you should calibrate soil by soil with a pressure volume (that is water content) curve, or by reading soil matric tables. you may see fig.6.7 of  Dr.Dingman reading, and search your soil types in regional literature,

 http://www.colorado.edu/geography/class_homepages/geog_3511_s11/notes/Notes_8.pdf

You need to determine the water retention characteristic of the soil. There is a relationship between volumetric water content and matric potential. By solving two simultaneous equations, you can get the other unknown term.

There is a realtion between matric potential and soil moisture.

Check the document I attach and see point 5 of it. Hope you can find it a useful guide for your work.

Regars

The soil moisture characteristic curve between matric potential and soil water content is to be drawn at different soil pressure head to know the relationship between two parameters

A widely used relation is from vanGenuchten, according to this formulation

relative moisture = 1 / ((1 + (alpha * psi) ^ n) ^ m).

where alfa, n m are the vanGenuchten curve parameters

psi is in cm

to convert the relative moisture in SWC (soil volumetric water content, m3/m3)

you have to know the SWC at saturation and the residual SWC (SWC when the soil is totally dry in equilibrium with the air moisture)

SWC=SWCres + relative moisture * (SWCsat - SWCres)

normally these values are obtained when retention curve is determined