farm craft

Ah. Saki, who wrote sort of modern fairy tales, stories of the supernatural, of magic, of fantasy, "tales of wonder". Probably farm craft would be not agriculatural science, but rather the magic needed to make crops grow, the spirits who need to be propitiated to bring just enough rain, who could curse your crops or bless them, the inner spirits of animals, their supernatural guardians, the rituals you have to go thrugh to insure a bountiful crop or fat animals--things like that i.e. fantasy. "Passing lightly over that" probably means just hinting at these kinds of things, because it can be dangerous or disturbing to look deeply at them. (This is all fantasy, remember, Saki did not generally deal with cold, hard reality).