Producing sulfuric acid
Sulfuric acid is a basic ingredient of superphosphate. In 1881 the New Zealand government offered a bonus of £500 for three years to any firm or individual who could produce 50 tons of sulfuric acid a year.
Thomas Kempthorne and Evan Prosser had set up a drug and importing company in Dunedin in 1863. They researched the process, acquired a site at Burnside near Dunedin, built their works by August 1881, and soon made enough acid to qualify for the bonus. Superphosphate manufacture began in early 1882.
The Frasch process
German-born American chemist Herman Frasch invented a method for extracting sulfur from deep underground. Water is superheated to about 170°C and forced down into the sulfur to melt it. The sulfur–water mixture is lifted to the surface by compressed air. It is then put into bins, where the 99% pure sulfur is left to solidify. This process is much less costly than mining.
Rock phosphate is dissolved in the sulfuric acid.
Workers combined the sandy mixture of bone dust and guano with sulfuric acid in large, flat-bottomed wooden vats, using wooden hoes. The chemical reaction and the heat, steam and fumes made the work intolerable, but after about 30 minutes the reaction subsided and the hot, sticky product was dug out and barrowed to storage sheds, where it dried and matured.
More fertiliser works
Kempthorne, Prosser and Company built other works, and more companies formed as demand outstripped supply. By 1976 there were nine companies operating 12 plants, producing 1.8 million tonnes of superphosphate annually.
In 2007, two New Zealand companies – Ballance Agri-Nutrients and Ravensdown Fertiliser Co-op – were making superphosphate. Both are farmer-owned cooperatives. The six works are fully mechanised, with computer-aided control of most functions. The process includes analysing rock phosphates, blending differing rocks, grinding the rocks, producing granules to reduce the amount of fine material, and controlling emissions.
Sources of sulfur
Early sources of sulfur were volcanic deposits in Italy, and to some extent in the North Island, especially Whakaari (White Island), off the Bay of Plenty. For much of the 20th century the main source was ‘Frasch’ sulfur from Texas and Louisiana. From the late 1960s, sulfur was recovered from natural gas, mostly from Canada.
Guano is a natural fertiliser formed from bird droppings. High in phosphate and nitrogen, it has been used as an ingredient of superphosphate. Nauru Island had large deposits which were mined by Australia and New Zealand during the 20th century. By trading intensively, some companies became very rich, but exploited the local people and the environment.
Sources of phosphorus
Bones were an early source of phosphate, so it is no surprise that the first two fertiliser works, near Dunedin and at Auckland’s Westfield, were adjacent to freezing works (abattoirs or slaughterhouses). The Chemical Manure Workers’ Union, whose members operated the superphosphate works, was a branch of the Freezing Workers’ Union. Freezing workers often worked in the fertiliser plant in their off-season.
Other sources included guano from the Seychelles Islands, coprolites, and low-grade rock phosphates from Clarendon in South Otago.
For most of the 20th century New Zealand relied on high-grade rock phosphates from Nauru and Banaba Island in the Pacific, and later from Christmas Island in the Indian Ocean. Lesser amounts have come from North Africa, Florida and China. Morocco is now the major exporter to New Zealand, and some comes from Togo.
Quality control and research
New Zealand manufacturers conduct field testing, and have improved the quality and consistency of superphosphate. New Zealand-produced superphosphate is now among the highest quality and cheapest fertiliser products in the world.
The New Zealand Fertiliser Manufacturers’ Research Association (FertResearch) funds research into improving results and minimising the damage to the environment. FertResearch has developed the Code of Practice for Nutrient Management, and provides software for farmers to assess whether they are using the right amount of fertiliser.