Kilayim, Chapter Four, Mishnah Four
Our mishnah continues to discuss what constitutes a legal partition in order to separate seeds on one side from a vineyard on the other. If the partition is sufficient, it will be permitted to sow seeds on one side and a vineyard on the other.
1) A partition of reeds: if between one reed and another there are less than three handbreadths, [the space] through which a kid (young goat) could enter, it counts as a [legally effective] partition.
2) A [stone] fence which has been broken through up to [the length of] ten cubits, [the breach] is [regarded] as a doorway; more than that, opposite the breach it is prohibited [to sow seeds].
3) If there were many breaches in the fence, if that which remains standing exceeds that which is broken through, it is permitted [to sow there].
a) But if the breached sections exceed those which remain standing, it is forbidden [to sow opposite the breaches.]
Section one: For the partition of reeds to count, there must be less than three handbreadths between each reed. Three handbreadths is considered by the mishnah the amount of space that a kid (goat) would need to enter. (Goats would have been the pests farmers would have wanted to most keep out of their fields). Note that in this case there is more open space than filled space because the reeds are surely less than three handbreadths. However, since there is no open space more than three handbreadths, the entire partition counts as a fence.
Section two: A stone fence can have a breach up to ten cubits (about five meters) in length and still count as a fence. In such a case one could plant seeds even opposite the breached area, because this breach is treated as if it were a door. However, if there is more than ten cubits of open space, he cant sow seeds opposite the breach.
Section three: If there were many breaches each less than ten cubits, the rule is that for the fence to count, the standing portion must exceed the breached portion. If it does, then he can sow seeds even opposite the breached portion. However, if the breached portion exceeds the standing portions, then he cant sow seeds opposite the breached portions, although he can still plant opposite the standing portion.