I am working on the development of a low cost capacitive soil moisture sensor which uses dielectric permittivity changes to estimate moisture.

I found that interdigital capacitor based on FR-4 has more capacitance as opposed to coplanar one of the same size. However, I need some perspective as to the range of electric field each will emit since it determines my sensing range. Insights from practical experiences of users can help me asess this situation better.

A picture of the two probes 

[![enter image description here](https://i.stack.imgur.com/81CvP.png)](https://i.stack.imgur.com/81CvP.png)

Edit 1 :

I am currently experimenting with the operating frequency to be used but it will be in 5-10 Mhz range.What I gather from research papers is that pcb on a capacitor can work as soil moisture probe due its fringing field. 
The values that I got - two parallel capacitors around 14pf
, FEF (interdigital) - around 25pf

Soil thickness I am intend to measure moisture for is about 0.5m