Soil Moisture Sensors

What is the difference in soil moisture sensors and how do I get the best bang for my buck?
Below is a brief explanation of the main types of soil moisture sensors and where they are suited.

Capacitance
Capacitance sensors means the sensor sends out an electrical current from one end towards a receiving end. Water collects and stores current a lot better than air and soil therefore the amount of water in the soil can be determined. Because this method relies on the electrical conductivity of water the presence of ions for instance salt (soils closer to the sea) can give inaccurate results. These sensors may need calibrating to adjust their outputs according to a sensor of known accuracy.
The majority of the sensors on the market will be capacitance sensors, they are very common and are good because they are easy to install, read and are reasonably priced. However it is important to remember that not all of these sensors are equal ask what the warranty is on the probe, do a quick review online to try and access credibility and ask to have the display that you will read off set up so that you know the units being reported and your limits (target and trigger point).

EnviroPro – Takes measurements every 10 cm down the profile (can choose max depth from 40 to 160 cm), they compensate for salinity and the field of influence is said to be up to 55 cm from the probe itself which is impressive for a capacitance sensor.

Aqua Check – Takes measurements every 10 cm down the profile (max of 40 or 60 cm) which helps especially on cropping farms determine what the soil moisture is lower in the profile. If installed in a crop then they will need to be moved before harvest and redrilled into another paddock the following year.

Decagon 10HS and Terros – Decagon (now known as the Meter group) have a number of probe varieties but these are two of the most common. The company is very good at testing and refining their sensors, these would be some of the most reputable on the market. They have prongs and need to be inserted at the desired depth in the soil (generally one at 10 cm and another at 30 cm). They need to be inserted into the side of a hole and then filled back in so they are best used in pasture settings where they can stay in the ground for multiple years and the soil has time to settle around them and give accurate readings.

Wild Eye – Another classic two prong probe which is inserted at the desired depth in the soil (generally one at 10 cm and another at 30 cm). Installation and settling is the same as for the Decagon/ Meter group sensors above. This probe will be a lot cheaper than the decagon sensors and as a result appears to have had a lot less testing and supporting information available but could mean it is affordable to put more sensors in on one farm.

Time domain transmission

The Aquaflex is the most well known time domain transmission sensor in NZ, designed in the country and well suited to NZ soils. The time taken for an electrical pulse to travel down the length of the Aquaflex and back again is measured. The time taken is used to determine how much water is in the soil. These sensors are very useful where the average soil moisture over a large sampling area is required, they are not overly expensive and are a good permanent solution. The downfall is that they need contact with the soil to work and must be dug into the soil at the required depth which is invasive, and the soil takes some time (length of time depends on the installation) to settle again and give an accurate reading.

Neutron Probe
A probe is lowered into a tube installed in the soil and high speed neutrons are emitted. They collide with hydrogen atoms in soil water and slow down, the number of slow neutrons returning to the source detector are counted and the soil moisture content is determined. This process is repeated at various depths down the tube to give moisture content down the profile.

It is highly accurate across a range of soil types and crops, the length of tube can be installed to suit the crop. A good option for cropping farmers who have high value crops and want to have their soil moisture decisions looked after by someone else. Pitfall is there is only one reading a week instead of continual real time monitoring.

Version 1
19/11/2021
Prepared by Nicole Mesman