Uktzin, Chapter 3, Mishnah 1. This mishnah rounds off the subject of produce growing in planters by supplying two measurements: how large a hole must be for a pot to count as pierced, and what happens when a pot is filled with earth to its very rim.
The Background
Earlier masechtos have already laid down the basic distinction. A perforated planter, an atzitz nakuv, is viewed by halachah as an extension of the earth itself, so that whatever sprouts inside it holds the status of mechubar lakarka, joined to the soil, and tumah has no grip on it. Seal that pot up, an atzitz she'eino nakuv, and it becomes talush, severed from the ground, so that produce growing in it is indeed susceptible to tumah. Since the whole distinction turns on that perforation, the Mishnah opens with the natural question.
How Big Must the Hole Be?
"Kamah hu shiuro shel nekev?" How much of an opening does the halachah demand? The answer given is a modest one: "kedei sheyeitzei bo shoresh katan", room for a thin root to come out. No wide breach is called for; a passage narrow enough for one slender root to work its way through and reach the soil beneath is all that the halachah requires.
From this measure we understand something further. Vessels made of gelalim, hardened animal dung, and vessels of unfired earth, adamah, may show no hole at all, yet small roots can work their way through their walls. Precisely by this standard they are counted as porous, and they carry the standing of an atzitz nakuv: whatever grows in them is regarded as attached to the ground.
A Pot Filled With Earth to the Rim
A second case follows: "mileihu afar ad sefaso". Someone has in front of him an unperforated planter, an atzitz she'eino nakuv, and he packs earth into it until the soil reaches its uppermost edge. How is such a pot to be classified?
The ruling: "harei hu ketavla she'ein lah lizbez", it is like a board that has no rim. Because he packed the space that would normally hold and receive its contents, the beis kibul, entirely with earth, the vessel no longer has any receptacle to speak of. It is nothing more than a flat surface with no lip at all.
The consequence is that anything sprouting there is judged mechubar lakarka, joined to the soil below, and tumah cannot take hold of it. In the usual case a sealed pot would leave its produce open to tumah; here, however, the earth has swallowed the interior, so the pot no longer functions as a utensil at all, and what grows in it stays bound to the ground.