Rambam - 1 Chapter a Day

As stated in Halachah 6, this and the following halachot apply when the mixture came about unintentionally. If the mixture was made intentionally, even the slightest amount is forbidden.

When quoting this law, the Shulchan Aruch (Yoreh De’ah 297:5) speaks of 24 se’ah of barley.

By removing the kernels.

So that the amount of wheat will no longer be considered significant.
The Jerusalem Talmud (Kilayim 2: I) refers to the general principle (Beitzah 4b) that we do not nullify the existence of a prohibited substance. Why then are we allowed to add more barley and thus nullify the presence of the wheat?
It explains that the mixture is forbidden only because of the appearance created, i.e., as stated in Chapter 3, Halachah 5, mixed substances are forbidden only if the mixture is noticeable to the eye. Hence, if the barley is added before the mixture is sown, when the crops grow, the mixture will not be noticeable. Hence, there is no prohibition involved (Rambam LeAm).

There are those who interpret the Jerusalem Talmud as implying that in this instance,
the prohibition is merely Rabbinic in origin. Hence, they question this ruling. According
to the explanation in the previous note, however, there is no difficulty.

There are six kab in a se'ah. Thus there is not enough of any one different species for it to be considered as a forbidden mixture on its own. Nevertheless, since when combined, there is a significant amount of seeds of other species, the mixture is forbidden.

The seeds of garden vegetables are far smaller than those of grains and legumes. Thus a far lesser amount is necessary to produce a crop. We must take this factor into consideration in these calculations [Rav Ovadiah of Bartenura (Kilayim 2:2)].

I.e., a square 50 cubits by 50 cubits, or any area of 2500 sq. cubits.

According to the Rambam’s statement in Halachah 1, the intent is not a se’ah, but one-twenty-fourth less.

One sixth of a se'ah.

I.e., we are speaking about sowing seeds that are bulkier than the seeds of grain and a larger amount is necessary to be sown in the area in which a se'ah of grain seeds would be sown.

I.e., one twenty-fourth of two se'ah, double the ordinary amount.

I.e., after the two species were mixed together accidentally, the person sowed the mixture with the intent of benefiting from both species (Kessef Mishneh).

I.e., with the intent that each one grow in a separate place.

And he does not desire to benefit from the intermingled species (Turei Zahav 297:3).

In the initial halachot of this chapter, a Scriptural prohibition was involved, as evident from the fact that the person was punished by lashes. For those halachot were speaking about instances where the person desired to benefit from the forbidden mixture. In this instance, we are speaking about an instance where he did not desire to benefit. Hence there is only a Rabbinic prohibition involved (Turei Zahav 297:4). Alternatively, the explanation given in note 3 can be employed here as well.

A pungent herb also known as fenugreek.

Instead of for animal fodder, as is also done on occasion.

I.e., it is not sown in a haphazard manner [the Rambam’s Commentary to the Mishnah (Kilayim 2:5) based on Bava Kama 81a].

That place has to be flat and smooth so that the grain can be threshed (ibid.).

For anything more than one species would be considered kilayim.

I.e., grafting vegetables onto any other tree is also forbidden, as stated in Chapter 1, Halachah 5.

If, however, they are not tied as a bundle, more stringent laws apply as indicated in the conclusion of the halachah. The rationale is that the fact that the vegetables are tied in a bundle indicates that they were placed in the earth for storage in that era, there was no refrigeration and not so that they would grow.

The Rambam mentions a vine, because that is the wording used in his source (ibid.), but it applies to other trees as well.
From the connection to the initial clause of the halachah and from the fact that other trees aside from a vine are mentioned, we can conclude that this clause also focuses on grafting. For there is no prohibition against planting vegetables in an orchard of trees other than a vineyard. What grafting is involved? It is possible to explain that the intermingling of the roots of the vegetable and the roots of the tree or vine could be considered as grafting (Rambam LeAm).

Nor those involving the Sabbatical year, the tithes, or the Sabbath labors (ibid.).

For this is not the ordinary way to plant these vegetable.

The Rambam’s ruling is based on his interpretation of Kilayim 2:5. The Ra’avad objects, offering a different understanding of that mishnah. The Shulchan Aruch (Yoreh De’ah 297:12) follows the Rambam’s understanding.

For after these steps, the seeds that were planted initially will no longer grow [the Rambam's Commentary to the Mishnah (Kilayim 2:3)].

For the plowing will destroy the plants that have grown. In contrast to the previous halachah, in this instance, he does not have to wait three days, because we do not think that the seeds will produce another plant.

This is the time when plants first begin to grow and it is possible to see whether one’s fields and orchards have mixed species growing in them or not.

And hence, they would be lax in doing so themselves.

See the Sefer Meirat Einayim 273 :27 which states that according to the Shulchan Aruch (and the Rambam) the court’s agents must declare the field ownerless. According to the Tur, by contrast, if a person sees mixed species are growing in a field, he may take possession of it, for it is considered ownerless even though the court has not made a declaration to that effect as of yet.