Peak Phosphorus

You’ve heard of peak oil. You haven’t? Peak oil is just another way of saying that the rate at which we are extracting fossil fuels such as oil, coal and gas is much, much faster that the rate at which they are replaced. It took many millions of years to produce the large deposits that we have in just a couple of centuries significantly depleted. They will run out. The only question is when. Peak oil is the moment when the global production of oil starts to decrease. Year on year reductions until there is none left. Time for solar power. Or nuclear. Or wind. Or something else.

Just like oil, we are currently consuming mineral phosphorus at a much greater rate than its natural replenishment. In fact, given that we get most of our phosphourus from rocks which take geological timescales to renew, we can assume that it doesn’t get renewed at all. And so at some point we will reach peak phosphorus. This is actually something of a big deal. Potentialy much more so than peak oil. Why?

Phosphorus is present in all cells in all forms of life because part of the backbone of DNA is phosphorus. You can’t make DNA without phosphorus. We get our phosphorus by eating plants that have drawn up phosphorus through their roots or by eating animals that ate plants. Or expensive tablets. Many plants do just fine by consuming the natural levels of phosphorus in the soil. Modern intensive farming quickly sucks up phosphorus which needs to be continually replaced. We can’t do much about the amount of sunlight a field gets, but we can significantly alter how much water is delivered via irrigation and sprinklers. We use pesticides to kill bugs that would eat the crops and we spray weed killer to reduce competition from other plant species. The rapid industrialisation of agriculture is known as the Green Revolution and it seems clear that if there hadn’t been the significant increase in yields from the 1960s onwards then there would have been serious risk of millions, perhaps billions of people starving. However, all these measures to improve crop yields will be worthless if there isn’t enough phosphorus in the soil. This is an example of Leiblig’s Law of the Minimum. Think of a leaky bucket that has lots of holes all over it. If you want to raise the water level, then you need to plug up the lowest holes. If you keep growing high yield crops on land with intensive farming practices then you are going to come up against phosphorus limitation. Until you plug that hole, yields will not increase and in fact will dramatically decline.

Farmers never had this problem in the past did they? Well they did, but they solved it in different ways. First, they fertilized their fields with phosphorus by using animal waste. Manure. Horse shit. Or cow. Pig or chicken. Manure will have nitrogen, phosphorus and other goodies that plants greedily gobble up. Farmers would also rotate the types of crops grown on a particular field as well as not grow anything on a particular field for a year. This form crop rotation has been successfully used since Roman times. There are many good things about crop rotation. But in terms of short term yields, intensive farming of monoculture (growing the same crop all the time) wins.

One way of understanding why it wins is because we make up for the greater inefficiency of the high intensive system. Putting on fertilizers, irrigation, pesticides – all this requires large amounts of energy. Think of how much energy it takes to dig up rocks that have mineral phosphorus in it. Then you grind up that rock, sieve and filter it. Chemically process it. Put it in a truck or ship or sometimes both and transport it potentially many miles. Load it onto a spreader and tow it behind a tractor so that it finally gets onto a field. Digging up stored energy (fossil fuels) allows us to dig up phosphorus and put that on fields in order to increase the amount of food we can grow.

If or rather when phosphorus runs out we will either have to eat less (something difficult to do as the global population will continue to expand until at least the middle of the century) or decrease the amounts lost from the system by increasing the amount of phosphorus that is recycled. Recyling phosphorus from human and animal waste and water runoff will take energy. Most probably a lot of energy as there will need to be the deployment of significant amounts of new infrastructure. We currently have the energy sources for this, but will we when phosphorus scarcity really starts to bite. When will that be? Depends on who you ask. Upper estimates of mineral phosphoros resources are about 300 hundred years. Lower estimates for reserves are decades. I think it’s fair to say no one really knows. Also, much hinges on what exactly do you mean by ‘runs out’. The only thing that seems certain is that limitations in phosophorus supply will decrease the supply and increase the cost of food.

And here’s the double whammy. Some estimates give a date of peak phosphorus around the middle of this century which is when global population will reach its maximum of some 8-9 billion. Just when we have the greatest number of mouths to feed, our reserves of easy to obtain, low cost mineral phosphorus may start to run out. The worse case scenario is that many people will starve. Avoiding that outcome will require more recycling and more efficient farming practices. Getting up and running on that will require energy. Where will that energy come from in the middle of the century?

Fossil fuels are a limited supply you know. Have you heard about peak oil?

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