From The Collaborative International Dictionary of English v.0.48:
Soil \Soil\ (soil), v. t. [imp. & p. p. Soiled (soild); p. pr.
& vb. n. Soiling.] [OF. saoler, saouler, to satiate, F.
so[^u]ler, L. satullare, fr. satullus, dim. of satur sated.
See Satire.]
To feed, as cattle or horses, in the barn or an inclosure,
with fresh grass or green food cut for them, instead of
sending them out to pasture; hence (such food having the
effect of purging them), to purge by feeding on green food;
as, to soil a horse.
[1913 Webster]