The nails corrode, and the use of a DC current aggravates the problem due to electro-migration.
This is true but there is an other effect that also reduces the sensitivity of gold probes. As a DC current is applied you get some electrolysis around the probes. That is the water is broken down into hydrogen and oxygen and forms microscopic bubbles on the surface of the electrodes. This reduces the surface area in contact with the soil and hence reduces the resistance measured. The only way round this is to use AC.