Using soil for purification of the najaasah of a dog
Question

Is it essential to purify the najaasah of a dog using soil? Or can we use other means such as soap?.
Praise be to Allah.In the answer to question no. 41090, we have explained how purification from the najaasah of a dog is done, and that it is obligatory to wash it seven times, one of which should be with soil. 
The scholars differed to whether something else may be used instead of soil, such as soap or other cleaning materials. 
Imam al-Shaafa’i was of the view that it is obligatory to use soil, and nothing else will do, because the Prophet (peace and blessings of Allah be upon him) mentioned soil specifically and enjoined using it. 
The view of Imam Ahmad is that it is permissible to use something other than soil, such as soap etc. 
See: al-Majmoo’, 2/600; Rawdat al-Taalibeen, p. 16; al-Mughni, 1/74; al-Insaaf, 2/248. 
It says in al-Mawsoo’ah al-Fiqhiyyah (10/139): 
If a dog licks a vessel, then in order for this vessel to be purified it must be washed seven times, one of which must be with soil. This is according to the Hanbalis and Shaafa’is.  
If something else is used instead of soil, such as potash (a substance used for cleaning in the olden days) or soap etc, or it is washed eight times, the correct view is that this is not acceptable, because this is the way of purification that has been enjoined in this case and we should do it as instructed as a sign of obedience to Allah, hence nothing else can take its place. 
Some of the Hanbalis said that it is permissible to use something else instead of soil if no soil is available, or if it will damage the place to be washed. But if soil is available and it will not cause any damage, then it is not permissible to use something else. This is the view of Ibn Haamid. End quote. 
Shaykh Ibn ‘Uthaymeen said in al-Sharh al-Mumti’ (1/292) concerning the view that nothing else will do instead of soil: this is subject to further discussion because of the following: 
1-The Lawgiver mentioned soil in particular, and it is obligatory to follow the text.
2-Lotus leaves and potash existed at the time of the Prophet (peace and blessings of Allah be upon him), but he did not refer to them.
3-Perhaps there is some substance in soil that kills the germs in the dog’s saliva.
4-Soil is one of the two things that purify, because it takes the place of water in the case of tayammum if water is not available. The Prophet (peace and blessings of Allah be upon him) said: “The earth has been made a place of prostration and a means of purification for me.”
The correct view is that nothing can take the place of soil, but if we assume that there is no soil available – which is an unlikely possibility – then using potash or soap is better than not using them. End quote.