Organic fertilizers are fertilizers derived from animal matter, human excreta or vegetable matter. (e.g. compost, manure). In contrast, the majority of fertilizers is extracted from minerals (e.g., phosphate rock) or produced industrially (e.g., ammonia). Naturally occurring organic fertilizers include animal wastes from meat processing, peat, manure, slurry, and guano.
Sources of organic fertilizer
The main organic fertilizers are, in ranked order, peat, animal wastes (often from slaughter houses), plant wastes from agriculture, and sewage sludge.
The main source of organic fertilizer is peat, an immature precursor to coal. Peat itself offers no nutritional value to the plants, but improves the soil by aeration and absorbing water.
Mined powdered limestone, rock phosphate, and Chilean saltpeter are inorganic (not of biologic origins) compounds, which can be energetically intensive to harvest.
These materials include the products of the slaughter of animals. Blood meal, bone meal, hides, hoofs, and horns are typical precursors.
Chicken litter, which consists of chicken manure mixed with sawdust, is an organic fertilizer that has been shown to better condition soil for harvest than synthesized fertilizer.
Processed organic fertilizers include compost, humic acid, amino acids, and seaweed extracts. Other examples are natural enzyme-digested proteins, fish meal, and feather meal. Decomposing crop residue (green manure) from prior years is another source of fertility.
Although night soil is a traditional organic fertilizer, the main source of this type is sewage sludge.
Recycled sewage sludge (aka biosolids) as soil amendment is only available to less than 1% of US agricultural land. Industrial pollutants in sewage sludge prevents recycling it as fertilizer.
Animal sourced urea and urea-formaldehyde from urine are suitable for organic agriculture; however, synthetically produced urea is not. The common thread that can be seen through these examples is that organic agriculture attempts to define itself through minimal processing (e.g., via chemical energy such as petroleum), as well as being naturally occurring or via natural biological processes such as composting.