Co-application of DC field and Microwave Irradiation on Soil
It won't affect the flow of DC current (as a first order electrical effect).

However the strong microwave signals will be picked up by the electrodes and cables, and carried back to the DC power supply circuits. Or leakage from the microwave generator/coaxial cables might couple to the DC power supply you stacked it right on top of.

RF might upset the operation of the power supply: the voltage and current go up or down.

If this is the case you need to do something about it. quarter wave stubs, coiled leads, shielding, ferrite sleeves. Entirely solvable if it happens.



A tip: old fashioned coil+needle meters might be more reliable indicators of current/voltage under strong radio fields, than digital types.



An issue with what you are trying to do is getting power into the soil, and not reflected back to the transmitter by mismatch. Reflected power/VSWR meters show you this.