Reference for Bava Metzia 217:2

<big>גמ׳</big> אמר אביי בקורות שקמה אין לו בשבח שקמה יש לו ורבא אמר אפילו בשבח שקמה נמי אין לו

HE MUST NOT SOW IT WITH FLAX,<span class="x" onmousemove="('comment',' Because it greatly impoverishes the soil, which does not regain its fertility until after seven years. This can apply only to a lease on a fixed rental, for if on a percentage basis, the lessor himself profits thereby (Rashi); v. p. 597. ');">2