There are a couple different things that can contribute to a sensor having a reading below 0. Please refer to this previous post for details (http://www.decagon.com/forums/soil-moisture/show/6#post6).

If your installation is sound, a custom calibration can frequently improve your readings and bring them back into line with what you would expect. Negative readings can frequently occur in very organic soils or soils with a very low density. If you have the space in your datalogger it is usually a good idea to save the raw data (especially if your calibration is a polynomial equation). That way you can always go back and apply a different calibration to your data.