How to get distinct output voltages in LM3914 dot/bar cimparator IC?
Replace the LED you want to monitor with an opto-isolator. The photo-transistor will give you the logic levels you require - although they may be inverted from the way you would like.







simulate this circuit &ndash; Schematic created using CircuitLab

This also has the benefit that you can isolate the two circuits. The soil probe negative rail can not be grounded without interfering with the circuit operation.