Phosphate minerals are a valuable natural resource found in rock deposits throughout the United States and the world.
Phosphate minerals were first discovered in central Florida, the "Bone Valley" area, in the early 1880s. However, many of the phosphate deposits are now mined in north Florida in Hamilton County and further south near the Peace River in the vicinity of Fort Meade in Polk County.
Florida provides a good majority of the nation's phosphate supply. According to the U.S. Department of the Interior, ".Florida and North Carolina accounted for more than 85 percent of the total U.S. production with the balance being produced in the western states of Idaho and Utah." The U.S. marketable phosphate rock production for the 2012 crop year (July 1, 2011 to June 30, 2012) was 28.8 metric tons.
The phosphate that is produced from mining is ultimately made into phosphoric acid, which is in turn used to create fertilizer and other products. Fertilizers are an important part of agriculture worldwide and have a huge impact on what we see harvested. "Modern farmers routinely use chemical fertilizers and animal feed ingredients, derived from mineral sources of phosphate, to augment organic sources of phosphate. This practice has increased yields of crops and livestock to levels that greatly exceed natural population growth rates," Central Florida Phosphate District (CFPD).
Two types of phosphates can be found in Florida: pebble and hard-rock. The pebble type is the more common of the two. "Hard-rock phosphate was mined in the late 1800s and early 1900s because it was concentrated and easier to mine," said Florida Geological Survey District Geologist, FDEP, Harley Means.
Means explained that hard-rock phosphates occurs within paleo-sinkholes where groundwater has dissolved the phosphate from upgradient pebble phosphate deposits and transported it into sinkholes where it replaced the limestone in the walls and fill within the sink. The majority of this type of phosphate has been mined out in Florida.
Pebble phosphate deposits form in a different way. It is believed that the fluctuating sea levels across the state, through geologic time, played a role.
"Nutrient rich sea water covered parts of Florida's carbonate platform (primarily during the Miocene epoch) and organisms utilized the nutrients, proliferated and ultimately did so in great abundance," said Means, who explained that this led to eutrophic conditions (meaning that the oxygen was removed from the water) and the remains of the organisms accumulated and over geologic time were converted to phosphate in the form of the mineral francolite. As sea levels fluctuated, these phosphate deposits were reworked by streams and rivers and became concentrated into placer deposits.
The phosphate mining process in Florida is conducted via a dragline. The miners first remove the soil, sand and clay atop and then bring in the draglines. The largest particles are first removed and then slurry, using large water cannons, is run to remove the clay. The material is then pumped into the beneficiation plants where sand and clay are separated for processing.
In order for phosphates to be used as a water-soluble fertilizer, the substance must first be chemically processed. While phosphate mining allows more productive crop harvests, there are a few drawbacks. Phosphate is nonrenewable like most things that are mined.
"At some point in the future we will reach the production limit and there will be less and less phosphate produced in Florida. However, as the economics change it may be economical to go to new areas or back into spoil piles and reprocess some of the material that was previously not economically feasible to mine," said Means.
Another downside is the byproduct known as phosphogypsum that is produced by the process. There is an abundance of this material stacked across the state. However, some initiatives are being taken to put it to good use, such as using it for fill material in road construction.
Lastly an issue arises when the fertilizers are applied in karst regions. These regions are areas that were created by the dissolution of soluble rocks, including limestone and dolomite, and contain aquifers capable of providing large supplies of water.
"The excess that is not taken up by the plants can make it down into the groundwater and cause significant contamination, especially if it makes its way to springs and surface drainage features like rivers," said Means, who added that the Suwannee River is a good example of a spring-fed river system that is terribly contaminated with excessive nutrients due to agricultural applications of fertilizer.
While phosphorous is an important part of root and flower development, it quickly becomes depleted in soils so the substance must be replenished regularly to maintain the impact.