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until the early 20th century all fertilizers were effectively natural. Potassium and phosphorus are mined from mineral deposits called potash and rock phosphorus, respectively. Nitrogen comes from mostly animal-associated materials (manures and guano, blood meal, bone meal, fish meal, feathers, etc). For most crops, manure must be composted before being applied as a fertilizer to prevent the spread of human pathogens (like E. coli or salmonella). Approximately 5% of all US crops are fertilized with manure and manure-related materials like compost. See also nitrogen fertilizer. |