Severus closed his eyes with a nod at her confused expression, finally cottoning on. "I meant that you didn't have to copy it down now, since I'm lending you the book and all. You can copy it down later, at your leisure."
When she asked about the Pritchard number, however, he frowned. He could have sworn they'd already covered this in Herbology, though perhaps he was merely remembering incorrectly. He did already have all the annotations in his book, but that didn't necessarily mean anything, because he already had annotations in his mother's N.E.W.T.-level Potions text, and he hadn't even sat for his O.W.L.s yet. This time around, anyway.
But he was sure that Pritchard number question about monkshood and wolfsbane had come up on his O.W.L.s, or had it been his N.E.W.T.s? Damn! No, it had to be, because he had it in his notes that Professor Sprout was discussing why dragon dung wouldn't work as fertilizer for belladonna. Now he was confusing himself, as well. Maybe they ought to move on to another area soon?
"It's the number to classify plants according to the strength of fertilizer they can tolerate, yes," he answered, in response to her question. "The Pritchard Scale is a measure of fertilizer strength, and a plant's Pritchard number tells you what strength of fertilizer you can use without killing it. Though, amongst all the suitable fertilizers with the right Pritchard number, there are several one can choose, for other desirable properties." God, he sounded as if he were giving a lecture, now. Too many years in front of a Potions class, obviously.