An application of organic fertilizer can take many forms. Organic fertilizer is a soil amendment derived from something that is or once was living, either a plant or an animal. Inorganic fertilizer is typically made from synthetic chemicals and minerals containing phosphorous, nitrogen and potassium.
There are many types of organic fertilizer. They include compost, manure, composted manure, kelp and other seaweed, fish meal, egg shells, brassin, guano, tea grounds, coffee grounds, bone meal, bloodmeal, leaves, worm castings, vermicompost, slurry, peat, humic acid and others. Some of these are self-explanatory. Others may need explanation. Vermicompost is a combination of worm castings and compost. Slurry is a nutrient-rich, muddy mixture of insoluble matter, such as mud from a pig pen. Brassin fertilizers are made from plant extracts. Humic acid is a combination of many acids that result from the biodegradation of dead organic matter. Humic acid is often used as a complement to other fertilizers.
Even living plants can be a type of "green manure" to contribute nitrogen and phosphorous to the soil, as in the case of cover crops.
You may also find blends of organic fertilizer under various brand names.
The purposes of applying fertilizer in the garden are manifold. Fertilizer amends the soil, making it more fertile in the short- and long-term. Organic fertilizer makes the soil more biodiverse, increasing both nutrients and living organisms. It increases production and yield and makes plants healthier. Healthier plants along with fertile, balanced soil decrease the incidence of diseases and insect infestations in the garden. Organic fertilizer has been used perhaps since the first garden on earth. Native Americans commonly used Menhaden fish as "fertilizer fish."
According to the USDA, "Organic agriculture is an ecological production management system that promotes and enhances biodiversity, biological cycles and soil biological activity." All of the procedures and practices of organic agriculture are intended to "enhance ecological harmony."
You may apply organic fertilizer directly to soil, spray it on leaves ("foliar feeding"), or apply it as a "side dressing." A side dressing is as the name implies, an application about 4 to 6 inches to the side of the plant, so that it feeds the roots.
Prior to planting, organic fertilizer is usually cultivated into the soil. There is little danger of nitrogen burn as there would be with harsh synthetic fertilizers. Organic fertilizers slowly release nutrients into the soil and to the roots of the plant, rather than all at once.
Compost tea is one popular method of applying organic fertilizer. It is made from steeping compost in hot water, usually out in the sun. Most experts recommend using fresh water, not treated water from the tap. The resulting liquid is applied to the sides of plants. Compost tea may damage plants if applied directly to leaves.
The use of organic fertilizer has many benefits as compared to synthetic fertilizers. Both have been shown to increase yield and growth of plants. However, run-off from nitrogen-rich synthetic fertilizers is known to contaminate ground water, rivers, creeks and oceans. Nitrates from synthetic fertilizer cause dead zones in the ocean as well as excessive algae bloom, massive fish kills and the phenomenon known as "red tide."
There are a number of advantages of organic fertilizer: they slowly release nutrients, retain water in the soil, increase soil biodiversity, improve soil structure, and prevent topsoil erosion. A chief advantage of organic fertilizer is that it avoids the nitrous oxide problems caused by synthetic fertilizers in the environment, both on land and in the atmosphere.