Gemara

(Rav Huna): If one sold a Se'ah of grain, and more than a Rova of Pesoles (foreign matter) was found in it, the seller must give grain in place of all of the Pesoles.

Version #1: This is letter of the law. A buyer pays for good Peros. One does not bother to remove a Rova (or less) of Pesoles. If it is more than a Rova, one toils to remove it. Once he toils, he removes all the Pesoles.

Version #2: This is a fine. A Rova is common, but more than this is not. We assume that he mixed in Pesoles. Therefore he is fined. He must give a full measure of good Peros.

Question (Mishnah): If a Se'ah of grain has a Rova of another species mixed in, one must diminish the other species (lest he plant Kilayim).

Assumption: A Rova regarding Kilayim is like more than a Rova regarding a sale (mid'Oraisa, it is not Batel).

Rejection: No, a Rova regarding Kilayim is like a Rova regarding a sale (mid'Oraisa, it is Batel). It is a stringency of Kilayim that he must diminish it.

Question (Seifa - R. Yosi): He must remove all of it.

If a Rova regarding Kilayim is like more than a Rova regarding a sale, we can say that Chachamim fine what is permitted (less than a Rova) due to what is forbidden (a Rova), and R. Yosi does not fine.

However, if a Rova regarding Kilayim is like a Rova regarding a sale, why must he remove all of it?

Answer: R. Yosi is stringent because it looks like he intentionally sows Kilayim.

Kidushin 39a: Rav Yosef mixed diverse seeds and planted them together.

Question (Abaye - Mishnah): Kil'ayim applies in Chutz La'aretz mid'Rabanan.

Answer (Rav Yosef): That refers to Kil'ai ha'Kerem (Kilayim with grape seeds or in a vineyard). Since one may not benefit from Kil'ai ha'Kerem in Eretz Yisrael, Chachamim forbade it in Chutz La'aretz. One may benefit from Kil'ai Zera'im in Eretz Yisrael, so Chachamim did not forbid it in Chutz La'aretz.

Kilayim 2:1 (Mishnah - R. Yosi): One must remove other species, whether one or two other species are mixed in;

R. Shimon says, this was said only regarding one species;

Chachamim say, whatever is Kilayim joins to a Rova (quarter of a Kav) in a Se'ah (six Kavim).

Rishonim

Rambam (Hilchos Kilayim 2:1): If seeds were mixed with another species, if they were one part in 24, e.g. a Se'ah of wheat in 23 Se'im of barley, one may not plant the mixture until reducing the wheat or adding barley. If he planted it (without doing one of these), he is lashed.

Radvaz: In Bava Basra, we require removing all of the other species. If he does not, he transgresses theft mid'Oraisa. The Halachah does not follow R. Yosi, who requires removing all of the other species.

Question (R. Akiva Eiger): The Gemara concludes (at least according to one version) that letter of the law, it is Kilayim only when there is more than a Rova. It is a stringency to reduce exactly a Rova. If so, why is he lashed for this?

Answer (Mutzal me'Esh 40): Torah law forbids planting the mixture when there is exactly a Rova. However, the Isur to keep what grows is only due to Mar'is Ayin (lest onlookers think that he grows Kilayim). If one reduced, what grows will not look like Kilayim, therefore, he may keep it.

Rambam (2): Everything that is Kilayim with the main species joins to one part in 24. E.g. if 23 Se'im of wheat were mixed with two Kavim (a third of a Se'ah) of barley, two Kavim of lentils and two Kavim of beans, one may not plant the mixture until diminishing from a Se'ah of the other species, or adding wheat. This is because barley, lentils and beans are all Kilayim with wheat.

Kesef Mishneh: The Rambam rules like the first Tana in Kilayim. The Yerushalmi asks, if one was Mevatel an Isur b'Mezid, it is forbidden! It answered that that refers to an Isur Torah. Here, one adds to avoid Mar'is Ayin. Chachamim and the first Tana argue about a Rova of oats and spelt in a Se'ah of barley. The first Tana requires diminishing them. Chachamim do not. They hold that oats and spelt do not join, for they are not Kilayim in a Se'ah of barley. It seems that the Rambam rules like Chachamim, for he discusses three species mixed in that are all Kilayim with wheat. This implies that if one of them were not Kilayim with it, he would not need to diminish them. He rules like Chachamim because their opinion was taught in the plural. Also, since Kilayim is due only to Mar'is Ayin, we follow the lenient opinion. Also, since the entire Rova is not of one species, there is no Mar'is Ayin.

Mutzal me'Esh (40): Why does the Yerushalmi ask about being Mevatel an Isur? Before it is planted, there is no Isur! Because he intends to plant them, this is considered being Mevatel an Isur. I prefer to answer that the Yerushalmi asked according to the opinion that the mixed seeds fell on the ground, and still one may add to be Mevatel the Isur.

Radvaz: The Rambam discussed many species, to disagree with R. Shimon, who says that we are concerned only for one species. The Rambam initially discussed barley. Why did he switch to wheat? It seems that he discussed a normal case. It is common to mix many species with wheat, for wheat is expensive. This is only when the other species became mixed by themselves in the granary. One may not intentionally mix in even one wheat or barley; one is lashed for this.

Question: All seeds do not sprout equally. Perhaps a species was less than a Shi'ur when it was planted, and when it sprouts it will be more than a Shi'ur!

Answer (Radvaz): We calculate whether or not there is a Shi'ur based on what their volume will be after they sprout.

Poskim

Shulchan Aruch (YD 296:1): One who plants different kinds of seeds together in Eretz Yisrael is lashed for "Sadcha Lo Sizra Kilayim."

Beis Yosef (295 DH Harkavas): Kilai Zera'im is permitted in Chutz la'Aretz.

Shulchan Aruch (5): If seeds were mixed with another species, if they were one part in 24, e.g. a Se'ah of wheat in 24 Se'im of barley, one may not plant the mixture until reducing the wheat or adding barley. If he planted it, he is lashed. Everything that is Kilayim with the main species joins to one part in 24. E.g. if 24 Se'im of wheat were mixed with two Kavim of barley, two Kavim of lentils and two Kavim of beans, one may not plant the mixed together until diminishing from a Se'ah of the other species, or adding wheat. This is because barley, lentils and beans are all Kilayim with wheat.

R. Akiva Eiger (Kilayim 2(6)): When there is more than a Rova, letter of the law he must remove all of the diverse seeds! In one answer of Tosfos, this is only according to Version #2 (Rav Huna's law, that one must remove all the Pesoles, is a fine). However, the Tur (CM 229) and many rule like Version #2! We can say that since it is a Safek which version we follow, we do not force a seller to remove all the Pesoles when we know that he did not intentionally mix in. We are lenient about Kilayim, for mid'Oraisa it is not Kilayim even when there is more than a Rova. However, Tosfos' other answer holds that even Version #1 (it is not a fine) holds that the Torah requires removing all the Pesoles when there is more than a Rova. We can say that we are stringent due to a Sefek Sefekah. Perhaps we hold like Version #2. Even if we hold that it is not a fine, perhaps even Version #1 requires removing all the Pesoles! Why didn't the Acharonim discuss this?