Measuring a varying capacitance using the Wien bridge
Wien bridge oscillator depends on two capacitors changing. The Schmitt inverter Oscillator only depends on one shunt capacitor and a feedback resistor. If you want to measure a wide dynamic range then I would suggest the Schmitt trigger with an R much lower than soil R. Then consider dual oscillators for different depths to indicate depth of moisture.

What is your R and C expected range with water and fertilizer affecting R and C independently?

What is your current or power or uWh budget?