SOIL CONDUCTIVITY METER

 

A friend from Australia questioned me about a means for measuring water content in soil, one easy method is to use the relation between soil moisture and conductivity.  Using two electrodes, a current can be made flow to measure de voltage drop across them.  Although conductivity depends on many other factors such as: soil chemestry, temperature, density, areation mainly; a cheap in situ relative measurement can be made.  Using my SerialDAQ, a current amplifier pnp-transitor stage, and a probe made from two graphite rods (commonly found in alkaline batteries) I set up a sampling system.

 

 

This is the picture of the system:

 

I placed a plastic bottle on top of the soil and made a tiny hole on the bottom so water can slowly drip.  I ran a sampling every second and recorded data to a log file.  This is the Excel plot from the log file.

 

 

There are other methods that aproach the issue in different ways.  One is the TDR (Time Domain Reflectometry) wich is also used to detect shorts and open circuits on long paired wires.  It applies to detect the reflection of an inyected pulse to a pair of electrodes and the resulting pulse yields the water content.   Another one is measuring the soil’s dielectric coeficient, altough this requires high end detectors and amplifiers. 

 

This is one cheap way to log data to a PC.  The software can be made to trigger an output when the signal drops below a setpoint.  My friend from Australia, I hope this could help.  I will make more efforts to obtain a satisfactory capacitive probe.  I am also testing the TDR technique at job to detect shorts and open circuits on faulty modbus+ cables. Your inquiring pushed me to seek for possible solutions, thanks buddy!