Precision Agriculture

I need to develop a $5 USD sensor that reads Temperature, Moisture. and Salinity of soil to be planted about a foot deep, 4 to 6 sensors per acre, wireless. This would greatly help with the food and fuel crises.

A $5 price tag suggests you are aiming at the poorer farmer.
Does such a farmer want such a sophisticated system ?.

Would it be better to concentrate on whatever is most important,
perhaps moisture content ?.

Temperature and moisture would be a good start since several things can be determined by these. I would like to focus on low power wireless and energy harvesting. I would also like to write the code in RobotBasic.org

I have chosen to use a plastic bottle preform as the housing, it is the shape of a lab test tube, and a standard bottle cap. This would be a good strong cheap weatherproof housing that could be easily dropped in a hole drilled with a auger bit.

http://www.teachersource.com/Chemistry/PreformsAndSodaBottles/SodaBottlePreformsandCaps_30pk.aspx

The reason I selected a $5 USD target is that Ratnesh Kumar of Iowa University has claimed this as a goal.

I am looking at using an ARM micro controller and a TI radio chip set.