Orlah, Chapter Two, Mishnah 2. The Mishnah continues with the subject taken up in the previous Mishnah: different prohibited substances that cause one another to become nullified. Terumah has the ability to nullify orlah, and orlah has the ability to do the same to Terumah. How so?
The ratios of nullification and the measures to keep in mind:
Terumah is nullified in one hundred and one.
Orlah is nullified in two hundred and one.
One se'ah consists of six kavim, and this fact is needed in order to understand our Mishnah.
The case in the Mishnah:
One se'ah of Terumah fell into one hundred se'ah of chullin.
Afterward, three kavim - which is half a se'ah, half the amount of the Terumah that fell - of orlah or of kilei hakerem fell into the mixture.
In this scenario, the Mishnah says, the Terumah will have the ability to nullify the orlah, and the orlah will likewise nullify the Terumah. How does it come about that they nullify one another?
The explanation according to Rabbi Yehoshua:
The simplest way to explain this is according to the opinion of Rabbi Yehoshua, mentioned in the previous Mishnah and originating in the first chapter of Terumos. Rabbi Yehoshua holds that Terumah is nullified in one part of a hundred - that is, in a hundred and a bit more.
And what does that mean in practice: if there are ninety nine and a bit more measures of chullin, and one measure of Terumah falls into them, the Terumah is nullified, since when the Terumah is combined with the chullin the total comes to a hundred and a bit more. According to this calculation we include the Terumah itself together with the chullin and relate to the total, and "one in a hundred" means that the total is a hundred and a bit more.
How the orlah helps nullify the Terumah:
According to Rabbi Yehoshua, when a se'ah of Terumah falls into ninety nine se'ah of chullin, and then three kavim, which is half a se'ah of orlah, fall into the mixture, the ninety nine se'ah of chullin together with the half se'ah of orlah amount to ninety nine and a bit more. It turns out that the Terumah has fallen into ninety nine and a half.
Had the Terumah fallen into ninety nine alone, there would not have been a sufficient measure there to nullify it. But now, when it falls into ninety nine and a bit more - and the addition is the half se'ah of orlah - the orlah joins with the chullin and helps to nullify the Terumah.
How the Terumah helps nullify the orlah:
For there are only ninety nine se'ah of chullin here. In order for the orlah, which is half a se'ah, to fall into two hundred parts - that is, into two hundred half se'ah measures - the Terumah is needed. The Terumah contributes its share, because ninety nine se'ah of chullin together with one se'ah of Terumah create a total of one hundred se'ah, and those hundred se'ah are two hundred times the half se'ah of orlah. And so the two of them nullify one another.
In summary: in this Mishnah we learned the scenario in which a se'ah of Terumah fell into chullin and afterward half a se'ah of orlah or of kilei hakerem, and according to Rabbi Yehoshua - who holds that Terumah is nullified in a hundred and a bit more - each of the prohibited substances joins in nullifying the other: the orlah completes the measure needed to nullify the Terumah, and the Terumah completes the hundred se'ah that are two hundred times the orlah.