When you think about a thriving ecosystem, it’s easy to see the symbiotic relationship between plants and animals. Plants feed animals and, in return, animals fertilize plants through scat and decomposition. For the home gardener, you can replicate that benefit to plants by applying fertilizer to grass, vegetables, flowers, houseplants, trees and shrubs. While you can buy fertilizer, once you know what’s in them, you can easily duplicate the process with items that are easy to find.