Organic Fertilizers may be defined as substances used to improve the quality of soil, derived solely from the remains or by-products of living creatures. Bat Guano, cottonseed meal, blood meal, fish emulsion, manure and sewage sludge are examples of organic fertilizers. Some organic materials like leaf compost are sold as soil conditioners rather than fertilizers, although small amounts of nutrients are present in them.
Like chemical fertilizers, organic fertilizers also contain plant macronutrients -- Nitrogen, Phosphorus, or Potassium (abbreviated to NPK), in different proportions. Some organic fertilizers which are low in any of these macronutrients may be fortified with other organic materials. For example, rock phosphate can be added to increase phosphorus, or greensand to increase potash.
Organic fertilizers depend on soil organisms to break them down to release nutrients; therefore, most are effective only when soil is moist and warm enough for the microbes to be active.
 Types of Organic Fertilizers
There are many types of Organic Fertilizers. Here is a list of the most important ones.
Cottonseed meal, a by-product of cotton manufacturing, is a slightly acidic fertilizer. Consequently, it is good for acid-loving plants such as azaleas, camellias, and rhododendrons. It contains roughly 7 percent nitrogen, 3 percent phosphorus, and 2 percent potash.
Blood meal is a rich source of nitrogen, and is made using dried, powdered blood collected from cattle slaughterhouses. It should be used with caution, as it is so nitrogen-rich that it may burn plants if used in excess. Blood meal supplies some essential trace elements including iron, in addition to nitrogen.
Fish emulsion, another nitrogen-rich fertilizer, is a partially decomposed blend of finely pulverized fish. It often smells very strongly, but the bad odour dissipates within a day or two. Recently, deodorized brands have been developed. Used best in late spring, when garden plants have sprouted, fish emulsion followed by a deep watering boosts plants' early growth spurt. Like Blood meal, this can also burn container plants if used in excess.
Manure is a complete fertilizer, even though it is low in the amount of nutrients it supplies. Manures vary in nutrient content according to the animal source and what the animal has been eating. A NPK fertilizer ratio of 1-1-1 is typical. Commonly available manures include horse, cow, pig, chicken and sheep. Fresh manure has the highest nutritional concentration, but also has the maximum salts that can potentially damage plant roots. So gardeners prefer to use composted forms of manure. Because of its low concentration of plant nutrients, manure is best used as a soil conditioner instead of a fertilizer.
Earthworm Castings, or compost derived from Vermiculture, is high in useful minerals and bacteria. It has a high NPK ratio, and almost 60 trace minerals that make it an ideal additive to soils of all types.
Sewer sludge, a recycled product of municipal sewage treatment plants is available in two forms, activated and composted. Activated sludge has higher concentrations of nutrients (approximately 6-3-0) than composted sludge. It is usually sold in a dry, granular form for use as a general purpose, long-lasting, non-burning fertilizer. Composted sludge is used primarily as a soil amendment and has a lower nutrient content (approximately 1-2-0). Many farmers question the long term effects of using sewage sludge particularly around edible crops. Some sludge may contain harmful heavy metals like cadmium, and its regular usage may cause their build up in the soil. Possible negative effects vary with the origin of the sludge and with the characteristics of the soil where it is used.
 Organic Vs Chemical Fertilizers
There is an increasing awareness that toxic chemicals in synthetic fertilizers are contaminatng groundwater on every inhabited continent, endangering the world's most valuable supplies of freshwater. Organic Fertilizers are an eco-friendly option as they do not contain toxic chemicals.
Chemical fertilizers, unlike organic fertilizers react with minerals in the soil. New compounds formed from these reactions glut or overload the plant, causing it to become unbalanced. Other compounds remain in the soil, many in the form of poisons. Whereas organic fertilizers do more than provide organic nutrients. They improve the soil structure, or tilth, and increase its ability to hold both water and nutrients.
Plants that are chemically fertilized may look lush, but are actually more susceptible to disease. Their tissues contain a high concentration of water which causes the protein quality to suffer. In contrast when organic fertilizers are applied, microbes in the soil break them down into an inorganic soluble form. This slow release of nutrients is a healthier situation for plant growth. It is important to understand that there is no fundamental difference in nutritional quality between organic and inorganic fertilizers.
 Some Diasdvantages of Organic Fertilizers
Since organic fertilizers need to be broken down by microbes into soluble minerals, the process of their absorption is long drawn. One potential drawback is that they may not release enough of their principal nutrient when the plant needs it for growth.
Organic fertilizers are usually more expensive than chemical fertilizers. Also they may not be as easy to source.
- To see online catalogues of Organic Fertilizers, go to Environmentally Friendly Lawn & Garden Supplies
- To learn about the use of different Organic Fertilizers for specific crop and garden problems, go to Organic Fertilizer and Soil Amendment Guide
- Organic Materials as Nitrogen Fertilizers