<P> NPK rating is a rating system describing the amount of nitrogen, phosphorus, and potassium in a fertilizer . NPK ratings consist of three numbers separated by dashes (e.g., 10 - 10 - 10 or 16 - 4 - 8) describing the chemical content of fertilizers . The first number represents the percentage of nitrogen in the product; the second number, P O; the third, K O. Fertilizers do not actually contain P O or K O, but the system is a conventional shorthand for the amount of the phosphorus (P) or potassium (K) in a fertilizer . A 50 - pound (23 kg) bag of fertilizer labeled 16 - 4 - 8 contains 8 lb (3.6 kg) of nitrogen (16% of the 50 pounds), an amount of phosphorus equivalent to that in 2 pounds of P O (4% of 50 pounds), and 4 pounds of K O (8% of 50 pounds). Most fertilizers are labeled according to this N-P-K convention, although Australian convention, following an N-P-K-S system, adds a fourth number for sulfur, and uses elemental values for all values including P and K . </P> <P> The main micronutrients are molybdenum, zinc, and copper . These elements are provided as water - soluble salts . Iron presents special problems because it converts to insoluble (bio-unavailable) compounds at moderate soil pH and phosphate concentrations . For this reason, iron is often administered as a chelate complex, e.g., the EDTA derivative . The micronutrient needs depend on the plant . For example, sugar beets appear to require boron, and legumes require cobalt . </P> <Table> Top users of nitrogen - based fertilizer <Tr> <Th> Country </Th> <Th> Total N use <P> (Mt pa) </P> </Th> <Th> Amt . used for feed / pasture <P> (Mt pa) </P> </Th> </Tr> <Tr> <Td> China </Td> <Td> 18.7 </Td> <Td> 3.0 </Td> </Tr> <Tr> <Td> India </Td> <Td> 11.9 </Td> <Td> N / A </Td> </Tr> <Tr> <Td> U.S. </Td> <Td> 9.1 </Td> <Td> 4.7 </Td> </Tr> <Tr> <Td> France </Td> <Td> 2.5 </Td> <Td> 1.3 </Td> </Tr> <Tr> <Td> Germany </Td> <Td> 2.0 </Td> <Td> 1.2 </Td> </Tr> <Tr> <Td> Brazil </Td> <Td> 1.7 </Td> <Td> 0.7 </Td> </Tr> <Tr> <Td> Canada </Td> <Td> 1.6 </Td> <Td> 0.9 </Td> </Tr> <Tr> <Td> Turkey </Td> <Td> 1.5 </Td> <Td> 0.3 </Td> </Tr> <Tr> <Td> UK </Td> <Td> 1.3 </Td> <Td> 0.9 </Td> </Tr> <Tr> <Td> Mexico </Td> <Td> 1.3 </Td> <Td> 0.3 </Td> </Tr> <Tr> <Td> Spain </Td> <Td> 1.2 </Td> <Td> 0.5 </Td> </Tr> <Tr> <Td> Argentina </Td> <Td> 0.4 </Td> <Td> 0.1 </Td> </Tr> </Table> <Tr> <Th> Country </Th> <Th> Total N use <P> (Mt pa) </P> </Th> <Th> Amt . used for feed / pasture <P> (Mt pa) </P> </Th> </Tr>

Where does the nitrogen in fertilizer come from