Remember when I wanted to create a moisture meter for uC's for gardening a couple years back? Some people suggested using steel wool as a conductor, was also looking into cotton rope, but they beat me too it and it might actually work.
http://www.ebay.com/itm/Soil-Moistu...354?pt=LH_DefaultDomain_0&hash=item2a234fbf7a
http://www.ebay.com/itm/Soil-Moistu...354?pt=LH_DefaultDomain_0&hash=item2a234fbf7a