(Mishnah): If a sharecropper received a field to plant barley, he should not plant wheat. However, if he received it to plant wheat, he may plant barley.

Rabban Shimon ben Gamliel: Both are prohibited.

To plant legumes, he should not plant grains; but to plant grains, he may plant legumes.

Rabban Shimon ben Gamliel: Both are prohibited.

(Gemara) - question: It is understandable that if he took it to plant barley, he should not plant wheat (as wheat weakens the earth); but (according to Rabban Shimon ben Gamliel,) to plant wheat, why not plant barley?

Answer #1: The Mishnah follows R. Ila who holds that even barley damages.

Answer #2: We are referring a place that does not produce double the amount of barley compared to the wheat. However, if it does, he may plant barley (as it does not weaken the land).