Chapter 9, Mishnah 3. This Mishnah also opens with the same topic, except here we are dealing with a situation of tenancy: the parties agreed to divide the yield of the field between them - let us say fifty percent to each side. And what is the law when the tenant does not want to work and is not willing to invest the required effort? Regarding this the Mishnah says that he is "movirah" - from the word 'bor', an unplowed field - meaning, he leaves it desolate and neglected.
The law of the Mishnah:
The landowner is entitled to sue him in a beis din. The beis din evaluates how much the field would have been fit to produce had the tenant done what was required of him, and the tenant pays out of his pocket the portion that was supposed to go to the landowner.
"Shekach kosev lo":
The Mishnah continues and brings the standard text of the obligation: "im ovir velo a'avid - ashalem meimeitav" - if I leave the field fallow and do not work it, I will pay as if it produced a crop.
Why did the Mishnah need the text of the contract?
The text that the Mishnah brings is the standard agreement between a tenant and a landowner, and it does not mean that a contract was actually signed between them in practice. If they had a signed contract in hand, there would be no need for the Mishnah at all, since it is clear that the contract is binding and it regulates the relationship between them. The novelty of the Mishnah is that since the standard agreement - when they get around to writing it - stipulates that the tenant will work and divide the yield with the owners, it implies that even in the absence of a contract he is expected to work; and if he did not do so, he can be sued and obligated to pay the landowner what he would have received otherwise.
In summary: In this Mishnah we learned the law of the tenant who left the field fallow - we evaluate how much it was fit to produce, and he pays the landowner his share. The basis of the obligation is the standard condition in tenancy contracts, "im ovir velo a'avid ashalem meimeitav", and from the standard text we learn about the actual obligation even in a case where a contract was not written.