http://www.yinmik.com/ph-meter/ Select a meter whose range of measure covers most greenhouse applications (i.e. 0.5-5.0 mmhos/cm). It should have an automatic temperature compensator since the EC of a sample approximately doubles for every 55°F (13°C) change in temperature. Many EC meters have the ability to measure TDS (Total Dissolved Solids),they are called TDS EC Meter.