http://www.xs4all.nl/~sbolt/e-groeneVinger.html
Solar powered "green thumb" monitors plant water level. Full construction details. Looks like a basic BEAM circuit applied to a conduction probe.
http://www.frontgate.com/jump.jsp?itemType=PRODUCT&itemID=18479
This Soil and Sun Probe automatically takes scientific sun/soil information generated from a probe you insert into the soil. Log on to its partner web site to learn what plants are recommended for that particular spot on your lawn.