ATmega - measure the soil humidity

(Deutsche Version) Today, I want to present a humidity sensor. The basic idea is to measure the soil humidity at plants to know when you have to water the plants.

To construct the sensor, you don't need much stuff. I used a small piece of stripboard, two nails and some pin strips. The sensor looks like this:

The next step is to measure the resistivity in between the two nails (when the sensor is in the earth) and then you can estimate the soil humidity. The electrical wiring is the following:

If the earth is humid, the sensor conducts well, therefore the ATmega measures a high voltage, if the earth is dry, the measured voltage is small. You can now think about putting units on the axis, in my opinion, it is sufficient to simply plot the "humidity" from 0 to 5 V and then check the voltage which corresponds to dry earth which has to be watered. This can now be displayed by an LED or by sending an automatic e-mail.