1. The Mishnah (84b) knows that the roots of the crop in the middle will not blend with the roots of the crops to its sides.
2. The verse regarding encroaching on a neighbor's field is also based on knowledge of agriculture.
3. The Gemara discusses early tribes that were experts in the agricultural properties of their land.
4. Rav Acha bar Yakov gives a different reason for the name of one of these early tribes.
5. Rav Ashi: The plot described in the Mishnah is referring to the planted area.
A BIT MORE
1. The Gemara proves at length that the Rabanan were very familiar with agriculture (as explained below), and therefore there is no reason to doubt how they knew this exact amount (see Daf 84, #5).
2. Rebbi Yochanan derives that the prohibition includes weakening a neighbor's field by planting things close to his border. This depends on expert knowledge of agriculture.
3. This is derived from the verse describing them as "dwellers of the land." Rebbi Yochanan explains that the Chori were able to know their land (i.e. what crops were best to plant where) by smelling it. The Chivi were able to obtain this knowledge by tasting their land.
4. The Chori were so called because they were liberated ("Chorin") from their land by the sons of Esav.
5. It was common practice to leave a small amount of unsown land for every area that was sown in order to facilitate watering and working the sown area. Rav Ashi points out that this is area is not included in the Mishnah's measurement of a six by six Tefachim square in which one may plant five different types of vegetables.