Geoscience Reference
In-Depth Information
3.2 Phosphorus
Phosphorus is one of the inorganic macronutrients in all known forms of life.
Inorganic phosphorus in the form of phosphate (PO
4
3-
) plays a major role in vital
biological molecules, such as DNA and RNA. Living cells also utilize phosphate
to transport cellular energy via adenosine triphosphate (ATP). Phospholipids are
the main structural components of all cellular membranes. Calcium phosphate salts
are used by animals to stiffen their bones, and adequate P supplies are necessary
for seed and root formation.
The principal phosphorus forms in the subsurface are Ca-phosphate, adsorbed
phosphates, occluded phosphates, and organic phosphate (Lindsay
1979
; Mengel
1985
). The principal phosphate mineral is apatite (or fluorapatite), Ca
10
(PO
4
)
6
(OH,F)
2
. Secondary minerals include silica, silicates, and carbonates, generally as
calcite or dolomite. Usually, these minerals are utilized as a raw material source for
the production of phosphorus fertilizers. Many soils in their natural state are low in
readily available P. For example, typical levels of phosphorus in subsurface solu-
tions of unfertilized soils range from 0.001 to 0.1 mg/L P (Hook
1983
). Therefore,
intensive agricultural activity requires fertilization to achieve maximum possible
yields. Application of phosphorus to soil as fertilizer, manure, or effluents results in
an immediate rise in the concentration of utilizable phosphorus. It was once thought
that P was completely immobile in soil, and therefore, farmers were encouraged to
increase phosphate fertilizer application without fear that P applied in excess of crop
requirements would be lost from the soil profile.
Phosphorus loss from soils occurs mainly through crop removal, runoff, and
leaching. The uptake of phosphorus by agricultural crops, for example, varies
greatly (i.e., 10-100 kg/ha/yr). While subsurface pathways can be significant in P
transfer to water, especially in soils with low P retention properties or significant
preferential flow pathways, it is reasonably well established that in most water-
sheds, P export occurs mainly by overland flow. Soils that have been used heavily
for agricultural crops are often deficient in phosphorus, as are acidic sandy and
granitic soils. In landscaped urban soils, however, phosphorus is rarely deficient,
and the misapplication of this element can have serious repercussions on plants,
the soil environment, and adjoining watersheds.
In ecological terms, phosphorus often is a limiting nutrient in many environ-
ments. However, the result of phosphate overfertilization is leaf chlorosis. Phos-
phorus is known to compete with iron and manganese uptake by roots, and
deficiencies in these two metal micronutrients cause interveinal yellowing.
Moreover, it has been demonstrated experimentally that high levels of phosphorus
are detrimental to mycorrhizal health and lower the rate of mycorrhizal infection
of root systems (Manna et al.
2006
). This mutually beneficial relationship between
the fungus and the plant roots allows the plant to more effectively explore the soil
environment and extract needed nutrients. Often in aquatic systems, an excess of
phosphorus may cause eutrophication. Eutrophication has been linked to many
aspects of water quality degradation, including fish kills, loss of biodiversity and
