Hello Guys (and Girls)
After long time, i found a good (and cheap) sensor, to measure the moisture of the earth from my plants. It is a capacitive moisture sensor, so it will not produce any electrolytic metal salts or other ions, which can be dangerous. Also the sensor will be long time stable and not dependent of the temperature.
Here is the link (GERMAN!!): https://www.mikrocontroller.net/topic/335407 I buy it here: http://www.ramser-elektro.at/shop/bausaetze-und-platinen/giess-o-mat-sensor-bausatz/ If you only need the PCB, you can buy it here: https://www.it-wns.de/themes/kategorie/detail.php?artikelid=1132&source=2
I hope i can help somebody with this informations.
Best regards, Nethacks