Soil moisture sensor
Soil moisture sensors measure the water content in soil. A soil moisture probe is made up of multiple soil moisture sensors. Since analytical measurement of free soil moisture requires removing a sample and drying it to extract moisture, soil moisture sensors measure some other property, such as electrical resistance, dielectric constant, or interaction with neutrons, as a proxy for moisture content. The relation between the measured property and soil moisture must be calibrated and may vary depending on soil type. Reflected microwave radiation is affected by the soil moisture and is used for remote sensing in hydrology and agriculture. Portable probe instruments are used by farmers or gardeners.
Technologies commonly used in soil moisture sensors include:
- frequency domain sensor such as a capacitance sensor.
- neutron moisture gauges, utilize the moderator properties of water for neutrons.
- electrical resistance of the soil
- Time domain transmission (TDT) and time domain reflectometry (TDR); water has a high dielectric constant; a higher water concentration causes a higher average dielectric constant for the soil. The average dielectric constant can be sensed by measuring the speed of propagation along a buried transmission line.
- heat dissipation sensors rely on the effective thermal conductivity of soil. Soil with additional water conducts heat more readily than dry soil. Heat dissipation sensors that include a porous intermediate water holding element are subject to errors of up to 30% during the wet and dry cycles.
Measuring soil moisture is important in agriculture to help farmers manage their irrigation systems more efficiently. Not only are farmers able to generally use less water to grow a crop, they are able to increase yields and the quality of the crop by better management of soil moisture during critical plant growth stages.
Besides agriculture, there are many other disciplines using soil moisture sensors. Golf courses are now using sensors to increase the efficiencies of their irrigation systems to prevent over watering and leaching of fertilizers and other chemicals offsite.
In urban and suburban areas, landscapes and residential lawns are using soil moisture sensors to interface with an irrigation controller. Connecting a soil moisture sensor to a simple irrigation clock will convert it into a "smart" irrigation controller that prevents an irrigation cycle when the soil is wet.
Simple sensors for gardeners
Cheap and simple devices are available for checking whether plants have sufficient moisture to thrive that do not require a power source. After inserting a probe into the soil for approximately 60 seconds a meter indicates if the soil is too dry, moist or wet for plants.
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- 229 Heat Dissipation Matric Water Potential Sensor, Instruction Manual, Logan, Utah: Campbell Scientific, Inc., May 2009, pp. 2–3
- Miscellaneous Methods for Measuring Matric or Water Potential, Textbook, Austin, TX: University of Texas, p. 652
- Irrigation Monitoring with Soil Water Sensors
- Practical Overview of Soil Moisture in 2010
- When2Water Sensors Tutorials about using sensors and tensiometers in irrigation scheduling
- "TDR (Time Domain Reflectometers): how they work, some literature on them, where to get them and how much they cost". Archived from the original on 2012-04-18.
- http://www.nrs.fs.fed.us/pubs/jrnl/2003/ne_2003_tyree_007.pdf Matric Potential, Melvyn T. Tyree, USDA
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