so if you dumped a high ph water in and it came out 5.8, that would mean... the water passing by molecules were getting acidified by whats in the soil and leaching them out. simple. first I would tell you to get some 6.5 water. Scoop out 2-3 tablespoons of soil from your pot. put it in a small glass. Cover the soil with the 6.5 water just untill its muddy, soup. Use as little water as you can. let it sit for 1-2 hrs. drain out the water into another glass and let it settle for 10-20 min, untill the water is somewhat clear. Now test the water that was sitting in with the soil. The molecules in the soil will bind to the water molecules and make them either acidic or alkaline. Im pretty sure if those were correct readings, you soil ph would be around 5.0 give or take.... maybe lower like 4.5. check back whendone