Leftover Fertilizer

Do you recommend using left over fertilizer from one year to the next? Does it loose its effectiveness over a winter for the following year application?

You can use "old" fertilizer as long as it is properly stored. Granular fertilizers need to be kept dry. Once they get wet they clump and it is impossible to spread at the recommended rates. I have used damaged fertilizer in my compost pile. Just break up the clumps and distribute through the pile in small amounts. Liquid fertilizers need to be stored in a dark are that is not subject to freezing temperatures. Both light and cold can interfere with the formulation. Properly stored fertilizers can be effectively used for many seasons.