SOME believe that dwindling supplies of potable water is humanity's great resource challenge; others think it is the imminent prospect of "peak oil".
But an equally important milestone in modern history will be an inevitable tightening of global supplies of phosphorus.
Phosphorus has underpinned the leaps made in agricultural productivity since World War II, and the world's economies and population levels have become dependent on a continous supply of the element.
Unlike nitrogen, which can by synthesised from the air, or the use of renewable energy to substitute for fossil fuels, there is no substitute for phosphorus. All the world's phosphate fertilisers come from mined phosphate rock, making it a finite resource.
Various analyses suggest "peak phosphorus" - the point at which supply falls behind demand - will occur around 2040, with all currently known reserves potentially exhausted within 50 to 100 years.
However, University of Technology Sydney researchers Dana Cordell and Stuart White warn that for most countries, a phosphorus squeeze is likely to come much sooner.
Demand for phosphorus is growing in line with population growth, and is being pushed higher by greater consumption of meat in countries like China and India.
(Based on European practices, the Swedish Environmental Protection Agency estimates that a vegetable-based diet uses 0.6 kilograms of phosphorus per person per year, compared with 1.6 kg for a meat-based diet.)
Few nations have access to enough phosphorus to supply their own agricultural needs: in fact, most of the world's known phosphate reserves are controlled by China, the United States and Morocco.
China has the largest reported reserves, but in 2008, at the height of the food crisis, China's central government introduced a 135 per cent tariff on exports to protect domestic supply.
The US, historically the world's largest consumer, importer and exporter of phosphate fertilisers, is now thought to have only about 25 years of domestic phosphate reserves left. US fertiliser manufacturers are importing large quantities of phosphate from Morocco.
Morocco supplies more than a third of the world's phosphate, but it is an industry that stands on politically unstable ground. Much of Morocco's phosphate comes from the disputed territories of the Western Sahara, an activity that has been condemned by the United Nations.
Phosphorus may be in finite supply, and that supply politically uncertain, but Australia's agricultural and food systems remain highly inefficient users of the fertiliser.
Dana Cordell calculates that only two per cent of phosphate fertiliser applied in Australia is eaten in locally-consumed food.
Up to 75 per cent of P fertiliser is locked-up in agricultural soils. About 20 per cent of applied fertiliser is exported in farm produce, and a minor amount is leached into waterways, contributing to nutrient overload or carried out to sea.
One researcher has estimated that of the billion tonnes of phosphorus mined since 1950, about a quarter now lies in water bodies or landfills.
Each year, Ms Cordell says, humans eat about three million tonnes of phosphorus, and excrete close to 100 per cent of it.
In some countries, that has led to serious investigation of recycled urine as a source of agricultural phosphorus.
Urine is "essentially sterile", and contains nitrogen, phosphorus and potassium in the correct ratios for plant growth.
If all human urine was recycled, according to Jan-Olof Drangert of Linköping University in Sweden, where Dana Cordell also studies, it could supply half the phosphorus needs of the world's cereal crops.
Two Swedish municipalities have mandated that all new toilets must divert urine away from solid waste. The urine is collected in tanks either at the house, or in a communal collection point, and picked up once a year by farmers who use it as fertiliser.
However, in countries like Australia, with a large land area and a relatively small population, nutrient recycling can at best provide five per cent of the nation's phosphorus needs.
* Visit www.phosphorusfutures.net