Phosphorus fertilizers are produced by acidulating phosphate rock. By itself, phosphate rock is not soluble and so cannot provide phosphorus in an available form for plant use. Many of the sources of phosphorus are sedimentary deposits on old ocean floors which were later uplifted by upheavals of the earth. These deposits can also contain many other minerals and so contamination with heavy metals such as cadmium can be an issue.  The other sources of phosphate rock are from igneous rock deposits, from molten lava, having volcanic origin. This rock is in general very low in contaminants.  Yara´s mines in Finland produces this type of phopate rock. To produce a phosphorus fertilizer, the rock is treated with acid; sulfuric, phosphoric or nitric. Each method has its advantages and constraints. The sulfuric acid route produces a low phosphorus fertilizer – single superphosphate - which is half gypsum. The use of phosphoric acid produces a higher concentration phosphorus fertilizer. The third manufacturing process is to use **nitric acid** to acidulate the rock phosphate. **This process is a cleaner process with no waste products and produces two fertilizers:** - Nitrophosphates which are combined with potassium to produce the complex NPK fertilizers - Calcium nitrate (from the nitric acid combining with the calcium in the rock phosphate) The limitation of this process is that the phosphate content of the fertilizer cannot exceed the nitrogen content.