Kilayim, Chapter Two, Mishnah Nine
This mishnah teaches how many different species of seed one might sow within one field the size of a bet seah (a field large enough to produce a seahs worth of grain).
1) One who wishes to make his field into square plots [each sown] with a different species, he should divide it into twenty-four square plots for a bet seah, a square plot per bet rova, and he may then sow in each whatever species he wants.
2) If there is one square plot or two [inside his field], he may sow them with mustard, but if there are three he may not sow them with mustard, since it looks like a field of mustard, the words of Rabbi Meir.
3) But the sages say: nine square plots are permitted, ten are forbidden.
4) Rabbi Eliezer ben Yaakov says: even though the whole of his field is a bet-kor, he may not make within it more than one square plot.
Section one: A bet seah is fifty cubits by fifty cubits, 2500 square cubits. If you were to divide this size field into twenty-four plots, each being able to contain a quarter of a rova (a rova is 1/6 of a seah), each plot would be 104 1/6 square cubits, 10.2 x 10.2. Each of these plots is considered a separate field and therefore he can plant whatever seed he wants in each. Since they are distinct plots and the whole field is set up this way, it is clear to anyone who sees the field that this is not kilayim.
Section two: The mishnah now refers to a case where there are a limited number of plots within a field planted with a single species. Rabbi Meir says that one can plant one or two plots of mustard within the field and this still doesnt look like kilayim. However, if he plants three plots of mustard this looks like a field of mustard mixed in with a field of grain, and there is a problem of looking like kilayim. This is a problem specifically with mustard because people dont typically plant this much mustard within a grain field. If it was another species, he could plant more plots and they wouldnt look like a full field of one species intermingled with another.
Section three: The sages rule more leniently than Rabbi Meir, allowing one to plant up to nine plots of a different species within a field the size of a bet seah. However, ten square plots looks like a field, and is therefore prohibited. Nine plots can be arranged in the following way so that each plot does not come into contact with each other:
Note that according to other opinions, there are other ways to draw this. In any case, what is most important is that since no two plots of one species are adjacent to each other, it doesnt look like two different kinds of fields intermingled.
Section four: A bet-kor, which is 75,000 square cubits, is much larger than a bet-seah, which is only 2500 square cubits. Nevertheless, Rabbi Eliezer ben Yaakov says that no more than one plot of a different species may be planted there. This explanation, while if fits the words, is hard to understand. Therefore, Albeck explains that Rabbi Eliezer ben Yaakov looks at the whole field as one square plot, meaning that even in a gigantic field such as a bet-kor, one can sow only one type of seed. He doesnt allow any plots of another species.