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.
Tomatoes and also onions and potatoes prefer a slightly acidic soil with a pH between about 6.0 and 6.8. Pine needles are acidic and will lower the pH of the soil. You may want to test your soil to see if the pine needles have lowered the pH too much.