John Lightfoot on Mark 12:2

[And at the season he sent to the husbandmen.] That is, in the fourth year after the first planting it: when it now was a vineyard of four years old; at least before that year there was no profit of the fruits. "They paint [or note] a vineyard of four years old by some turf [or clod] of earth, coloured; and that uncircumcised with clay; and sepulchres with chalk."

The Gloss is this: "On a vineyard of four years old they paint some marks out of the turf of the earth, that men may know that it is a vineyard of four years old, and eat not of it, because it is holy, as the Lord saith, Leviticus 19:24; and the owners ought to eat the fruit of it at Jerusalem, as the second tithe. And an uncircumcised vineyard," [that is, which was not yet four years old; see Leviticus 19:23] "they mark with clay, that is, digested in fire. For the prohibition of (a vineyard) uncircumcised, is greater than the prohibition concerning that of four years old: for that of four years old is fit for eating; but that uncircumcised is not admitted to any use. Therefore, they marked not that by the turf, lest the mark might perhaps be defaced, and perish; and men not seeing it might eat of it," &c.