The core of the project is to achieve the ability to sense the moisture of the soil on a crop field. The reason for choosing the capacitive way over the resistance method is to avoid the rust on the electrodes, dielectric terminals doesn't need electrical conductivity, therefore, they can be isolated and protected from water, air and dust.

The capacitance or the dielectric permittivity of the ground changes with the level of water content in it. There's 5 main ways to sense those changes:

1. Using a 555-like timer, and use the changes of capacitance on the ground to change the frequency of the output signal.

2. Using a dual opamp in an astable oscillator mode and measuring the frecuency of the output signal.

3. Using an opamp in integrator mode, where the cut off frecuency is based on the capacitance of the ground.

4. Using a microcontroller to generate a signal above 1Mhz and using a low pass filter to alter the output voltage, based on the capacitance of the ground.

5. Using a capacitance meter IC.

I haven't decided which method i'm gonna test first.

Info about the methods:
http://electronicdesign.com/analog/use-analog-techniques-measure-capacitance-capacitive-sensors
http://wemakethings.net/2012/09/26/capacitance_measurement/