 5 locations across a field. The 5 x MP406 sensors are installed in channels 1 to 5 on a SCP1 Soil Water Content and Potential Meter. The MP406 sensors are giving output data as volumetric water content (%). A moisture release curve has been derived for this soil and, coincidentally, it is the same equation as Curve A above. The SCP1 Soil Water Content and Potential Meter has the flexibility to handle scripts and a script for the equation for Curve A above is entered. Channel 6, via the script, is now assigned as a virtual sensor, correlated with Channel 1, and its output data is soil water potential (kPa). Channel 7, also via a script, is now assigned as a virtual sensor, correlated with Channel 2, and its output data is also soil water potential (kPa). Channels 8 to 10 are also assigned as virtual sensors correlated with channels 3 to 5 respectively. Now the SCP1 Soil Water Content and Potential Meter is simultaneously recording soil water content and soil water potential data at 5 locations in this hypothetical cotton field.
In summary, through ICT International’s SCP1 Soil Water Content and Potential Meter, soil water potential can be measured with a soil water content sensor. A moisture release curve needs to be generated in order to derive a mathematical relationship between soil water content and water potential for a particular soil type. This mathematical equation can be either directly entered into the SCP1 and data output generated via a virtual channel, or data output can be generated direct on a sensor channel. At all times, raw millivolt data can be recorded so any alterations can be applied to the measured data from the field.
For more information please contact ICT International:
+61 2 6772 6770
Dane, JH and Hopmans JW (2002). Water retention and storage. In: Methods of Soil Analysis, Part 4 Physical Methods (Editor-in-Chief SSSA: WA Dick). Pp 671 – 720. Soil Science Society of America, Inc, Madison, Wisconsin, USA.
|Analogue Channels||5 single ended|
|Minimum Logging Interval||1 second|
|Delayed Start||Suspend Logging, Customised Intervals|
|Communications||USB, Wireless Radio Frequency 2.4 GHz|
|Data Storage||MicroSD Card, SD, SDHC & SDXC Compatible (FAT32 format)|
|Software Compatibility||Windows 8, 8.1 & 10. Mac OS X|
|Data Compatibility||FAT32 format for direct exchange of SD card with any PC|
|Data File Format||Comma Separated Values (CSV) for compatibility with all software programs|
|Memory Capacity||Up to 16GB, 8GB microSD card included.|
|Temperature Range||-40°C to +80°C|
|Upgradeable||User Upgradeable firmware using USB boot strap loader function|
|Internal Battery Specifications|
|960mAh Lithium Polymer, 4.20 Volts fully charged|
|External Power Requirements|
|Bus Power||8-30 Volts DC, non-polarised, current draw is 190mA maximum at 17 volts per logger|
|USB Power||5 Volts DC|
|Internal Charge Rate|
|Bus Power||60mA – 200mA Variable internal charge rate, maximum charge rate of 200mA active when the external voltage rises above 16 Volts DC|
|USB Power||100mA fixed charge rate|
|Internal Power Management|
|Fully Charged Battery||4.20 Volts|
|Low Power Mode||3.60 Volts – Instrument ceases to take measurements|
|Discharged Battery||2.90 Volts – Instrument automatically switches off at and below this voltage when no external power connected.|
|Battery Life varies|