Plants require nourishment to grow and flourish. Native species survive in areas where the organic and mineral nutrients in the soil and climatic conditions favor their development. Non-native species need soil amendments and special treatment if they are to thrive. Fertilizers are necessary because humans tend to plant non-native lawn grasses, perennials, fruits and vegetables. Organic fertilizers represent an effort to make soil amendments more gently than would chemical fertilizers.
Fertilizers add the three basic building blocks of plant growth to the soil: nitrogen, to develop plant proteins, potassium for flowering and cell development and phosphorus for good root growth. These three nutrients are basic to healthy growth, but other minerals are required in varying amounts by plants, depending on variety and soil composition. Plants probably need fertilizer when they are non-native or when they are repeatedly grown in the same soil, as in a garden plot or flower border.
Organic fertilizers are composted before use to allow their contents to become completely rotted and allow the bacteria that complete the process to die. They are also dried to make them easier to handle. Ripening and drying also kills off harmful bacteria and reduces odor.
Manure and Homemade Compost
Manure is rich in nitrogen, the major element of the "big three" needed for plant growth. Manure may come from farm animals, poultry or even bats. Manures also contain smaller amounts of potassium and phosphorus and traces of calcium, manganese and sulfur. Some, like dog manure, are high in nutrients (in this case, phosphate) that should be limited in application because of possible run-off hazards. Compost of rotted garden materials contains nitrogen but it varies in value, depending on the nature and age of the pile. Its main value is as a soil conditioner.
Fish and Blood or Bone Meal
Dead fish were once dug into the ground in the fall to nourish the next spring's crops. Fish fertilizers and blood meal provide high percentages of nitrogen and are used before planting for a fast start. Bone meal, often used for perennial bulbs, is high in phosphates and calcium with smaller amounts of nitrogen, potassium and manganese.
Scrap from food processing is recycled, after composting, into fertilizer. Some by-products, like hair and feathers, contain a high percentage of nitrogen. Grape skins contain only potassium. Peanut meal contains a good balance of nutrients, including calcium, manganese, sulfur and chlorine. Kelp is a "complete" food for plants, containing a full range of nutrients, minerals and amino acids and even useful as a mulch.
Fertilizer ingredients must be balanced for specific uses and soil types. The benefit in using an organic fertilizer is that it can be formulated for a specific usage. Before choosing an organic fertilizer, have your local university agricultural extension test your soil and make recommendations as to ingredients you need in an organic fertilizer. If you can't find a general fertilizer that fits the bill, you can always add components to meet your garden's needs.