A natural way to fertilize is done by working a dressing of compost in the soil around each plant. Compost can provide a young plant with nutrients that could be missing in your soil. It can easily be purchased from any local garden center or you can make compost yourself as nature provides all of the natural ingredients needed.
Leaves, twigs, grass, plant cuttings and vegetable waste can be used to make your own compost pile. You can also make compost tea that can be used to water your plants.
Combine a pound of compost for each gallon of water in a large bucket. Stir it once a day for at least 5 days, strain out the solids and you are left with compost tea.
Your plants will love it.
A natural fertilizer you can purchase is concentrated fish emulsion. Follow the directions on the bottle to mix the correct strength and use this to water your plants, but be careful using this kind of fertilizer. If you can stand the smell... beware that fish emulsion can attract cats, dogs, and wild creatures. You would not want your cat or dog to roll around in this or dig up your plants.
I like the make-at-home type of fertilizers you can create using common ingredients you have around the house. Epsom salts are a good source of magnesium sulphate. You can sprinkle it around your plants or mix 1-2 tablespoons per gallon of water. Roses benefit from either a side dressing worked into the soil or you can spray it on the leaves. Tomato and pepper plants prefer either a side dressing or a good watering of Epsom salts.
Azaleas and blueberry bushes love acidic soil and nitrogen. You can fertilize these plants with leftover coffee grounds that provide nitrogen. A simple solution of 1-2 tablespoons vinegar to 1 gallon of water can be used to water and acidify the soil for these acid loving plants. Save those egg shells you throw out all the time.
Let them dry out, crush them or pulverize them in a blender and the result will be a great source of calcium for your vegetable plants.
It is a good idea to research your plants and find out if they require high, medium or low fertilization. Most vegetables have a high requirement but herbs have a medium requirement. Most evergreen trees and shrubs have a low requirement and some flowers such as Nasturtiums require no fertilization and will actually flower better if they are not fertilized. Testing your soil is a reliable method to find out exactly what your soil may be missing. Of course the more natural ingredients you use the better, but some gardeners do well using a balance of natural and synthetic fertilizers to satisfy the needs of their plants.