There are three [types of] beis hapras. One who plows a grave, [where the law is that] any species of tree can be planted, but no seeds can be sown except for seeds [from plants] that are cut; and if he pulled out [the plant], he piles its produce into [the field itself] and sifts it through two sifters. These are the words of R’ Meir. How-ever, the Sages say: grain with two sifters and legumes with three sifters. The straw and the bean chaff are [then] burnt. [The field] spreads tumah through touch or carrying, but does not render tamei through ohel.