Yes, NITRIFYING is a Scrabble word!

Letter values in Scrabble US: N 1, I 1, T 1, R 1, I 1, F 4, Y 4, I 1, N 1, G 2.

Is it a valid word?

NITRIFYING is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

convert into nitric acid, nitrous acid, or nitrate, especially with the action of nitrobacteria
treat with nitrogen or a nitrogen compound
treat (soil) with nitrates

Not playable in Scrabble US: NITROGENIZE, NITROGENISE.