משנה על תרומות 9:1

If one’s 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. He turns [the soil] and then he may sow [the barley], if it [the wheat] had already grown. 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. How much must he plow [when overturning the soil]? Like the furrows [that are plowed after the [first] rainy season. 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.