When asked to think of a critical mineral resource, one that nations fiercely compete for, most people might mention oil and gas, or copper, or rare earth elements. These economic resources are well known for their importance to modern industrial society, and most people know something about where they form and how they are mined.

However, there is another mineral resource that is just as important, but much less well known, at least to the general public: phosphate.

Since ancient times humans have used phosphates to fertilize their crops. And until recently, the main source of phosphorus was certainly not glamorous.

Guano is a word from the language of the Quechua, the native people of coastal peru. These people are one of the first to harvest the white material, which is made up of the droppings of marine birds. The Quechua collected guano from the hyperarid coastal regions of Peru, where the droppings of birds were not diluted by rain. Later, the Inca treasured guano so much that disturbance of seabirds was punishable by death!

Although the Quechua are known to have used guano as a fertilizer for over 1,500 years, its use didn’t really take off until the dawn of modern high-input agriculture in the early to mid 19th century.

To understand why guano mining is possible in some locations but not others, we need to take a step back and look at the conditions required for the formation of guano.

Since guano is a biological byproduct, created by birds, it will only form in commercial quantities in areas with large numbers of birds. Seabirds, in turn, only live in places with plentiful marine life. Since the areas of the oceans with the highest concentrations of life are zones where upwelling brings deep nutrients to the surface, it is these zones of upwelling where birds were able to create the largest deposits of guano.

But there’s a catch – the valuable components of the guano, specifically the phosphates, are easily leached away by rainfall. That means that commercially viable deposits tended to form only in extremely arid locations.

In the end, guano-based phosphate deposits can only form in exceptionally dry areas of upwelling.

European mining of guano was carried out for much of the 19th century in locations that met the required conditions. The best places for guano mining ended up being islands off the coast of Peru, where upwelling combined with hyperarid conditions allowed deposits of guano up to 50 metres thick to form. Many of these islands were claimed by the United States or European countries, strip-mined, and abandoned once the phosphates were gone.

In modern times, it’s estimated that 50% of global agricultural yield is supported by the intensive use of fertilizer. However, the guano islands of the pacific were largely exhausted by the end of the 19th century.

We now source most phosphates from phosphorite deposits. These hard-rock deposits, which form on the seabed and are later uplifted, provide a reliable source of phosphorous for our agriculture needs. However, as they too are depleted, we’ll need to find a new source for this essential resource.