Bava Batra 38 - Owner's Protest
Presumptive ownership (chazakah) is only valid in the absence of the alleged owner's protest. If the owner protests at least once in three years, the occupant is put on notice, and his chazakah is ineffective. In other words, if the occupant really bought the field, he should keep the deed, and his negligence to do so in inexcusable in the face of a protest, thus his three years of occupancy don't help.
It is sufficient to protest in front of two witnesses, without the occupant present. These witnesses have friends, their friends have friends, and the word travels till it reaches the occupant. If, however, the owner is in a province that has little communications with the province where his field is, he will not bother to protest, since his protest can't reach the occupant anyway, and thus the chazakah can't be established at all. Example of such regions are Judea and Galilee, which at the time had little travel between them.