פירוש על בבא מציעא 9:8

המקבל שדה מחבירו לזרעה שעורים – in a tenancy – such-and-such barley or wheat or money.

If one leased a field from his fellow with the condition that he sow barley, he may not sow wheat; [But if he leased the field with the condition that he sow] wheat, he may sow barley. Rabban Gamaliel forbids this.
If one leased a field from his fellow with the condition that he sow grain, he may not sow beans; [But if he leased the field with the condition that he sow] beans, he may sow grain. Rabban Gamaliel forbids this.
Both sections of this mishnah in essence teach the same law, so we will explain them together. If one leased a field with the stipulation that he sow a certain type of produce he may not sow something else that is more damaging to the soil of that field. He may, however, sow something that is less damaging to the soil. For instance, if he promised to sow barley he may not sow wheat since wheat is more damaging; but if he promised to sow wheat he may sow barley, since barley is less damaging. Similarly, if he promised to sow grain he may not sow beans which are more damaging; but if he promised to sow beans he may sow wheat which is less damaging. This is the anonymous opinion expressed in the first two clauses of each section. Rabban Gamaliel disagrees and states that one may not change the condition under any circumstance, even if the new seed will be less damaging to the field. Although there may not be exist a specific reason why in certain cases one should not change the conditions of the agreement, one is in general forbidden to do so.

לא יזרענה חטים – as the wheat weakens/deteriorates the land [more] than the barley.

רשב"ג אוסר – that it is hard for the land when they sow it one year with one species and another year with a different species. But concerning the legal decision, everything is according to the land/property and according to the place. If he made a condition with him in a matter that weakens the land a bit, he is not able to change and to plant something that weakens it a lot, but the opposite is permissible.