The 555 timer IC can be used as a moisture meter IC when used in connecting with an appropriate sensor, as for example a gypsum block sensor. One of these sensors are produced under the name Watermark, which is a sensor encapsulated in stainless steel, that has a long lifetime.
The sensor is connected to the ‘input’ side of the 555 timer IC and it now works as a moisture meter IC, because the sensor causes a change in the power supply current when the soil moisture changes. This change in supply current can be measured and related to change in soil moisture.
The sensor is connected in such a way, that it modifies the oscillation frequency of the 555 timer IC, because the output oscillation frequency is determined by the components connected to the input side.
The 555 IC is normally used for timing purposes, or for creating delay, or constant frequency for e.g. generation of sound, but can just as well be used as a soil moisture meter IC.