By Torah law, seed kilayim is prohibited when a person sows two species together or willingly maintains them together. If they grew in his field without his wanting them, he has not violated a Torah prohibition. Nevertheless, because of marit ayin, the appearance of wrongdoing, the Sages prohibited a field owner from leaving kilayim in his field even when he does not want them there. Since the field is open and visible to all, passersby may think that the owner does not care about observing the mitzvot, and this would desecrate God’s name.
Therefore, if one wants to sow his field with wheat and finds that barley seeds have become mixed in with the wheat, he may do so only if the barley remains below a certain threshold. If the barley is not enough to produce one twenty-fourth of the field’s yield – 1/24, or 4.16% – he may sow the mixture as it is. With such a small amount, there is no concern for marit ayin, since anyone who sees the field will understand that the barley is growing there against the owner’s will.
Although one may not intentionally sow even a single barley seed in a wheat field, this applies only when the barley was deliberately added. If one intentionally mixed even one barley seed into the wheat, he may not sow the wheat unless he first removes the barley seed. If, however, the barley became mixed in unintentionally, he does not want it there, and removing it would be burdensome, he may sow the mixture as it is, provided the barley is less than one twenty-fourth of what will grow in the field. At such a small proportion, there is no concern for marit ayin, since anyone who sees the field will understand that the barley is growing there against the owner’s will. If the barley is enough to produce one twenty-fourth of the field’s yield, he must either reduce the barley or add wheat until he is certain that the barley will produce less than one twenty-fourth of the growth in the field.
The same rule governs a field that was sown with one species and later has another species growing alongside it. The determining factor is whether he wants the second species to remain. If he does, the mixture is kilayim, and he must uproot the second species entirely. If he does not want it there, and it makes up less than one twenty-fourth of the field’s growth, he need not reduce it. If it reaches one twenty-fourth, he must reduce it until it is less than that amount.[2]
Because this mitzva is important and those who violate it cause a grave desecration of God’s name, every year on Rosh Ḥodesh Adar, when grain and vegetables were beginning to sprout, the agents of the beit din would urge the Jewish people to be careful about kilayim (Mishna, Shekalim 1:1).
On the fifteenth of Adar, the agents of the beit din would inspect the fields. If they found kilayim, they would uproot the minority species and place it before the owner’s animals. The field owners benefited twice: their fields were weeded, and their animals were fed. They therefore stopped bothering to remove the kilayim themselves. The Sages then enacted that the agents would uproot the kilayim and leave it on the roads, where any animal could eat it. Still, because the field owners were happy that their fields had been weeded, they did not bother to uproot the kilayim. When even this did not solve the problem, since owners still benefited from the weeding, the Sages enacted that the agents of the beit din would declare everything growing in any field where kilayim was found ownerless. From then on, field owners took care to avoid kilayim (Mo’ed Katan 6b). Once the agents of the beit din declared the kilayim ownerless, the field owner no longer violated the prohibition because of it, since the prohibition against maintaining kilayim does not apply to ownerless growth.
[2]. Mishna, Kilayim 2:1; MT, Laws of Kilayim 2:1–10. Some maintain that because the field owner does not want the minority species to remain, it is nullified in the majority by Torah law. Nevertheless, because of marit ayin, the Sages prohibited sowing or maintaining the minority species once it amounts to 1/24, as observers may think that he mixed it in intentionally. When the minority species is less than 1/24, however, there is no concern for marit ayin. People do not normally mix in such a small amount intentionally; on the contrary, an amount that small is generally understood to have become mixed in by accident (Rash; Rosh; R. Ovadia of Bartenura; R. Shlomo Sirilio).
Others maintain that once the minority species amounts to 1/24, maintaining the mixed planting is prohibited by Torah law even when the owner does not want the minority species there. In their view, the Torah prohibition itself is appearance-based: at that proportion, it looks as though he wants kilayim in his field. This is how most commentators understand Rambam’s view. Nor do we say that the minority species is nullified in the majority, for nullification applies only when one component stands in halakhic opposition to the other, such as forbidden food mixed with permitted food. Here, by contrast, both species are permitted in themselves, so one is not nullified in the other. Moreover, a recognizable minority is not nullified. When it is clear to all that he does not want the second species to remain, such as when it is known to be harmful, he need not reduce it even if it reaches 1/24.
