As Rock Phosphate Runs Out, What is More Important – Food Crops or Fuel Crops? – by Professor Chris Rhodes (Oil – June 21, 2012)

(Please note that this article was published in June 2012)

World rock phosphate production is set to peak by 2030. Since the material provides fertilizer for agriculture, the consequences are likely to be severe, and worsened by the increased production of biofuels, including those from algae.


The depletion of world rock phosphate reserves will restrict the amount of food that can be grown across the world, a situation that can only be compounded by the production of biofuels, including the potential large-scale generation of biodiesel from algae. The world population has risen to its present number of 7 billion in consequence of cheap fertilizers, pesticides and energy sources, particularly oil.

Almost all modern farming has been engineered to depend on phosphate fertilizers, and those made from natural gas, e.g. ammonium nitrate, and on oil to run farm machinery and to distribute the final produce. A peak in worldwide production of rock phosphate is expected by 2030, which lends fears over how much food the world will be able to grow in the future, against a rising number of mouths to feed. Consensus of opinion is that we are close to the peak in world oil production too.

Phosphorus is an essential element in all living things, along with nitrogen and potassium. These are known collectively as, P, N, K, to describe micronutrients that drive growth in all plants and animal species, including humans. Global demand for phosphate rock is predicted to rise at 2.3% per year, but this is likely to increase in order to produce crops for biofuel production. As a rider to this, if the transition is made to cellulosic ethanol production, more phosphorus will be required still since there is less of the plant (the “chaff”) available to return as plant rubble after the harvest, which is a traditional and natural provider of K and P to the soil.

World rock phosphate production amounts to around 140 million tonnes. In comparison, we would need 352 million tonnes of the mineral to grow sufficient algae to replace all the oil-derived fuels used in the world. The US produces less than 40 million tonnes of rock phosphate annually, but to become self-sufficient in algal diesel would require around 88 million tonnes of the mineral. Hence, for the US, security of fuel supply could not be met by algae-to-diesel production using even all its indigenous rock phosphate output, and significant further imports would be needed. This is in addition to the amount of the mineral necessary to maintain existing agriculture.

In principle, phosphate could be recycled from one batch of algae to the next, but how exactly this might be done remains a matter of some deliberation. e.g. The algae could be dried and burned, and the phosphate extracted from the resulting “ash”, or the algae could be converted to methane in a biodigester, releasing phosphate in the process. Clearly there are engineering and energy costs attendant to any and all such schemes and none has been adopted as yet.

Cleaning-up the Environment.

There is the further issue of the demand on freshwater, of which agriculture already struggles to secure enough to meet its needs, and in a sustainable picture of the future, supplies of water appear uncertain against the countenance of climate change. It is in the light of these considerations that algae/algal fuels have begun to look very appealing, especially given the claimed very high yields that can be obtained per hectare as compared say with rapeseed and biodiesel.

Conventional algae production can be combined with water clean-up strategies, to remove N and P from agricultural run-off water and sewage effluent, both to prevent eutrophication (nutrient build-up in water), which causes algal blooms, and to conserve the precious resource of phosphate. Algae might also be “fed” with CO2 from the smokestacks of power stations to reduce carbon emissions.

For the rest of this article, click here: