Sheviis, Chapter 3, Mishnah 4. The Mishnah teaches us the concept of "medayer" - one who pens animals in his field, a word that comes from dir, an enclosure or pen. It was common practice to pen animals in the field, and once they stayed there for a while they produced a significant amount of manure and fertilizer. The question, then, is how this can be done in a way that does not look as though a person is fertilizing his field.
The size of the pen and how it may be expanded:
The Mishnah rules that one who makes a pen in his field makes a "sahar" - an enclosure - the size of a "beis sasayim", and limits the pen to that size. The animals stay in that space and produce a great deal of manure there. After they have been there for a while and the manure has accumulated, he may uproot three of the pen's sides and leave the middle one in place; the three walls he uprooted he sets up on the other side of that middle partition, thereby creating an additional pen on the other side.
The result is that he has penned an area of four se'ah in total: the first pen the size of a beis sasayim, and the second pen on the other side also a beis sasayim.
The area that may be penned:
The Tanna Kamma: "beis arba'as se'in" - this is the maximum area that may be penned.
Another opinion: "beis shemonas se'in" - according to this opinion he may do this on all four sides. And in truth the area involved is even larger than that: eight se'ah in addition to the area in the middle, for one who makes all four sides around the middle area reaches a total of ten se'ah.
When his entire field is that size:
According to the Tanna Kamma, who holds that the maximum area that may be penned is four se'ah, if a person's entire field is that size, it cannot be that he should end up fertilizing his whole field. In such a case he must leave over at least a small portion that is not included in the pen and has no manure in it, and this is "mipnei mar'is ha'ayin" - so that it should not look as though he set out to fertilize his entire field.
What is the law regarding the manure that accumulated in the pen?
May he leave it where it is? The Mishnah rules that he may not. He must remove the manure from the pen and place it in the field "k'derech hamezablin" - in the manner in which people usually make piles of manure. This refers to the halachah taught in the two previous Mishnayos: that one makes three piles for every beis se'ah. He may not leave the manure inside the pen, but must take it out and form from it the piles discussed earlier.
In summary: In this Mishnah we learned the law of the medayer - one who makes a pen for his animals in his field: he makes a sahar the size of a beis sasayim, uproots three sides and leaves the middle one in place, and thus ends up penning a beis arba'as se'in (and according to the other opinion a beis shemonas se'in, and in practice even more than that); if his entire field was that size, he leaves over part of it because of mar'is ha'ayin; and the manure that accumulated he must remove from the pen and form into piles in the manner of those who fertilize.