Principal forms of phosphorus
Phosphorus is the 11th most abundant element on Earth and like nitrogen it is essential for all plant and animal growth. It is part of DNA and every cell’s energy transfer. Phosphorus is also a major constituent of teeth and bones. Phosphorus differs from nitrogen in that it is highly reactive and many of its compounds are not readily soluble. Only a very small fraction of phosphorus occurs in soils as a solution since most is strongly bound to soil particles or the soil organic matter. It enters water primarily in combination with soil particles where it becomes available for plant growth (eutrophication).
Phosphorus is not found free in the environment but is widespread as a compound with different minerals. Phosphate is the naturally occurring form of phosphorus. Mineral fertilisers account for approximately 80% of phosphates used worldwide. Detergents account for 12%, animal feeds 5% and special applications, such as food and metal treatment 3%. The annual global production of phosphate is around 17 million tonnes derived from roughly 140 million tons of rock concentrate. More than 75% of the globally-available phosphate is surface mined, with the remainder obtained by underground mining. Because there is a finite amount of phosphate available (estimated at 150-250 years at the current rate of extraction) there is increasing interest in obtaining phosphate from other sources such as recovery from animal wastes and sewage sludge. A considerable amount of research has been conducted in recent years to develop economic methods of phosphate recovery from both wastewater and animal wastes.
The seriousness of the phosphate crisis means that stewardship of the essential element should be a consideration in all policy making. Isaac Asimov summarized the importance of P:
“…life can multiply until all the phosphorus is gone, and then there is an inexorable halt which nothing can prevent … We may be able to substitute nuclear power for coal and plastics for wood, and yeast for meat, and friendliness for isolation – but for phosphorus there is neither substitute nor replacement”. Asimov, I. (1974) “Asimov on chemistry” Doubleday Company, New York
More on this topic can be found at:
Sources of phosphorus
Diffuse sources of phosphorus include phosphorus attached to soil particles in surface water runoff, in animal slurries and particulate and soluble phosphorus in water flowing in field drains and ditches. Agriculture contributes about 50% of the phosphorus entering surface waters and may be the main source in some areas. The other major source of phosphorus is the treated water recovered by sewage treatment works. As improvements are made to the quality of sewage effluent discharges the relative contribution of diffuse agricultural sources of phosphorus increases in importance. Agricultural runoff only occurs in wet weather when dilution is greatest.
Animal wastes and treated sewage sludge are good sources of phosphorus (and nitrogen and potassium) and their use as fertilisers both saves the farmer money and is a good use of available resources. It completes the phosphorus nutrient-cycle by returning back to the soil the phosphorus taken from the soil by crops. Phosphorus from sewage sludge primarily comes from food wastes, urine and excreta, and detergents. Adults excrete 98% of the P in their diets because they are turning over cells, rather than laying down new ones.
Phosphorus in soils exists in organic and inorganic forms. Organic phosphorus exists in un-decomposed plant residues, microbes, and soil organic matter. Inorganic phosphorus is usually associated with aluminium (Al), iron (Fe) and calcium (Ca) compounds of varying solubility and availability to plants. Phosphorus can be rapidly fixed into relatively insoluble forms within soil and therefore be unavailable to plants. The extent to which this happens depends on soil pH and type including its Al, Fe, and Ca content. When phosphorus is applied in fertilisers as soluble compounds most of it is adsorbed by the soil. This sorbed phosphorus is in equilibrium with the soil solution, so when some phosphorus is taken up by plants, a small amount is desorbed into solution. Also specialised fungi extend out from the roots and extract phosphorus from the soil matrix and transport it back to the roots. Unlike nitrogen, most of the phosphorus that is lost from soil is associated with particles of soil (whereas nitrogen is lost in solution or as gas).
The term agricultural runoff encompasses two processes that occur in the field—surface runoff and subsurface flow. However, these can be interrelated processes where surface or overland flow infiltrates into soil during movement down a slope and then moves laterally through the soil as interflow reappearing as surface flow further down slope.
The loss of phosphorus in agricultural runoff occurs in sediment-bound and to a much smaller extent in dissolved forms. Sediment phosphorus includes phosphorus associated with soil particles and organic material eroded during rainfall and flood events and constitutes the vast majority of phosphorus transported in surface runoff from most cultivated land. Phosphorus can also be readily transported in sandy, acid organic or peaty soils which have low phosphorus fixation or holding capacities and in soils where the preferential flow of water can occur rapidly through macro-pores and fields drains. In contrast, surface runoff from grass, woodland, and uncultivated land carries little sediment and therefore is generally dominated by dissolved phosphorus. This is a clue to preventing sediment-bound phosphorus entering watercourses by planting vegetated margins (often called buffer zones) adjacent to water to filter out and trap soil particles. Although the phosphorus transported in sediment is generally not readily available to plants, it can be a long-term source of phosphorus for the aquatic environment where soil erosion transports soil particles into lakes, rivers and the marine environment.
Ways of reducing phosphorus losses
Ways of reducing phosphorus losses from land include:
- Using phosphorus fertiliser and manures according to the results of soil testing and nutrient balance assessments of inputs and off-takes. Soil analysis every fourth year is cost-effective and helps the nutrient balance to be maintained.
- Preventing soil erosion by:
- Growing cover crops in winter
- Improving water infiltration to soil, by improving soil infiltration capacity
- Improving soil water holding capacity
- Adopting minimum tillage systems, especially during risk periods, namely autumn
- Maintaining grass buffer zones/riparian zones on field boundaries
- Installing farm track sediment traps
- Moving gateways away from points of drainage
- Maintaining environmentally sustainable embankment of ditches or streams
- Fencing water courses, ponds and lakes to prevent animals from dunging in them, which they do if they drink from them.
In addition, animal wastes and manures are usually applied to meet crop nutrient requirements and to avoid leaching losses of excess nitrogen into water. This is because nitrogen is usually the primary growth-limiting nutrient and the Nitrates Directive controls the rate of application. However, applying manure to meet crop nitrogen requirements typically adds more phosphorus than the crop requires, contributing to the gradual build up of the amount of phosphorus in soil.
Low or no tillage systems, where there is little turning of the soil between crops, can help reduce erosion. However, these systems can also result in the build up of phosphorus in the surface layers of soil through manure addition, fertiliser application and crop residue decomposition, which then is more likely to cause pollution where erosion occurs.
To read more about nitrogen and phosphorus processes see:
Defra phosphorus programme:
Agricultural Phosphorus and Eutrophication , Second Edition. US Department of Agriculture:
Land use for achieving ‘good ecological status’ of waterbodies in England and Wales: a theoretical exploration for nitrogen and phosphorus:
Can Constructed Wetlands Reduce the Diffuse Phosphorus Loads to Eutrophic Water in Cold Temperate Regions?
Read more about optimising the use of fertilisers and manures:
PLANET is a newly developed, computerised version of Defra’s industry standard ‘Fertiliser Recommendations (RB209)’ book. It provides farmers and advisers with a quick and easy way of obtaining RB209 recommendations for arable, horticultural or grassland crops in each field, each year, taking account of the crop nutrient requirement as well as the nutrients supplied from organic manures, soil and fertilisers.
MANNER is a decision support system that can be used to accurately predict the fertiliser nitrogen value of organic manures on a field specific basis.
Phosphate concentrations in UK rivers in 1995-2008