1620. Soam means the iron links, which fasten a yoke of oxen to the plough.
"Minstrelsy of the Scottish border (3rd ed) (1 of 3)"
Walter Scott
Full thirty pleughs, harnes'd in all their gear, Could not his valiant noble heart make fear: But wi' his sword, he cut the foremost's Soam In two; and drove baith pleughs and pleughmen home.
"Minstrelsy of the Scottish border (3rd ed) (1 of 3)"
Walter Scott