Are you using any sensors now?
Mike is great. His is the most unique use of drupal I've seen. OPK needs more sensor drivers. I want to make a an OPK script for this moister sensor when they come in the mail. It should be good for 4" propagation pots or annual row crops. I've seen various projects use vegitronics for garden scale but not on farms.
http://vegetronix.com/Products/VH400/
There are other people working on open hardware moister sensor motes with gypsum block sensors. That would be more appropriate for perennial crops.