Gemara

Question: Why doesn't Kisuy ha'Dam apply to Kodshim (i.e. Melikah of birds)?

Suggestion: It does not apply due to R. Zeira's law;

(R. Zeira): "He will cover it in earth" teaches that earth must be put below (so the blood falls on it) and above (after the blood fell).

This cannot be done for Melikah (which is done on the Mizbe'ach)!

Question: Even if the blood cannot be covered from below, it can be covered above!

(Beraisa - R. Yonason ben Yosef): If a Chayah was slaughtered and a Behemah was slaughtered over it (and its blood fell on the Chayah's blood), no other cover is needed;

If a Behemah was slaughtered and a Chayah was slaughtered over it, the (Chayah's) blood must be covered.

Answer: Since Dam Kodshim cannot be covered from below, there is no Mitzvah to cover it above;

(R. Zeira): If a Minchah is Ra'uy l'Bilah (it could be mixed), mixing is not Me'akev. If it is not Ra'uy l'Bilah (e.g. it is too big), mixing is Me'akev (so it is Pasul).

Question: We should scrape the blood off the Mizbe'ach, and then cover it!

(Mishnah): Blood that spurted far away and blood on the knife must be covered.

One must scrape the blood off the knife to cover it. Likewise, we should scrape the blood off the Mizbe'ach!

Answer: "He will spill (the blood) and cover" teaches that the blood must be proper to be covered immediately after Shechitah;

For Korbanos, Kisuy cannot be done until the blood is scraped off the Mizbe'ach.

87a (Mishnah): If one covered the blood and it became exposed, he need not cover it again;

If the wind (blew something and) covered it, he must cover it.

30b: R. Yonah bar Tachlifa shot a checked arrow at a bird, and thereby slaughtered it.

Question: R. Zeira taught that one must put earth underneath beforehand!

Answer: R. Yonah was Mazmin (designated or prepared) all the earth of the valley to cover the blood below.

Rishonim

Rosh (6:10): Putting (spilling) the blood on loose earth is considered like putting earth below. He need not put earth by hand. Even if he found loose earth and slaughtered over it, this suffices. R. Yonah was Mazmin all the earth in the valley, i.e. he ground it up to make it loose earth.

Rashi (31a DH d'Mazmin): R. Yonah moved and ground up all the earth in the valley. Alternatively, it was loose earth, and he verbally designated it.

Tosfos (83b DH Tzarich): It is a Mitzvah to put the earth above. If he covered the blood and it became uncovered, he is exempt. If the wind covered the blood, blood and it became uncovered, he must cover it again.

Tosfos (83b DH Shachat): Why is one exempt when he slaughtered a Behemah over blood of a Chayah? Why do we exempt (from covering below) when he slaughtered a Chayah over blood of a Behemah? He should scrape off the blood and cover it, like blood on the knife!

R. Akiva Eiger: I answer like the Ro'oh, that it was clear to the Gemara that one must scrape. One cannot cover it while it is on the knife. Do not ask from covering without loose earth. There he need not scrape, for it is Ra'uy l'Bilah. This is why we need not scrape when he slaughtered a Chayah over blood of a Behemah. We asked from Kodshim, in which we seek to totally exempt since there is no earth below and it is not Ra'uy l'Bilah. We asked that we can scrape it and cover it, just like blood on the knife, which cannot be covered on the knife. The Rashba holds that blood can be covered on the knife. The Gemara inferred from the words 'he is obligated to cover.' It did not say 'he covers with earth.' Why didn't we infer this from the Beraisa, which says 'if he slaughtered a Chayah over blood of a Behemah, he is obligated to cover'? I say that the inference is only that it did not say 'he covers with earth', lest we think that he must scrape in order to put earth below. We can say that the Tana of the Beraisa relied on the case of Dam Behemah over Dam Chayah, which he (totally) exempts and does not require scraping. This shows that also for a Chayah over a Behemah, he must cover, but he need not scrape. A Chayah covered by Dam Behemah is exempt because it is like what was covered by wind. We can say that covering applies only with earth. Since we exempt, this shows that we do not require scraping at all for covering. Regarding blood on the knife, one could err and say that he relies on scraping. Therefore, it should have explicitly said 'he covers with earth.' We can explain the Sugya that initially we asked that we should cover at least on top. Whether we cover Dam Chayah slaughtered over Dam Behemah only on top, or we scrape and cover also on bottom, the same should apply to Dam Kodshim. We answered that there we cover without scraping. Here it is exempt, for it is not Ra'uy l'Bilah. We asked from blood on the knife, for which we scrape, so also here we should scrape! If so, the same applies to Dam Chayah slaughtered over Dam Behemah! Dam Behemah over Dsam Chayah is exempt because it is covered. Something covered through wind is exempt even if it was not covered with earth.

Poskim

Shulchan Aruch (YD 28:5): There must be loose earth below, and one must verbally designate it. Some say that one need not (designate it).

Beis Yosef (DH Keitzad): Rashi explains that he must slaughter over loose earth, and not over hard ground. Tosfos and the Rosh say that no designation is needed. Rashi and Ba'al ha'Itur require designation.

Bach (6): The Agur (1100) says that the custom is like the Rosh. The Agudah (Chulin 82) says that it is good to be stringent like Rashi and put loose earth or verbally designate it

Shach (12): Rashi connotes that one must verbally designate the loose earth when he found it there, for if not, he did not do anything. All agree that one must put the earth above. I say that they argue only about the ideal Mitzvah. B'Di'eved, all agree that one need not put even the earth above. We hold (Sa'if 11) that if the wind covered the blood, he is exempt. This is an explicit Mishnah!

R. Akiva Eiger: When the wind covered it, he did not fulfill the Mitzvah. However, when the blood is covered, there is no Chiyuv to cover. This is why if it became exposed, he must cover it. Likewise, if he did not put earth below, he is not obligated to cover (below), but if it became exposed (below), he must cover it! If so, the Mitzvah is specifically that he put earth. Similarly, we can say that when he found loose earth and slaughtered over it, he did not fulfill the Mitzvah of covering, and if it became exposed, he must cover it, and when necessary, he must scrape the blood and put earth below. A simple application is if he slaughtered over loose earth, whether he must scrape the blood and put loose earth. Whenever the blood is exposed and he must cover it, he must put earth below. It is truly as if there were no earth below; he must scrape the blood, like in Sa'if 7. The first opinion here holds similarly when there was earth below by itself. It is as if there was no earth below.

Shulchan Aruch (7): If he slaughtered and there was no earth below, he must scrape the blood and put it in loose earth and cover it with loose earth above.

Beis Yosef (DH Shachat): It seems that the Tur learned from 83b, in which we asked that we should scrape the blood off the Mizbe'ach, and then cover it (also below)! I later found that the Rashba learned this law from this Gemara.

Beis Yosef (DH v'Yesh): The Kolbo brings from Sefer Me'oros that blood that splashed or is on the knife must be covered, but one need not scrape it from there. One covers it in its place. This is difficult. Perhaps he learns from 84a, which exempts Korbanos from Kisuy due to "he will spill and cover", and Kisuy cannot be done until the blood is scraped off the Mizbe'ach. He holds that this rejects what was said above, that blood that spurted far away and blood on the knife must be covered. (I.e. the Halachah does not follow that Mishnah.) This is no proof. Tosfos and the Rashba say that Kodshim are different, for always the blood must be scraped (therefore they are always exempt, but Chulin must be covered, even in a case when it must be scraped first in order to cover it).