pH of the substratum (5 states)

1 on very acid substrata, such as lignum and conifer bark
2 on acid substrata, such on non-eutrophicated bark of Quercus
3 on subacid to subneutral substrata (e.g. on bark of Sambucus)
4 on slightly basic substrata, such as dust-covered bark
5 on basic substrata, e.g. pure limestone