I don't think a continuous length of wire surrounded by soil will show much resistance change between its ends, because the wire conductivity is likely to be much greater than the soil conductivity (wet or dry). It's more usual to sense the soil resistance between two separated probes, as mentioned above.
DC energisation of the wire/probes has the disadvantage of their rapid destruction due to electro-chemical effects. AC energisation, for brief periods, can largely overcome that. The wire/probes should be of a corrosion-resistant material.