(Mishnah): Whether a non-fruit bearing or a fruit-bearing tree, we view it as if there are figs - if they are capable of producing a fig cake the value of 60 Italian Maneh, one may plow the entire Beis Seah for them; less than that, one may only plow whatever is needed for them.

If one of the trees produces a fig cake's amount (of the size for three trees) and the other two don't produce at all; or two together produce a fig cake's amount and one doesn't produce at all - one may only plow whatever is needed, unless they all produce.

If there are between three and nine trees, (they must together produce a fig cake's amount); but if there are ten trees (in a Beis Seah) or more, whether or not they together produce (a fig cake's amount), one may plow the entire Beis Seah for them.

(What's the source for the first Mishnah's teaching that one may not plow a field before Shevi'is?) The pasuk states (Shemos 34:21), "You shall rest from plowing and harvesting''. The pasuk doesn't need to prohibit plowing and harvesting during Shevi'is (as a pasuk elsewhere prohibits it (Vayikra 25:4)). Rather, it's prohibiting plowing before Shevi'is for Shevi'is and harvesting Shevi'is produce on Motzai Shevi'is.

(R. Yishmael): (This pasuk refers to observing Shabbos.) Just as plowing is optional, so too the harvesting here is optional. This excludes harvesting barley for the Korban HaOmer, which is a Mitzvah and permitted on Shabbos.

(Gemara): (The Mishnah taught) Whether a non-fruit bearing tree etc. Why did they require calculating using fig trees? Since their fruits are large and they produce a large yield, as opposed to an Esrog tree that has large fruits but doesn't produce a large yield and an olive tree that produces a large yield but doesn't have large fruit.

(R. Chiya bar Ada): Every tree produces a large amount in the first year but less in the second year; a fig tree produces a large amount every year.

(The Mishnah taught that if one tree produces a fig cake's and the other two don't produce at all...one may only plow whatever is needed, unless they all produce.) R. Baybai said from R. Chanina - as long as any tree doesn't produce less than 1/3rd of a fig cake.

(Zeira bar R. Chanina): If there are nine trees, it's as long as it doesn't produce less than 1/9th of a fig cake. (Similarly, if there are four trees, it's as long as any tree doesn't produce less than 1/4 of a fig cake.)

If there were four trees with space in between them for the ox to pass with its plow and another two which don't have space in between them for the ox to pass, since it cannot pass, we view them as one single tree so that there are now five trees in total and each tree must therefore produce 1/5th of a fig cake.

If there were five trees without enough space between them, it falls into the dispute of R. Shimon and Rabbanan of the Mishnah in Kilayim - If a vineyard was planted with less than four amos between each row, R. Shimon said that it isn't a (Halachic) vineyard and he may plant within 4 Amos of them. Rabbanan said that it is a vineyard and we view the middle rows as if they don't exist and he may plow the entire Beis Seah.

The Mishnah cited the pasuk, "You shall rest from plowing and harvesting''. The pasuk is explaining the reason of the first clause of the Mishnah, that if they don't produce enough, 'he may only plow as much as is needed for them', because the pasuk says, 'you shall rest from plowing and harvesting', so there is only permission to plow before Shevi'is when it's needed by the trees, but not when needed for Shevi'is. The pasuk doesn't need (to prohibit plowing and harvesting during Shevi'is) etc. (See above (d).)

R. Yishmael (above (e)) follows his opinion that the barley for the Omer offering cannot be brought from Suria, only from Eretz Yisrael (and it must be harvested for the sake of the Omer, as the harvesting is an intrinsic part of the Mitzvah of the Omer. He understands that the pasuk was referring to Shabbos, and the pasuk only prohibits optional harvesting, but one may harvest for the Omer on Shabbos.