Siman 82

Regarding the matter of excrement, how moist must it be to be considered excrement? We concluded in the Gemara (25a), Rava said: The halakha is that excrement like a shard is forbidden, and urine—as long as it moistens. And what is the case of excrement like a shard? We concluded that even if it is so dry that if one throws it, it crumbles—it is forbidden until it crumbles by means of rolling, for it crumbles more easily by means of throwing than by means of rolling. But when it crumbles by means of rolling, then it is like dust, and it is permitted. Thus the Tur ruled. And the Rambam wrote in the third chapter that if it was drier than a shard, to the point that if he threw it, it would crumble—it is like dust, and it is permitted to recite opposite it. Thus far his words. And also according to the opinion of Rabbeinu Yonah, it crumbles more easily by means of rolling than by means of throwing. Our teacher, the Beit Yosef, ruled according to the words of the Rambam, and our teacher, the Rema, wrote that the primary view is like the Tur. (And the Magen Avraham wrote that the Rambam's opinion is also toward stringency, but he holds like the Ri that it crumbles more easily by rolling than by throwing, see there.)

And specifically crumbling is effective. But if it breaks into two—it is not effective (Magen Avraham). And even if it has no odor, as long as it is not crumbleable—it is forbidden (ibid. and Beit Yosef). And we have already explained that there is a distinction between it and excrement that has an odor, regarding the matter that one only needs to distance oneself from it four amot, as I wrote in siman 79, section 6. And in a time of cold when the excrement has congealed like a stone, since after the cold it will return to its former state—it is forbidden like moist excrement (ibid. subsection 2).

And that which they said regarding urine, "as long as they moisten," this means that even if they were absorbed significantly into the ground, if they would wet the hand—it is forbidden to recite [the Shema] facing them. And some say that it is not forbidden except in the case of "moist enough to moisten." That is to say: when one places another thing upon it, that thing becomes moist and can moisten others. And if there is not that much moisture—it is permitted to recite facing them. And one may rely on this, since it is only of rabbinic origin.