As discussed in comments, providing that the power source is capable of low amounts of power and there are no components that are likely to store energy (that can generate a spark) there will be no ignition or surface temperature side-effects that could ignite even the most volatile of gases such as hydrogen (19uJ) to ignite in the perfect gas-oxygen mix.
The cable feeding the sensor will have inductance but it is likely to be less than a ten microhenries BUT what is the chance that hydogen will be around? It would need a minimum of 40uH with 1A flowing from the battery pack to be potentially unsafe. This circuit would then need to be broken to produce a spark. No chance. If hydrogen is present then what sort of plants are you cultivating?
You might have some petrol around but you'd need a lot bigger energy spark than for hydrogen (the most volatile of the gases to sparks) but you could ignite petrol with a surface temperature of 250ºC however, this is unlikely to be produced by the sensor circuit and the 4AA batteries.
If you can't ignite the most volatile of gases and fluids with energy or hot surfaces you won't ignite anything known to man.
If you don't believe look up Intrinsic Safety techniques and look out for iginition temperatures of the most volatile gases and also check for hot surface temperatures.
Here is a simple site that should convey the basic principles
It's giving you units of "wetness", from 0 meaning "bone dry", and 950 meaning "sopping wet".
It's for an Arduino. It doesn't give you anything as technical as real units - just a rough scale between wet and dry. It's entirely up to you how you interpret that value.
It's about as accurate as sticking your finger in the soil to feel how wet it is.
You could divide the sampled value by 10 and use it as a percentage of sogginess.
Best Answer
A likely mechanism for the operation of the sensor is by creating a Galvanic Cell battery out of the probe electrodes, with the moisture in the soil acting as the electrolyte / "salt-bridge".
From the Wikipedia page referenced above:
The "sensor probe" would consist of two dissimilar metals such as Zinc and Copper, which @JimDearden correctly surmises in his answer.
However, the actual current flow due to Galvanic redox reactions is what would create the energy to deflect the needle of the sensor, rather than conductivity of the moist soil alone. The moisture here works merely as an electrolytic medium for the electrochemical reaction.
This sensor would fail if used in distilled or otherwise very pure water, or moisture containing no dissolved salts / electrolytes.