Food that contains protein contains nitrogen. Nitrogen is the most common component of fertilizer. It is usually the factor that determines crop yield in a garden.
I am fairly careful about wasting food. However, I still end up with the carton of sour milk, the brick of tofu several weeks past date, a can of beans that were opened for a burrito but got lost in the back of the fridge. These are all excellent sources of nitrogen for the garden.
Not all high protein foods are good fertilizers. High fat foods are not good because the soil flora and fauna have a hard time digesting fats. High salt foods are not good for the soil either. So cheese, salty canned things, and many preparations of meat are not good candidates.
One very good candidate is the annual batch of dyed Easter eggs that you or your neighbor might have. They are high in protein, low in salt, reasonable in the amount of fat, and they come at the right time of the year for gardening. In my next post, I will describe how to use hard boiled eggs as garden fertilizer.