[chbot] Just when I thought I had a good soil moisture solution...

[Robin Gilks]

... I discovered that rather than resistive I should be looking at high
frequency capacitive sensors.

Commercial ones run at up to 70MHz, this one based on am ATTINY441 AVR
chip runs at the system clock rate of 16MHz and has an I2C interface.

https://www.tindie.com/products/miceuz/i2c-soil-moisture-sensor/

Very simple mode of operation, its basically using the PCB as the two
plates of a capacitor and as the dielectric constant around it changes
(due to moisture) so does the capacitance.

This is measured by driving a high frequency clock to the cap through a
series resistor and measuring the peak voltage swing across the cap with a
couple of diodes and long(ish) time constant RC network providing a
differential output to the A/D inputs of the micro.

Think I'll go this route...

-- 
Robin Gilks