A rock and a hard place

What is the story with phosphate? (7 minute read)

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When Nauru makes the headlines these days, it’s because the tiny island 1300 kilometres north-east of the Solomon Islands is one of the locations where the Australian government sent asylum-seekers. At its peak, Nauru housed over 1,000 refugees and migrants, a lot of people when you consider that the total population of Nauru is just over 10,000. Today, just over 100 remain, most of whom have been recognised as refugees under the criteria set by the United Nations.

But Nauru is notable for more than just asylum seekers. In 1901, a New Zealand geologist discovered that the island had rich deposits of rock phosphate. In 1906, the first of that rock phosphate was mined. By 1968, when Nauru gained independence from Australia, a third of the island had been strip-mined and left desolate, with little of the profit going back to Nauru. Mining continued during the 1970s and 1980s, and for a brief period the people of Nauru were among the wealthiest in the world, on paper at least. But there was a heavy price to pay. Rock phosphate mining stripped the forest cover and soil from most of the island, leaving a maze of jagged limestone pinnacles. Then, the rock phosphate ran out, and poor investment and fraud left the island impoverished once again.

Phosphate mining in Nauru (image credit: Lorrie Graham/AusAID, CC BY 2.0)

The history of Nauru is inextricably linked to the story of rock phosphate. But what is rock phosphate, and why is it so important that an entire island was virtually destroyed to get it?

Rock phosphate is an imprecise term, and can refer to more than one thing. But, in general, it is a type of rock containing a large amount of the element phosphorus. Some rock phosphate, like that which was mined on Nauru, is derived from guano, or bird droppings. Other rock phosphate is derived from the fossilised skeletons and shells of marine animals. The phosphorus in rock phosphate is not in its pure or elemental form, but is in the form of minerals such as apatite, where the phosphorus is combined with oxygen and other elements.

Phosphorus is the chemical cousin of nitrogen, the gas which makes up most of the atmosphere and which I wrote about two weeks ago. But it’s a very different chemical. Whereas nitrogen is a stable gas, phosphorus is a solid. But even that is an oversimplification, as pure phosphorus has different forms with very different properties. In its white form, phosphorus is dangerously reactive, spontaneously bursting into flame on contact with air. In its red form, it is more stable although still reactive. When you strike a match against the strip on the side of a matchbox, that strip contains red phosphorus.

In its pure form, phosphorus is dangerous, but in the form of phosphate, it is essential to life. It is the sixth most common element in the human body, occurring in our DNA and bones, as well as in a remarkable chemical process called the ATP cycle. The ATP cycle is what carries energy within our body, and phosphate is an essential part of that. In short, ATP stands for adenosine triphosphate, and as you might guess from the name, it has three phosphate molecules as part of it. Inside cells, the conversion of ATP to ADP, which is adenosine diphosphate (you guessed it, with two phosphates), releases energy. There’s a video here which explains the basics if you would like to know more.

While phosphorus in its phosphate form is essential to life, it is not abundant. In fact, it is one of the two elements that often limit the productivity of natural environments (the other is nitrogen). If we didn’t add phosphorus to our agricultural soils, they would soon be depleted. And, unlike nitrogen, we can’t just extract it out of the air.

That is where rock phosphate, and places such as Nauru, come into the picture. We mine the phosphate, turn it into fertilisers such as superphosphate, and pour it onto our pastures. For most of the 20th century, New Zealand’s superphosphate came from Nauru’s rock phosphate. Once Nauru’s supply ran out, we turned to Morocco, and that is where we still get most our superphosphate from.

New Zealand uses a lot of superphosphate. In 2019 we applied 788,000 tonnes to our agricultural land, more than any other fertiliser. As a result, we are hugely dependent on the supply of rock phosphate from Morocco. If we didn’t have Morocco, there are other sources – the next most important producers are China, the United States, Russia and Saudi Arabia. But Morocco holds more of the world’s rock phosphate than all the other countries put together.

Rock phosphate is not a renewable resource, but exactly how much is left is debatable. The International Fertilizer Development Center estimates that there is enough for the next 300-400 years. Others have estimated that reserves could run out much sooner, in as little as 50-100 years. Either way, it’s not something that we want to waste.

There are other reasons to be concerned about phosphate fertilisers such as superphosphate. Rock phosphate contains cadmium, a toxic heavy metal that can accumulate in plants and animals, and ultimately the human body. At high concentrations, it can cause kidney disease, cancer and other health problems. So far, the amount of cadmium that has entered New Zealand’s soil due to phosphate fertiliser is small. The amount in our diet is not enough to cause health problems and is well below safe limits, as far as we know. Currently, cadmium levels are managed by voluntary limits on how much can be in fertiliser and a national cadmium strategy, so there is no immediate danger. But, so far, the only way to limit cadmium in fertiliser is to use rock phosphate which is low in cadmium. Even if there is plenty of rock phosphate for the next few hundred years, what is available may not be the rock phosphate we want.

There is another problem with phosphate fertiliser, and it’s very similar to the problem I wrote about with nitrogen. Excess phosphate applied to our fields and farms ends up in waterways, contributing to nutrient pollution, excess growth of algae and, at worst, dead zones in lakes and in the ocean. But there is a subtle difference between nitrate pollution and phosphate pollution. Nitrates are almost always soluble – they dissolve in water. As a result, nitrates applied as fertiliser are easily washed out of soil and into waterways. Phosphates, on the other hand, are mostly insoluble. While phosphoric acid, the form in superphosphate, is soluble, it quickly attaches to soil particles and isn’t easily washed away. But if the land erodes, that then takes phosphate from the land and into waterways, eventually ending up on the sea.

Once the phosphate is in the sea, most of it falls to the sea floor, and is effectively lost to us. There is a phosphorus cycle, where the deep ocean sediments eventually form rock and are uplifted until they form land again, but that is a process on a geological time scale.

There are many good reasons not to waste phosphate and nitrate fertiliser, even before you consider that using unnecessary fertiliser is a waste of money. So, why is it a problem? Surely there is an obvious solution – stop using too much. But is it really that simple?

When I decided to try and answer that question, I thought there was probably a range of complex reasons behind excessive use of fertiliser, but this doesn’t appear to be the case. A number of countries have cut their fertiliser inputs without sacrificing yield, with the implementation of better management practices on farms. In some cases, it is simply a lack of awareness on the part of farmers.

There are subtle differences in how to prevent nitrate and phosphate ending up in waterways. Because nitrate is soluble and easily washed out of the soil by water, applying smaller amounts more frequently is more effective than applying a large amount at one time. That is less of an issue with phosphorus, where preventing soil erosion is a major factor.

Despite what I have said here, the excessive use of fertiliser won’t go away immediately. Farmers in New Zealand and other wealthy countries have access to good information and resources such as soil testing, which allow them to apply fertilisers with much greater precision. This is not the case for farmers in countries which are less well-resourced. And it takes time to change the way people manage their farms. But, for once, I find myself feeling more optimistic as I write about an environmental problem where the solutions seem within our grasp.

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