He says: My mother has eczema on her body and performing wudu is difficult for her, whether with hot or cold water.

Should she perform tayammum? And is all soil pure?

This is something you should consult a doctor about, consult a doctor about it.

Ask the doctor, male or female: Does water harm me or something like that?

If the Muslim doctor who understands this says to her: Yes, perform tayammum, then she should perform tayammum. The doctor is the mufti here.

The doctor is the mufti in this matter. As for whether all soil is pure, that is the basic rule.

The basic principle regarding soil is that it is pure, the basic principle regarding physical objects.

The basic principle is that everything is pure except what evidence proves to be impure. This chair is pure, this ground is pure, this wall is pure, your cars are pure, this is the basic principle, everything is pure.

This is a rule among the scholars: the basic principle regarding objects or physical things in general is that they are pure.

Unless there is evidence proving their impurity.