Hello, in soil there is no need to check the runoff pH. Just make sure what she gets in is right. Although 5.8 is a little low for soil so I would start to bring that up closer to 6.5. Adding some beneficial bacteria and humic acids will help the condition of the soil. It will also break down organic matter like dead roots so the plant can absorb nutrients and oxygen properly. When using the meter, make sure your water has some ec value, 200-400ppm consisting of calmag mainly. Otherwise you meter will not be able to give a steady reading. Using RO and soft water causes issues with reading the meter. I hope this helps, good luck and happy growing