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Superphosphate

by  Arthur Duncan

Known as ‘super’, it’s New Zealand’s premium fertiliser. Superphosphate first went on sale in England in 1843 – the very time when British farmers began settling in New Zealand. It proved ideal down under, especially for sheep and dairy pasture, and became a driving force in the growth of agriculture.


Superphosphate history

Superphosphate is an artificial fertiliser, and is the most important fertiliser used in New Zealand. Farmers often shorten its name to ‘super’.

Improving plant growth

Most soils contain some nutrients and can support plant growth, even if this is slow. For faster plant growth, as long as soil is moist enough, extra nutrients can be added by applying fertiliser.

Nutrients

The main nutrients in superphosphate fertiliser are:

  • calcium (about 20%)
  • sulfur (11–12%)
  • phosphorus (9–10%).

Phosphorus, commonly called phosphate, comes from rock phosphate. It is needed for pasture and crop growth on New Zealand soils. It increases the strike rate of seedlings, stimulates root development and flowering, and improves plant growth.

Superphosphate is effective in a range of soils and climates, and does not affect the acidity of soil.

Early agricultural chemists

In the early 1800s, there were two important figures in the new science of agricultural chemistry.

The first was a French chemist, J. B. Boussingault, who around 1834 began an innovative series of experiments on his farm in Alsace. He created a nutrient balance sheet, comparing the total nutrients applied to a crop with the total taken up by the crop.

The second was Justus von Liebig, a German chemist, who patented an original, artificial manure. However, this failed in practice because the manufacturing process made the phosphate unavailable to plants.

The invention of superphosphate

These two threads of research soon came together, leading to the invention of superphosphate.

In 1843, entrepreneur and agricultural chemist John Bennet Lawes used Boussingault’s methods on his Rothamsted estate near London. Aware of Liebig’s failure, he made his own phosphate manure using a process he had patented in 1842. This involved treating mineral phosphates with sulfuric acid to make superphosphate. In this form, phosphate is rapidly released into the soil, where it can be used by plants. The fertiliser was first advertised for sale on 1 July 1843.

Early New Zealand

The British settlers who arrived in the 1840s and 1850s to take up farming would have heard about the new science of agricultural chemistry, and wondered how much of it was relevant to New Zealand conditions. They pressed for institutions that would offer agricultural education.

Canterbury Agricultural College

Canterbury Agricultural College was founded at Lincoln in 1878. One of the earliest moves by staff was to import superphosphate. It probably came from Adelaide, Australia, which had the first superphosphate manufacturing plant in the southern hemisphere. From then on, superphosphate set the pace for the development of agriculture in New Zealand.


Making superphosphate

Phosphate is a naturally occurring form of the element phosphorus, present in all living cells. It is found in the fossilised remains of ancient marine creatures, and in large build-ups of bird droppings (guano), which form sandy deposits known as rock phosphate.

Making superphosphate involves chemically treating rock phosphate, quarried in the US, Egypt, Tunisia and elsewhere.

The production process

  • One of the basic materials needed to make superphosphate is sulfuric acid. This is produced by burning sulfur and dissolving the fumes in water.
  • Rock phosphate is then dissolved in the acid.
  • The chemical reaction (which takes about half an hour) produces phosphoric acid and calcium sulfate. These are the main components of superphosphate. The ratio of acid to rock phosphate must be carefully judged: too much sulfuric acid produces excess phosphoric acid.
  • The steaming product is usually carried on a conveyor belt to storage for maturing. During storage, ‘free’ phosphoric acid continues to react with residual rock phosphate.

Too much liquid can cause the superphosphate to become like putty under mechanical pressure (often referred to as ‘green’ super). This is difficult to use in farm machinery. The problem can be overcome by artificial drying, but this is prohibitively expensive.

Purity of rock phosphate

Rock phosphates can range in purity from 62% to 88% tri-calcium phosphate content. This affects the amount of acid needed. The main impurities are silicates, oxides of iron and aluminium, fluoride, and carbonates. These may all complicate the process, or form undesirable emissions.


New Zealand superphosphate

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.

Making superphosphate

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.

Modern methods

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.

Scooping poop

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.


Superphosphate pros and cons

High demand

Superphosphate has been a reliable and cost-effective fertiliser in New Zealand since 1882, especially on sheep and dairy farms. Once it was known that superphosphate was widely suitable for New Zealand pastures, suppliers struggled to meet the demand.

In the 1920s and 1930s the government promoted superphosphate, backed with research.

Developing hill country

After the Second World War (1939–1945), New Zealand developed aerial topdressing. Fertiliser could now be efficiently and easily applied from the air, rather than by hand. There was strong support from soil conservation authorities, such as catchment boards that had been formed to combat erosion in hill country. The result was a huge increase in hill country farming, both privately and through government schemes.

Research revealed the need for other nutrients such as potash, cobalt, boron, selenium and molybdenum, and farmers were able to add these to the superphosphate as required.

Alternatives

There are alternatives to superphosphate as a source of phosphorus for crops and pastures. Superphosphate is not used on organic farms because it is a manufactured (chemical) fertiliser. Organic farmers use unmodified rock phosphates that will react with the natural acids in the soil to release phosphorus.

Cautions

As with all chemical fertilisers, there are some features of superphosphate that need to be treated with caution.

  • Damaging seeds. Superphosphate and other very soluble fertilisers can damage nearby germinating pasture seeds. To avoid this, each seed can be protected in a lime pellet, or fertiliser should be sown separately.
  • Fluorosis. Superphosphate contains fluorine, which can cause sickness (fluorosis) in animals that ingest it directly from recently fertilised pasture. The best practice is to ensure that animals do not graze pastures recently topdressed with superphosphate until rain has washed the foliage clean.
  • Cadmium. Cadmium, contained in most rock phosphates, is regarded as toxic to humans. New Zealand manufacturers take care to ensure that cadmium does not exceed 280 milligrams per kilogram of phosphorus. This is to avoid any excess cadmium entering the food chain.
  • Impact on waterways. Phosphate is strongly held in the soil by clinging to clays. It is then released slowly as plants require it for growth. Very little is carried by runoff water into streams. The phosphate that gets into streams and damages the environment is mostly attached to fine sediments. So if the loss of these sediments can be reduced, then the amount of phosphate entering waterways will also be reduced. But if superphosphate is dropped straight into water it will release phosphate rapidly.This could raise the levels of the phosphate in farm waterways or dams. So it is important that fertiliser is applied in a way that does not allow this to occur.

External links and sources

More suggestions and sources


How to cite this page: Arthur Duncan, 'Superphosphate', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/superphosphate/print (accessed 7 December 2019)

Story by Arthur Duncan, published 24 Nov 2008