If I buy second hand clothes from non muslim or charity shop, do i need to purify them by soil in assumption that the previous owners might have touched dogs in times of wearing them?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallAllaahu 'alayhi wa sallam, is His Slave and Messenger.

Scholars have held different opinions regarding the ruling on the clothes of non-Muslims. Some considered them pure unless proven otherwise. This is the opinion of the majority of scholars. Others viewed that the issue is based on a number of factors, saying that the clothes which do not touch their private parts are pure while those which touch their private parts are impure. In any case, all scholars unanimously agreed that it is permissible to use the clothes of non-Muslims once they are washed because they become (ritually) pure. The difference in opinion among them was only regarding the purity of the clothes before washing them; but once they are washed, they become pure and there is no harm in wearing them at all. For more benefit, please refer to Fatwa 132656.

On the other hand, they are not to be purified with soil; only water is needed. You simply need to wash the clothes with water as if they were the clothes of a Muslim that became impure. The Standing Committee for Academic Research and Issuing Fataawa was asked, "Is it permissible for a Muslim to wear clothes that had been worn by the People of the Book (Europeans Westerners) then they donated them to poor countries and they were washed then people made much demand for them because they are cheap? Is it permissible for Muslims to wear such clothes?" The answer reads, "It is allowable for a Muslim to wear the clothes of a disbeliever unless they are known to be impure (najis). If it is not known whether they are pure or impure, then the assumed original state is that they are pure. If they are washed before wearing them, then that is better."

Finally, the possibility that the clothes are impure due to touching a dog should not be considered, and the assumed state is that it did not happen.

Allaah knows best.