The next example of an agricultural restriction is plowing, mentioned in tractate Moed Katan 13a: "A field that has been cleaned of thorns during the shmitta year can be planted the next year. But if it has been improved during the shmitta year it must not be planted the next year. And R' Yosi ben Hanina says that we hold that if the farmer who improved the field died-his son may plant." Cleaning the field of thorns or rocks is forbidden during shmitta, but our Rabbis did not impose a penalty on violators. If the field was improved more than that, our Rabbis imposed a penalty on that particular farmer that he may not plant seed the next year on his field. He may not rent it out either. However, if the farmer died, the penalty vanishes and even a son who received the field by inheritance may plant. After looking at the Rambam (Hilchos Shmitta and Yovel ch. 1 halacha 11), it seems that there is a story behind this penalty. The Rambam wrote that the non-Jewish kings demanded that the farmers produce food for their armies. When the shmitta year came, the Jews were forced to plow and plant. However, they could only do so for the king and not for themselves. Anyone who "improved" his field more than he was forced to do, was penalized by our Rabbis. The RA'AVAD on the Rambam had a different interpretation. He wrote that the non-Jewish kings "taxed" the farmers by taking their produce. If there was no produce to take, so they stole their belongings. So the Rabbis permitted planting for the king, but not for themselves. The Rambam seems to permit planting (when forced by the king) in all areas of the land of Israel. The RA'AVAD forbids within the smaller boundaries of the land of Israel that were in force during the second Temple, but permits planting (when forced by the king) outside those boundaries. There is a gemorra in Sanhedrin (page 91a) about Alexander Mukdon threatening the descendants of the Cananites that came from Africa to demand the land of Israel back. They realized the danger and fled. The Jews took the produce in their fields and the gemorra adds that it was the shmitta year. This is one proof that the Jews during the period of the second Temple kept the shmitta year in spite of the difficulties involved.