Water quality is threatened by a long history of fertiliser use on land.

A study led by researchers quantifies for the first time the maximum amount of nutrients specifically, phosphorus that can accumulate in a watershed before additional pollution is discharged into downriver ecosystems. That average threshold amount is 2.1 tonnes per square kilometre of land, Beyond this, further phosphorus inputs to watersheds cause a significant acceleration of loss in runoff.Phosphorus, an element in fertiliser, is essential to the growth of plant food. But the mineral is also harmful when overused. When it gets into surface water, it can lead to excessive plant growth in lakes and rivers and proliferation of toxic algae, harmful to human and animal health.

The two main sources of phosphorus to watersheds, the land adjacent to tributaries, come from agriculture (fertilisers and animal manure) and from the human population (through food needs and sewage).In some countries, including China, Canada, and the US, phosphorus is so heavily used now that the saturation point is reached in as little as five years.Nutrient management strategies developed using novel creative approaches are urgently required for the long-term sustainability of water resources.

One possible mitigating measure would be to do what is already being done in some European countries, instead of adding more and more to help plants grow, phosphorus can be recycled and reused as fertiliser rather than accessing more of the raw mined material.