Measuring soil moisture and using a vintage DC ammeter as the indicator
The Weston 506 DC moving coil meter movement would have a full scale deflection of 1 mA or 50 mV. The coil resistance would be 50 Ω. For a 20A meter, the shunt resistance would be 0.0025 Ω and dissipate 1 W at 20A.
The shunt could be either internally- or externally-mounted across the meter terminals. If the shunt is not found outside, internal mounting may be confirmed by measuring the resistance across the meter terminals.
The shunt would have to be removed before the meter could be used as a moisture indicator. To remove an internal shunt, if any, the movement would need to be taken out by unscrewing 3 screws on its housing.