If earth is replaced by another substance (like soap or bran) in the required washing for dog/pig impurity, is the requirement fulfilled?
Chapter on what purification is achieved with regarding water
Al-Mughni
Book of Purification
Primary text
There are two opinions regarding substituting earth with another substance. One opinion states it is insufficient because the purification in this matter was commanded with earth, and nothing else can replace it, similar to Tayammum (dry ablution). The second opinion states it is sufficient because these other substances are more effective in removal than earth, implying the mention of earth was merely to indicate the type of cleaning required. This is analogous to using a stone for Istinja (cleansing after relieving oneself).
Supporting text
Regarding adding an eighth wash instead of using earth, the soundest position is that the eighth wash does not substitute for the earth. If the intent was to strengthen the water's efficacy, the eighth wash does not achieve this more than combining the required washing elements. If the command is devotional (ta'abbudi), substitution is forbidden. Some scholars state substitution is only allowed if earth is unavailable or spoils the washed item, but not when available without harm.