Can someone who rents a field decide to plant something other than what was agreed upon?
According to the last Mishna on today’s daf, if the agreement was to plant barley he cannot plant wheat, but if the agreement was to plant wheat, he would be allowed to plant barley. Similarly, if the agreement was to plant grain, he cannot plant legumes (kitnit), but if they had agreed that he would plant kitnit, he can choose to plant grain. In both of these cases Rabban Shimon ben Gamliel disagrees, ruling that no change can be made.
The Tanna Kamma‘s reasoning is that certain crops weaken the ground more than others. Since the individual who is planting is only renting the field, he cannot sow a crop that will weaken it more than what they had agreed upon. If, however, he wants to sow a crop that will do less damage to the ground, he would be permitted to do so.
Although there are some manuscripts that have the Tanna Kamma permitting the planting of grain instead of the agreed-upon legumes, the reading that appears above would not seem to be the correct one, since kitniyot – leguminae – are well known as adding to the well being of the soil. The root of leguminae has bacteria that allow them to extract nitrogen from the air, making it more fertile.
The Gemara explains Rabban Shimon ben Gamliel’s position as being based on the passage in Zephaniah (3:13) forbidding people from being deceptive under any circumstances, even if in the end something positive will come out of it. Another explanation raised in the Gemara is that the owner of the field may have a specific reason why he wants a certain crop to be planted. According to Rashi, he may want the field to grow the same crop every year. Tosafot suggest that the owner may want to rotate his crops, in order to be sure that the soil retains different types of nutrients.