If you pre-mix nutrients, over time they can chemically react and go into forms that are unusable to plants, unsafe for plants, or just fall out of solution.
Kind of like how if you use the same dirt over and over you are going to get weird interactions between whatever nutrient residues are in it.
And as Zandor says, N comes in lots of different forms of which some are NOT good for your plants.
stinkyattic Reviewed by stinkyattic on . Shelf life for nute mixes? Since I only have a handful of plants and feed my plants every two weeks or so, I won't be going through a gallon of the nute mix very quickly. When should I throw out the old stuff and make a new batch? Once a month? Once every three months? Every 6 months? I happen to use fox farm products and they are used for soil applications. Rating: 5