Soil Moisture Sensors

There's been a lot of salt damage to fruit trees after two years of drought using poor quality water and not doing adequate leaching.  The situation could be improved by knowing when to irrigate.  Here's some guidelines on selecting soil moisture measuring devices.

Soil moisture sensors fall into two broad categories, volumetric and tensiometric methods. One tells you how much water is in the soil and the other tells how difficult it is to remove water.  Volumetric methods require a calibration of the sensor to the soil, whereas tensiometric is good to go when installed.  The case in both methods is the grower learns to keep soil moisture within a given range of values and in theory, the plant is kept in a better condition with improved health and yields. 

The most common volumetric methods rely on measuring the dielectric constant of the soil which determines the velocity of an electromagnetic wave or pulse.  These sensors have become widely used because they have a good response time, do not require maintenance and can provide continuous readings, allowing for automation.  There are several different methodologies used Time Domain Reflectometry,  Frequency Domain Reflectometry (Capacitance), Amplitude Domain Reflectoremtry (Impedance), Phase Transmission, and Time Domain Transmission.  There is quite a range in prices for these different devices, but they generally do not require maintenance once installed and not require close soil contact.

The tensiometric methods include Tensiometers, Gypsum Blocks, Granular Matrix Sensors, Heat Dissipation and Soil Psychrometer.  These techniques rely the sensor to come into equilibration with the soil moisture and generally are unaffected by soil salinity. Gypsum blocks and Granular Matrix are not very responsive in sandy soils and require good soil contact.

Volumetric Sensors

Tensiometric Sensors