There are three basic types of fertilizers

Organic residues such as manure, which are lower in nutrients compared to more advanced forms of plant food and are also called soil amendments because they are mixed into the natural soil.

Naturally occurring yet inorganic minerals that are medium to high in nutrients, including insoluble elements- limestone, rock phosphate, granite dust, greensand, and natural sulfur, as well as soluble elements- Chilean nitrate, natural potassium magnesium sulfate, Epsom salts, and natural potassium sulfate.

Inorganic synthetic fertilizers, which are highly soluble and have mainly high levels of nutrients- urea, synthetic sodium nitrate, ammonium nitrate, liquid ammonia, ammonium phosphate, ammonium sulfate, triple phosphate, and purified potassium chloride.

For any soil amendment to be certified ‘organic’ the ingredients in the product must be naturally occurring; such as manure, peat, and, seaweed. Of the three types of fertilizers synthetic fertilizers are commonly used to maximize crop yield while organic fertilizers have been shown to produce higher quality plants with lower quantity of crop yield.

Fertile Soil: A Grower’s Guide to Organic and Inorganic Fertilizers. Robert Parnes, Ph.D., 1990.