In the soilwater documentation, optioanl parameters are referred to, eg:
Solutes in Rainfall Solutes can be applied to the soil via rainfall using the optional parameter 'rainfall_XXX_conc' in parts per million,
where 'XXX' is the solute name.
It is not possible to create these in the UI - only in old con/par files.
A user (kevin kodur) has requested this.