Extending the wires of soil moisture sensor without voltage drop
If you need to reduce the voltage drop over a given distance, you need to increase the size of wiring being used.

In this case, with a load of 35mA and a distance of 15m, the voltage drop using 16-18ga wire would be negligible.  As in less than a half percent.

Here is a voltage drop calculator that you may find useful: 

Voltage Drop Calculator