Organic fertilizers are fertilizers derived from animal matter, animal excreta (manure), human excreta, and vegetable matter (e.g. compost and crop residues). Naturally occurring organic fertilizers include animal wastes from meat processing, peat, manure, slurry, and guano.
In contrast, the majority of fertilizers used in commercial farming are extracted from minerals (e.g., phosphate rock) or produced industrially (e.g., ammonia). Organic agriculture, a system of farming, allows for certain fertilizers and amendments and disallows others; that is also distinct from this topic.
Organic fertilizers are used to augment the concentrations of plant nutrients and organic substances. They can restore the soil fertility and enhance the crop yield and quality. They are slow but efficient approaches to improve farming production and maintain a sustainable environment in the long term.
As people realize the importance of soil health and the preservation of our environment, organic fertilizers have entered the public view as means to enhance the soil quality and increase crop production. But organic fertilizer is not a recent innovation; rather, its history can be traced back to thousands of years ago. In the ancient China, animal wastes were main sources of organic fertilizers. In the ancient Egypt, the river of Nile, which was enriched in mineral ingredients, was the main source of organic fertilizers. People lived in the Roman Empire Columella had a variety of sources of organic fertilizers and summarized their applications in details. From the eleventh to thirteenth centuries, farmers in the western Europe relied on manure for plant nutrients. Although artificial fertilizers provide convenient and cheap choices for farmers to manage their lands in recent decades, organic fertilizers still play an important role in agriculture and crop production.
Examples and sources
By many definitions, minerals are separate from organic materials. However, certain organic fertilizers and amendments are mined, specifically guano and peat. Other mined minerals are fossil products of animal activity, such as greensand (anaerobic marine deposits), some limestones (fossil shell deposits), and some rock phosphates (fossil guano).
Peat, a precursor to coal, offers no nutritional value to the plants, but improves the soil by aeration and absorbing water. It is sometimes credited as being the most widely use organic fertilizer and by volume is the top organic amendment.
Animal sourced materials include both animal manures and residues from the slaughter of animals. Manures are derived from milk-producing dairy animals, egg-producing poultry, and animals raised for meat and hide production. They may contain biological and chemical contaminations and are greatly impacted by the diet of the livestocks. Consequently, they need to be processed carefully and properly.
When any animal is butchered, only about 40% to 60% of the live animal is converted to market product, with the remaining 40% to 60% classed as by-products. These by-products of animal slaughter, mostly inedible -- blood, bone, feathers, hides, hoofs, horns, -- can be refined into agricultural fertilizers including bloodmeal, bone meal fish meal, and feather meal.
Processed organic fertilizers include compost, humic acid, amino acids, and seaweed extracts. Other examples are natural enzyme-digested proteins. Decomposing crop residue (green manure) from prior years is another source of fertility.
Other ARS studies have found that algae used to capture nitrogen and phosphorus runoff from agricultural fields can not only prevent water contamination of these nutrients, but also can be used as an organic fertilizer. ARS scientists originally developed the "algal turf scrubber" to reduce nutrient runoff and increase quality of water flowing into streams, rivers, and lakes. They found that this nutrient-rich algae, once dried, can be applied to cucumber and corn seedlings and result in growth comparable to that seen using synthetic fertilizers.
Treated sewage sludge
Sewage sludge, also known as biosolids, is effluent that has been treated, blended, composted, and sometimes dried until deemed biologically safe. As a fertilizer it is most commonly used on non-agricultural crops such as in silviculture or in soil remediation. Use of biosolids in agricultural production is less common, and the National Organic Program of the USDA (NOP) has ruled that biosolids are not permitted in organic food production in the U.S.; while biologic in origin (vs mineral), sludge is unacceptable due to toxic metal accumulation, pharmaceuticals, hormones, and other factors.
With concerns about human borne pathogens coupled with a growing preference for flush toilets and centralized sewage treatment, biosolids have been replacing night soil (from human excreta), a traditional organic fertilizer that is minimally processed.
Organic Fertilizer vs. Chemical Fertilizer
Organic fertilizer significantly increases the microbial population and soil enzyme activity compared to chemical fertilizer. It generates a higher crop yielding rate in the long term.  The application of chemical fertilizers, on the other hand, could eventually leads to the destruction of soil structure and surrounding environment and higher susceptibility of plants to diseases and pest. Because organic fertilizer usually has a lower concentration of plant nutrients compared to chemical fertilizer, however, a larger amount of fertilizers needs to be applied and hence increases the cost of crop production.  In order to minimize the damage caused by chemical fertilizers and maximize the benefits brought by organic fertilizers, an optimum approach is to apply a combination of both fertilizers. It is proven that such combination indeed would improve the soil quality. 
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