Closing the ring of nutrients — the science and society of nitrogen, phosphorus and the organic cycle

Food is also a vehicle that carries nutrients. The nitrogen and phosphorus drawn up from the soil of the fields change shape into rice and vegetables, are carried to the city, pass through our bodies and, for the most part, flow out as sewage. In the instant a toilet is flushed, nutrients that came from a field enter the drainpipe and, by way of a treatment plant, head for the river and the sea. On the farm the soil grows lean; at the river mouth nutrients run to excess — the same elements, short in one place and overabundant in the other. This is the one-way street of nutrients now running between the modern city and its farmland. The flow that once described a ring back to the fields has become, today, almost a single straight line.

This one-way street is hard to notice precisely because it is invisible. We take note of food loss and kitchen scraps, but we do not usually register the quantity of nutrients dissolving away into the sewers. And yet what underpins the world's food production is exactly these nutrients — nitrogen and phosphorus — and they are not inexhaustible. One is made from air at the cost of vast amounts of energy; the other is a finite resource dug from a limited number of mines. That is why the pattern in which nutrients reach a dead end in the city and are thrown away is not merely wasteful. This column begins, first, with the science of those two elements — nitrogen and phosphorus.

Nutrients failing to return to the fields is a problem with two faces. One is the resource side. To make up for the nutrients that crops carry off, we synthesise nitrogen fertiliser in factories and keep digging phosphate ore from mines. Manufacturing nitrogen fertiliser demands a great deal of energy, and phosphate rock is a finite resource whose reserves are concentrated in a few countries. As long as we keep topping up by mining rather than closing the ring, the supply remains a tightrope walk. The other is the pollution side. The nitrogen and phosphorus that escape from fields and sewers into waterways become, there, an excess of nutrition, breeding explosive blooms of algae and oxygen-poor 'dead zones'. The US Environmental Protection Agency, too, explains that excess nitrogen and phosphorus cause harmful algal blooms and a lack of oxygen in the water.

The same nutrient, then, is short in the field and overflowing in the water. This asymmetry is the clearest possible proof that the ring has been cut. In a natural ecosystem, fallen leaves, carcasses and excreta are all broken down and returned to the soil, becoming the nutrients of the next life; matter travels in a circle, with no dead ends. Human cities, however, have become a device that cuts this circle open somewhere and pushes nutrients in a single direction. The key to the problem is to reconnect the broken circle — to prevent, to return, and to keep it turning. In what follows we first trace, along the flow of nutrients, exactly where that circle is cut.