3,638. Taking Challah From Chadash and Yashan

Hilchos Bikkurim 7:3

Two doughs belonging to the same person and made from the same type of grain combining is as follows: If a wheat dough touches a spelt dough, they combine (because they’re similar). If it touches any other type of dough, they don’t combine. Similarly, if a barley dough touches a dough of spelt, oats or rye, or if doughs of spelt, oats or rye touch one another, they combine.

Hilchos Bikkurim 7:4

A dough of this year’s grain doesn’t combine with a dough of last year’s grain even if they’re of the same type. This out of concern that people mistakenly think that terumah may be taken from new grain for old grain. Accordingly, challah isn’t taken from the place where the two doughs meet. Rather, one must bring another dough, from either this year’s grain or last year’s grain, and add it to combined dough in order to form the requisite volume of one or the other. This is the case when two doughs touch. However, if a person combines flour from the different types of grain and makes a single dough of them, then all five types of grain combine to form the requisite volume to require that challah be taken, as has already been discussed.