NPK: Industrial Fertilizer Production in the USA
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As most gardeners know, the three basic elements of fertilizer are nitrogen, phosphorus, and potassium, often represented by their symbol on the periodic table of elements: N, P, K. 
N: Nitrogen (blue flags on map)
Nitrogen is prevalent in the earth’s atmosphere, but in order to be absorbed into plants, it has to be fixed in the soil. This limitation on agricultural production was overcome by technology in the early 1900s with the Haber process, which uses gas to make ammonia, a highly reactive form of nitrogen. The Haber process created the synthetic fertilizer industry, and a global boom in agricultural productivity, dependent on fossil fuels extracted from the earth. In the USA, dozens of companies produce nitrogen-rich fertilizer components at refineries that are fed by the nation’s network of natural gas pipelines.
P: Phosphorus (red flags on map)
Phosphorus is essential to root growth and other plant functions. It is naturally in most soils, but in limited amounts, and is therefore commonly enhanced with fertilizers (as the P in NPK). Phosphorus can be found in things like bone meal, but the largest source is from phosphate rock deposits, associated with ancient sea beds. The United States was the world's largest producer of phosphate rock from the end of the 19th century until 2006, when US production was exceeded by China. Today the US still supplies more than 20% of the world’s phosphates, and 75% of that comes from one region in central Florida called the Bone Valley.
K: Potassium (orange flags on map)
Potassium facilitates growth, photosynthesis, and other critical functions in plants. For fertilizer, it is mostly extracted from potash, which is found in salt deposits underground and on the surface (potassium’s elemental symbol K is derived from kalium, Latin for alkali). Of the millions of tons of potash fertilizer used in the USA annually, most comes from other countries, especially Canada. 15% or so comes from domestic mines, and nearly all of that from three underground mines in southeastern New Mexico, with a little more from around the Great Salt Lake.