Soil thermometer - Soil thermostat

Stick the soil thermometer in your pot's substrate and read the value of temperature. 

This will let you know if you can water your plants at that moment or if it's too hot and you could cook the roots by irrigating then. Besides, as temperature rises the amount of oxygen available for the roots decreases, lowering nutrient uptake.

If water it's too cold, root's functioning will stop. Therefore, this is also a useful device for the winter since it will let you know whether you must bring your plants inside or leave them in the balcony.

Substrate must be between 18°C and 25°C when you water them. However, it can be a real challenge to have that temperature during the summer, including at night time.