I question your obsession with 555? It is a handy chip, but if you are using Arduino, it seems to violate your requirement for "easiest".
This page had a pretty good explanation of why they make sensors out of plaster. And a quite straightforward construction tutorial.http://www.cheapvegetablegardener.com/2009/03/how-to-make-cheap-soil-moisture-se...
PS: You might want to re-consider your avatar. Using the same one that is already used by several other people is confusing.
To use alternating DC, simply connect each side of the sensor to an Arduino digital pin and an Arduino analog in pin. Then you can ground one pin and pull the other up and make a voltage reading. Alternating pins on the next reading, etc.