15. One should not toss kernels of grain to fowl on damp ground [on Shabbos], because it is possible that some of them may remain and sprout [due to the damp conditions].
(1) One of the 39 "Avos melachos" (prohibited categories of activity on Shabbos) is planting ("Zoreah"). This category covers any activity that enhances growth, therefore, throwing seeds, on Shabbos, into an area in which they may potentially sprout is prohibited. The Shulchan Aruch (336:4) permits throwing grains to fowl if the grains will be totally consumed within two days, because it takes at least 3 days for grains to sprout. Furthermore, the Shulchan Aruch permits throwing these grains into an area where people walk, because the grains will not sprout in that sort of environment.