Phosphate: staid and steady but critical

Phosphorus is the eleventh most common element in the Earth’s crust, although P levels in most rocks are low. Agricultural supplies of P come from soil pools, from manures and other organic sources, from manufactured fertilizers and from industrial and urban wastes. In the past, bone meal was also an important source of P, but this is not effective on all soils and, depending on the origins of the bones, can meet with societal resistance.

With regard to P losses, most P transported from soil to water is in eroded soil particles to which P is adsorbed. Residual organic forms of P from manure are free to move with soil water, and they can be leached. Phosphate leaching is only a problem on soils that are well supplied or oversupplied with P, especially where they have inadequate capacity to immobilize P. Maintenance of good soil cover is the best protection against such losses. Since P movements are almost exclusively physical, the impacts of P on the environment and effective responses are often different from those that concern nitrogen.

P overenrichment of surface waters can upset the ecosystem balance as P is often the limiting nutrient in aquatic ecosystems, which are frequently sensitive to much lower levels of P than terrestrial systems are. The P involved may come from crop or livestock agriculture, wilderness lands, urban wastes or industrial effluents. However, the effects of P lost from agriculture must be considered relative to its bioavailability. The immediate bioavailability of P associated with soil particles may be small, whereas phosphorus lost from livestock production is more likely to be in solution.

Different sources and forms of phosphorus mean that decision makers must craft effective responses to the particular case of each watershed. Possibilities include:

• Adapting agricultural practices with relation to specific farming systems, soil types and hydrology to reduce specific hotspots, taking into account whether there are permanent features fostering P losses or temporary contributing factors;
• Addressing industrial effluents, which are often “point” sources and therefore fairly easy to contain;
• Removing nutrients from urban wastes, although few sewage treatment plants, even in developed countries, include tertiary treatment to remove P from the effluent.