פירוש על חולין 6:7

Introduction The Torah states that the blood should be covered with “earth (afar).” The rabbis expand this to include anything in which plants grow, just as they grow in earth. The blood may not be covered with something in which plants do not grow.

With what may one cover up [the blood] and with what may one not cover it up?
One may cover it up with fine dung, with fine sand, with lime, with white clay, or a brick or an earthenware stopper [of a cask] that have been ground into powder. All of these substances fit into the category of “earth” because plants may grow in them. Hence, they can be used to cover the blood.

But one may not cover it up with coarse dung or coarse sand, or with a brick or an earthenware stopper [of a cask] that have not been ground into powder. In contrast, these substances are coarser or completely solid and hence cannot be used to cover the blood. We see that the criterion for whether a substance is usable is not merely the nature of the substance itself but also the state that it is in.

Nor may one cover it with a vessel. Covering the blood with a vessel does not fulfill the mitzvah.

Rabban Shimon ben Gamaliel stated a general rule: one may cover it with anything in which plants would grow; but one may not cover it with anything in which plants would not grow. This is the general rule that governs the reasoning behind the laws above.