Update 2011-09-05: I have created a new blog about electronics only – check it out here: Electronic measurements, NSLU2 and soil moisture sensor
The 1-wire soil moisture sensor circuit I use in my garden is designed by Eric Vickery from http://www.hobby-boards.com.

(c) 2006 Hobby BoardsĀ Designed by Eric Vickery
Title: Moisture Meter 3
Rev: 3
Date: 10/27/2006 06:11:30p
(Click the picture for a larger version.)
Here’s a bit of explanation on how the humidity sensor circuit works.
The circuit is connected like this:
and the circuit works like this:
- Watermark sensor gets wet
- The time constant of oscillator IC 555 changes
- 555 IC oscillates faster and its supply current goes up
- Current in resistor R2 goes up causing the voltage across R2 to go up too
- The value of the current register of IC DS2760 changes
- Current register of IC DS2760 is read via 1-wire network
In my case I got the following values during calibration. I let the Watermark sensor dry in the wind and recorded the current register value, and afterwards the sensor was soaked in a bucket of water:
Dry = -0.2368
Wet = -1.400
These numbers form the 0 and 100 % limits of my soil moisture readings.