Physan 20 is not a fertilizer. It is a relatively broad spectrum fungicide, bactericide and overall sanitizing agent, to put it briefly. I'm not familiar with the shelf life of this product, even thought I do use it occasionally and keep a 12 oz spray bottle within reach for spot treatment . As far as any fertilizer mixes, (NPK, minor elements, etc.) my understanding is they are all very stable even after being dissolved in water if kept tightly sealed. In other words, don't worry about it.