Why the future for phosphates lies in recycling

Why the future for phosphates lies in recycling

Phosphorus is the eleventh most common element on Earth, essential to all living organisms. In particular, alongside nitrogen, it is one of the main plant nutrients. In nature, phosphorus always occurs combined with oxygen and other elements, forming phosphates.

In the past, the elements necessary for plant and animal growth were recycled in primitive agricultural communities. Crops were consumed by animals and man close to their place of production. The resulting animal and human manures, as well as crop wastes, were then applied to cultivated land, returning the nutrients to the soil.»

The need to restore the phosphate cycle

Phosphorus input into natural systems comes only from the weathering of certain rocks and is comparatively scarce. Phosphorus is therefore easily depleted in soils and the sustainability of traditional agriculture depended on respecting the phosphorus cycle.   The growth of cities and the intensification of farming have broken this nutrient cycle. Human wastes (containing the nutrients) are concentrated in urban sewage and, where this sewage is treated, various factors mitigate against spreading of the resulting sludges on agricultural land. These factors include geographical concentration of sludge production (resulting in long transport to farmland, and therefore costs) and  contamination of sludges with physical and chemical pollutants present in urban waste waters. Similar problems apply to animal wastes in the case of concentrated, intensive livestock production (pigs, chickens ...). In order to obtain high crop yields and to produce enough food for growing world populations, intensive modern agriculture requires large quantities of nutrients in a form readily available to plants : mineral fertilisers.   Phosphate mine at Khouribga, Morocco

Around 80% of phosphates produced by the worlds industry today are used in fertilisers, with a further 5% being used to supplement animal feeds. These phosphates are manufactured from phosphate-containing rock mined from deposits in several countries. Around 140 million tonnes of phosphate rock are extracted each year across the world. Traditional P-Cycle: in the past phosphorus was recycled back to the land by local agriculture The phosphorus cycle has thus largely been replaced by a linear throughput system: phosphates are extracted from a non-renewable resource (phosphate rock), pass through crops, animals and man, and end up either in landfill (of raw or incinerated sewage sludge) or in rivers and the sea (if sewage or animal wastes are not adequately treated). Comparatively little is restored to agricultural land. Phosphates from fertilisers and manure may also build up in agricultural soil, in some circumstances towards or beyond saturation levels. Part of the applied phosphates may tend to run off into surface water rather than being retained in the soil and crops.

Modern society's throughput P-system: consumption of a non-renewable resource