From this "capacitive soil moisture sensor" you will get a voltage and if you measure the voltage with the high resistance of the ADC and a 10-100nF capacitor directly close to the ADC-pin every second, then you can use in theory a very long wire.
- The biggest problem are the magnetic fields in the environment, they will induce current in the wire and create a voltage ripple on the wire.
The solution is a twisted wire (GND+Vout) or a shielded wire. You can use a coaxial cable or simply twist your wire like they have done it with each pair of wires in a network cable.
If you still have noise, then you should use a little LC- or RC-filter. You can use a 10k Ohm resistor from Vout of the sensor to the ADC-pin and directly very close to the ADC-pin a 100nF capacitor.
The cutoff frequency is here: f0 = 1 / (2 * Pi * R * C) = 1/(2 * Pi * 10000 Ohm * 0,0000001F) = 159Hz
If you only want to measure every minute, then you can use higher values for a lower cutoff frequency.