NOTES

משנה ג

שתתליע - This takes three days in damp soil, longer in dry conditions (רע"ב, רדב"ז, רש"ש), the מלאכת שלמה says that when replacing barley with wheat, a shorter wait is required as barley germinates faster  The reason why one must wait for this is because if the soil is turned sooner, the wheat seeds will survive and then take root (רדב"ז). 

ויופך - The ראב"ד brings a תוספתא that says these as alternatives: either wait for the wheat seeds to germinate, or turn the soil.  While not adopted as the הלכה, it is possible that the reasoning behind this is that once it has begun to extend roots, the wheat is no longer a 'seed' prohibited as כלאי זרעים (although once the plant has begin sprout, it is once again susceptible to the prohibition).  Thus, one has a choice:  either plant in that moment when the wheat do not have the status of זרעים and cannot create כלאים, or plow the field.  Although plowing too early will not kill off the wheat seeds, doing so is sufficient according to the ראב"ד to evince the famer's intent to not have wheat grow there.

לא יאמר אזרע ואחר כך אופך - The הוה אמינא here is less than obvious, as if one cannot plant a new species without first plowing, and then only after the seeds of the old species have germinated, why would one think they could do so after the old species has sprouted?  The רדב"ז says that because the wheat has sprouted and is visible, one can completely eradicate it by plowing (which is not as easily done with seeds still beneath the ground, even if they have begun to germinate), and since you can completely eradicate the old crop you might think that you can wait do so until after you've planted the new species.  Note that according to the ראב"ד above, this makes perfect sense:  The משנה first gives you the option of either plating while the wheat does not have the status of seeds, or else plow up these seeds. However, the משנה now tells us, once the wheat has begun to sprout you cannot plant like you could when they were germinating, and you must plow, and plow first.

כתלמי הרביעה - The רע"ב says that these are large furrows that were plowed after the first rainfall; the רמב"ם says that they were plowed before the rain to help the field retain water.  Note that this tilling of the soil will not eradicate every single wheat seed, but is still deemed sufficient.

שלא ישייר רובע לבית סאה - Abba Shaul's position is that the field should be plowed so that 1/24 of the seeds that were planted there do not survive.