Organic fertilizers are becoming increasingly common thanks to greater environmental awareness. If you were to use organic pesticides and fungicides on fruits and vegetables in order to ensure a healthier and more environmentally friendly harvest, it's only natural that the products used to fertilize these same fruits and vegetables would have organic sources as well.
Nitrogen Source Ingredients
Nitrogen is the element all plants need for growth and is especially important in those species with large, leafy green foliage. Urea is an organic chemical byproduct created by all living creatures when their bodies' create protein. It is excreted during urination, and remains behind as a salt-like material after all the other chemicals have evaporated. Because it is very soluble in water, urea can often be lost from soil after a hard rain. Blood meal is a powder created from dried blood, which has a high nitrogen content. Crab meal is made by crushing and drying the castoff shells and viscera from the crabbing industry.
Potassium Source Ingredients
Potassium is needed by plants to maintain cellular integrity, thus fighting off infections. It's also important for the production of fruit and flowers. Kelp meal, and ground and dried kelp work well to provide this nutrient, but it can be expensive and is not often found in inland areas. Traditionally, potassium has been put into soil by potash, which is the ashes of burned plant and tree matter. This is very cheap and easy to find under the name "muriate of potash," but should be used sparingly because it is an extremely alkaline substance that can throw of the pH balance of the soil.
Phosphorous Source Ingredients
Phosphorous is important for a plant's healthy root system and doubly important for tubers like yams and potatoes. Rock phosphate, a naturally occurring phosphorous, has become difficult to obtain because of the cost of mining it and the fact of its insolubility. Because it's insoluble, it will stay in the soil for a very long time and continue to provide plants with nutrients, but grinding it into powder for dispersal is a laborious task. Bone meal has a small amount of nitrogen and a high amount of phosphorous. It's cheap, easy to obtain, and disperses through the soil slowly for month or year-long fertilization. When immediate fertilization is required, bat guano is the best organic choice.