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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.
Not sure, but why don't you scale the mix down to what you will be using?
If you are talking about your mixed nutrients then only mix what you need.
If you are asking how long store bought nutrients will last before you need to replace the unused portions then about a year for a liquid is fine but much longer then that and some nutrients like N will start to break down.
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.
one thing i can say, is if youre talkin about "mixed nutes" id agree with zandor... just divide your gallon formula by 4 and use quarts to feed instead of mixing gallons....... but if you have to... just keep your mixed nutes out of the light...
store only in te dark, once i mixed some nutes in a 20oz. bottle and left it in the room under the lights and it started to grow a floating mold in it.....
yuck