Chapter 9, Mishnah 8 continues to deal with the relationship between the landowner and the tenant farmer in the field. The Mishnah discusses a case where the farmer received the field in a tenancy agreement in order to plant a specific species in it, and he seeks to change and plant another species.
Receiving a Field to Plant Barley:
The Mishnah states: "Hamekabel sadeh mechaveiro lizor'ah se'orim - lo yizre'ennah chitim" - One who receives a field from his fellow to plant it with barley may not plant it with wheat. This is not a prohibition for fundamental reasons, but for economic reasons: barley depletes the field's resources less than wheat. The agreement between the parties is for a set and fixed amount - it was agreed in advance that the tenant would give the landowner, for example, ten se'ah. The landowner made his calculation and agreed to this return based on the degree of depletion that the field is destined to absorb. But if the farmer plants wheat, the field's resources will be depleted more, while the landowner will receive the exact same set amount. This is a bad deal for him, and therefore the tenant is not permitted to do so.
On the other hand: "Chitim - yizre'ennah se'orim" - If he received it to plant wheat, he may plant it with barley. If the original agreement was that the landowner would receive ten measures of wheat, and from this we assumed that the farmer would plant wheat, he is permitted to plant barley. Why? Because ultimately he is still obligated to pay the ten measures of wheat even though he planted barley, and he will buy the wheat from the market. The landowner therefore receives his payment either way, while the loss caused to him by the depletion of the field's resources is smaller, since barley depletes it less than wheat. For this reason, the matter is permitted, since the landowner stands to gain.
The Opinion of Rabban Shimon ben Gamliel:
Rabban Shimon ben Gamliel argues with this and forbids it. In his opinion, there is more here than the simple economic consideration of that year itself: depending on what was planted the previous year, the landowner might want crop rotation or specifically to avoid it - meaning to diversify the crops or not to diversify them - since this contributes to the long-term fertility of the field. It turns out that the landowner could have additional considerations besides the simple question of what pays more this year, and therefore the tenant farmer is not permitted to change.
Grain and Legumes:
"Lizor'ah tevuah - lo yizre'ennah kitnit" - If he received it to plant grain, he may not plant it with legumes. If the original agreement was that the farmer would plant grain, he is not permitted to plant legumes such as beans and peas in its place, since they deplete the land more than the landowner expected. And when from the beginning of the agreement the landowner receives a set amount of wheat, he will not agree to have legumes grown in his field that deplete its resources.
"Kitnit - yizre'ennah tevuah" - If he received it to plant legumes, he may plant it with grain. If the agreement was for legumes from the outset, the Tanna Kamma says that the farmer is permitted to plant grain in their place, since it depletes the field less. However, Rabban Shimon ben Gamliel forbids it even here: one may not deviate from the agreement, since it is possible that the landowner has considerations of crop rotation and of what was planted the previous year.
The Halachah:
The halachah is generally decided like the Tanna Kamma, meaning that in principle the tenant farmer is permitted to deviate from what was agreed upon as long as the change is solely to the benefit of the landowner. However, in practice, everything depends on two factors:
The language of the contract: If a contract was drawn up defining what the farmer will plant, it is understood that he must abide by it.
Local custom: What is accepted and expected in that society - that also determines the matter.
Therefore, in daily life, it is difficult to say that this law will come into practical application.