Fertilizer baskets are used to contain your slow release fertilizers. These baskets allow you to place your fertilizer exactly where you want it without worrying about it rolling or washing away. These work great to prevent many animals from taking the fertilizer away, but a dog may figure out how to pull them and break them open.
Just fill, close then push the spikes into the soil. Each time you water some of the fertilizer will leach out of the basket and fertilize your tree.