If you are designing your own intelligent “Graden Computer” with an optional digital plant moisture sensor/water ... user when it is time to water a potted plant, and/or turn on a water pump to quench ...

This is a simple arduino project for a soil moisture sensor that will light up a LED at a certain moisture ... in the dry soil and measure the resistance value and then pour water and measure it again ...

The easy way is to use the Arduino-powered Norman climate simulator from [934Virginia ... difference between your target climate and actual sensor readings in your greenhouse.