Although the Torah clearly states that the blood must be covered with afar – with dirt – the Mishna on today’s daf offers a number of substances that will suffice for this mitzva, including lime, fine sand, fine granulated manure, or crushed potsherd, brick, or earthenware. Rabban Shimon ben Gamliel concludes by teaching that as long as plants can grow in it, it can be used for covering the blood. The Gemara teaches that ashes can be used, as well.
There are other mitzvot that also involve the use of dirt or ashes. Rava teaches –
As a reward for our father Abraham having said: “I am but dust and ashes” (see Bereishit 18:27) his dchildren merited two commandments: the ashes of the Red Heifer, and the dust used in the ceremony of the sota.
The Gemara asks why Rava counts these two commandments and does not include also the dirt used for kisuy ha-dam.
In response, the Gemara distinguishes between the commandment of kisuy ha-dam where the dirt serves no purpose beyond covering the blood, as opposed to the para aduma and sota where the dirt and the ashes play a central role in purifying the defiled and establishing the innocence of the woman.
One point raised by the commentaries is why these commandments are considered a unique reward in response to Avraham’s statement; surely such commandments would have been given in any case in order to deal with these particular situations! The Maharsha suggests that in the case of the para aduma, Avraham’s merit was that these situations could be dealt with relatively simply, rather than through some cumbersome mechanism. In his Etz Yosef, a commentary on the Ein Ya’akov, Rabbi Ḥanokh Zundel explains that with regard to the sota, it is possible that a woman suspected of adultery would have automatically been forbidden to her husband; Avraham’s merit led to the creation of a mitzva whose purpose was to establish marital peace and harmony.