A carton of old milk in the fridge doesn't need to go down the drain or in the trash, instead, use it in your backyard vegetable garden. Using milk as a natural fertilizer can help give your vegetables a huge boost. In addition to feeding them to help with growth, milk also helps with calcium deficiencies -- yes, plants have that, too! -- and helps fight viruses. Milk contains proteins, B vitamins and sugar that are good for plants. It also enriches the soil and is a good natural alternative to chemical fertilizers. To feed your vegetables leftover milk, simply make a 50-50 mix using water.