Both your water pH of 5.1 and buffer pH of 6.7 are indicating that you have free H+ in soil solution and residual acidity that will need neutralized.

(1) Germination and early seedling growth of Pinus contorta and Picea glauca were examined, using distilled water (pH 6·3), eight acidic buffer solutions (pH 2·2), and eight alkaline buffer solutions ...