The Sefer Yerei’im says that if someone throws seed or pits in a damp field that is normally used for planting he is Chayav for Zorei'a. If the field is dry and it is during the rainy season it is forbidden d'Rabbanan to throw pits in the field because when it rains it will sprout.
The Eglei Tal explains that it is not forbidden Min ha'Torah to throw the pits in a dry field because it is not a certainty that it will rain on Shabbos and therefore it is regarded as an unintentional Melachah. However it is forbidden d'Rabbanan because when it rains after Shabbos it will sprout and a Melachah that begins on Shabbos and is completed after Shabbos is forbidden d'Rabbanan.
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