The Kilcher Homestead is a 600 acre Pioneer Homestead that has been protected for perpetuity via a ConservatIon Easement. The Kilcher Family has also preserved the Historic Cabin and site as a LIving Museum.
Yes, negative pH is possible, but not in water as a solvent. The range of pH in water is 0 to 14 (to be more precise, it is -1.7 to 15.7, but for simplicity's sake, we start with the usual values.) Thus, what pH indicates is the ability of the acid to form H3O+, which can then donate a proton.
When your soil have pH dependt charge clays this effect becomes more proeminent (organic matter also has this kind of charge). If you had a pH decrease with increasing soil salinity, it means your soil has predominance of negative charges in soil (predominance of negative charges in pH dependent charges). It is perfectly expected for your soil.