Kilayim, Chapter Two, Mishnah Three
This mishnah deals with a person who had planted his field with wheat and then changed his mind and decided that he wanted to plant barley. What must he do so that he can plant barley?
Note that I have explained this mishnah according to Albeck. There are other quite different explanations.
1) If ones field was sown with wheat, and he changed his mind and decided to sow it with barley, he must wait until it [the wheat] rots.
2) He turns [the soil] and then he may sow [the barley], if it [the wheat] had already grown.
3) He should not say: I shall [first] sow [the barley] and, then turn [the soil] rather he must first turn [the soil] and then sow.
4) How much must he plow [when overturning the soil]? Like the furrows [that are plowed after the [first] rainy season.
5) Abba Shaul says: [one should plow] so that one does not leave [unplowed] as much [ground] as holds a quarter [kav] to a bet seah.
Section one: Before he plants the barley, he must wait for the wheat seed to start to rot underneath the soil, meaning until it stops being seed. According to the Tosefta, this takes three days in moist ground and more than that in dry ground. After the wheat has rotted, he can plant barley and according to the mishnah the wheat wont grow.
Section two: If the wheat has started to sprout, he must first overturn the soil and then he may plant the barley.
Section three: He shouldnt say that he is going to first sow the barley and then turn over the soil because this would be prohibited. Rather what he must do is first overturn the soil and then he can plant the barley.
Section four: The mishnah now turns its attention to how much one must plow for it to be considered as if he had properly turned the soil. The first opinion is that he must plow furrows like those that are plowed after the first rainy season. These furrows are spread apart one from the other, meaning the mishnah is rather lenient. He need not overturn all the soil.
Section five: Abba Shaul holds that he must make sure that no more than ¼ kav within a bet seah (a field large enough to grow a seah of wheat) is left unturned. Since there are six kav to a seah, this means that he must not leave more than 1/24 unturned. Abba Shaul is clearly stricter than the anonymous opinion in section four.