Milwaukee EC Tester

PS: If you are in organic farming you will not really need it in principle.

The Milwaukee EC Conductivity Tester is designed for hydroponics and more to help make sure your plants are getting the right amount of nutrients. 

Measures conductivity and temperature with automatic temperature compensation
Ideal range for hydroponics (0 to 20,000 µS / cm)
Units in µS / cm easily convertible to ppm
Factory calibrated with the ability to recalibrate if necessary
350 hours of continuous use (3 x 1.5V batteries included)