Measure Soil Moisture

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There are many different ways to check the soil moisture in your garden. This is a summary of the different instruments and techniques available measuring soil moisture. Also included are links to resources for making your own sensors.

Manual Methods

Plant Appearance

When you see noticeable signs plant distress like wilting, it means the plant has run out of water. This is not good for the plant and is a poor way to determine when to water.

Fingers

Just feel the soil. It can be hard to judge other than the surface moisture unless you dig a hole.

Wood Probes

If the soil is soft you can use a thin wood stick such as a skewer to probe the soil. Insert the stick and when you can pull it out you can observe or feel it to determine how moist the soil is.
Amazon - 12-Inch Bamboo Skewers

Notched stick

Inside urban green soil probe.jpg To use this type of probe, you push into the soil, rotate, and pull out. You can then access the soil moisture by observing and feeling the soil samples retrieved in the notches. See this article for more details A Better Way To Check Soil Moisture

Soil cores

A hollow tube you push into the soil to extract a column of soil for examination. These come in various lengths up to several feet long
Tubular Soil Sampler - From Amazon

Bottom Wick

For containers, Al Tapla suggests putting an adsorbent wick hanging down from the bottom of the pot. You can just feel it where it exits the pot to determine if the soil is still moist. See Also: Container wicks

Weight

For containers, if you are familiar with the plant, you can just lift or weigh it to get a feel for how much water is in the soil.

Soil Meters

There are many different types of meters to measure soil moisture. They range in cost from a few dollars up to a few thousand dollars. The low cost meters are almost all conductivity meters. The most expensive ones are radar based.

Soil Electrolyte Meters

These meters have a probe with two dissimilar metals separated by an insulator. When the probe is inserted into the ground the two disimular metals and the moist soil form a simple battery and the output of this battery is displayed on a meter to indicate soil moisture. The more moist the soil, the higher the battery output and the higher the reading on the meter. The main disadvantage of these meters is erroneous readings when the soil contains too high a concentration of salts or minerals or when the probe metals oxidize or otherwise no longer make good contact with the soil. Sometimes cleaning the probe with steel wool can help, but check manufacturer instructions frist. The advantage of these meters is that they require no battery, and are low cost and very simple to use. These meters are often combined with probes for PH.

Examples of these meters are:


Soil Resistance (or Conductivity) Meters

These meters typically have a battery and work by the electrolyte effect described above or measure the resistance of the soil by sending a small current through the soil. Moist soil is more conductive (lower resistance) and so will conduct more current. The results are displayed on an analog or digital meter. The disadvantages of this meter type are: batteries required, erroneous readings when the soil contains too high a concentration of salts or minerals or when the probe metals oxidize or otherwise no longer make good contact with the soil. Sometimes cleaning the probe with steel wool can help, but check manufacturer instructions first. The advantages of these meters are: usually low cost and very simple to use.

Examples of these meters are:

DIY Do It Yourself Sensors

Soil Capacitance Meters

The capacitance of soil depends on how moist it is. If two parallel conducting probes are placed in the soil, then they form a capacitor with the soil as the dielectric. As the moisture of the soil changes, the relative permittivity of the soil changes and as a result the capacitance between the probes changes. The capacitance values are very small on the order of a few pico farads and so require a little effort to measure. One common method is to create an oscillator using the soil capacitance to set the frequency. As the to soil moisture changes, the capacitance changes and the oscillator frequency shifts. This frequency shift is measured and displayed to show the soil moisture.

These meters have the advantage of being reasonably independent of the soil mineralization, and the metal in the soil probes can be insulated and so are less prone to oxidize or otherwise deteriorate from being in direct contact with the soil. Since the measurement is affected by the soil both to the sides and between the probes, this method is more of an overall indication of the soil moisture rather than just the soil directly between the tips of the probes as in the case of the soil conductivity meters.

These meters were once very expensive, but prices have dropped with the advent of micro processors.

Consumer Products

System Components

  • Vegetronix makes several different models that are relatively low cost and easy to inferface to micro processors and other systems.
  • Decagon Devices makes a range of different professional grade soil sensors

DIY Do It Yourself Sensors

It is not difficult to fabricate you own soil capacitance sensors.

Chemical Color Change

Some materials will change color depending on how moist they are. Some materials may have a limited useful lifetime. Sensors that use this effect:

Daphne's Plantdoc sensor is leaf shaped with the stem inserted in the soil


Frequency domain reflectometry (FDR)

FDR systems send an oscillating signal into a buried transmission line and then measure the frequency of the signal that is reflected back. The amount of moisture in the soil changes the dielectric constant of the soil and as a result the frequency of the reflected signal. So the difference of the sent and received signals changes depending on the soil moisture.

Time Domain Reflectometry (TDR)

Measures the speed of a radio wave through transmission line buried in the soil. The amount of moisture in the soil changes Dielectric constant of the soil and as a result the speed of the signal. TDR sensors send a electromagnetic pulse into the buried transmission line and then measure how long it takes to see the pulse reflected back from the end of the buried transmission line.

Time Domain Transmission (TDT)

Measures the one-way time for an electromagnetic pulse to travel over the length of a Transmission Line. This is similar to the TDR technique, but rather than measuring the reflections of the pulse, it measures the one-way transit time of the pulse over the length of the transmission line by using equipment at both ends of the transmission line. Since soil moisture changes the speed of the signal, the amount of water in the soil can be calculated from the transit time of the signal. Typically these sensors are permanently installed in the soil.

  • AQUAFLEX soil moisture meters The Aquaflex sensor loops the transmission line so that physically all of the electronics are at one end of the sensor.

Coaxial Impedance Dielectric Reflectometry

Similar to FDR and TDR sensors described above. The sensor sends an oscillating signal into the soil and measures the signal reflected from the soil itself to determine the soil moisture.

Gravimetric

Take some soil weigh it, put it in the oven, heat it to drive out all the water, weigh it, the difference is the amount of water that was in the soil. Accurate, but not practical for frequent measurements.

Soil Water Tension

Tensiometer diagram

These sensors (Tensiometers) measure how hard it is to extract water from the soil, i.e. they measure soil water tension. This is the same water tension that plants must overcome to extract water from the soil. Typically these sensors are a sealed water filled pipe with a ceramic cup at the soil end, and a vacuum gauge at the top. When the soil is dry, water travels through the ceramic cup into the soil and creates a vacuum at the top of the pipe which can be measured with the vacuum gauge. When water is added to the soil, water travels back from the soil through the ceramic cup into the pipe decreasing the vacuum at the top of the pipe. It is possible to make you own Tensionmeters although I have not seen anyone do it recently.


Neutron Thermalisation

Measures how much water in the soil slows down neutrons. Expensive.

Thermal Dissipation

Measures how heat dissipates in the soil which changes with soil moisture. Depends on a dry soil heating up faster than a moist soil. The sensor is a porous block with an embedded heater and temperature sensor. To make a measurement, the temperature is measured, the header turned on for a fixed time period, then the temperature is measured again. The difference between the beginning and ending temperatures is a function of how much moisture is in the soil.

Example: AquaSensor

Soil Thermocouple Psychrometers

This method requires putting a sample in sealed chamber and indirectly measuring water that evaporates from the soil with a thermocouple.

References

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