Page 137 - Soil and water contamination, 2nd edition

124 Soil and Water Contamination

bound phosphorus to inorganic phosphates. The turnover rate of organic phosphorus is
rapid in conditions favourable for microorganisms. If the C: organic P ratio is about 200:1 or
smaller, phosphorus is readily mineralised and released into solution and becomes available
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for plant uptake . If the C: organic P ratio is larger than 300:1 (less than about 2 g kg P
in organic matter), the microorganisms use most of the phosphorus and immobilise it in
their cells instead of releasing it for plant uptake (Miller and Gardiner, 2008). Part of the
phosphorus, however, remains in organic form. Some of the organic phosphorus is present
in complex humus polymers, but most (about 60 percent) is present in the form of small
molecular compounds, such as inositol phosphates. Inositol phosphates are primarily
bacterial in origin and occur mainly in the form of insoluble Ca, Fe, and Al salts. They are
only slowly mineralised. In general, the soil organic phosphorus correlates well with organic
matter content and organic nitrogen , but the C: organic P ratio displays more variation than
the C:N ratio, because organic phosphorus is less associated with large humus polymers than
organic nitrogen (Whitehead, 2000).
As mentioned above, the availability of phosphorus is generally low, because of its affinity
with iron , aluminium , sesquioxides , and calcium . Phosphorus availability depends on soil
pH , redox conditions, and organic matter content . Phosphorus available for plant uptake is
greatest at pH values between 6 and 7, or under anaerobic conditions.

6.3.3 External sources and sinks

The major external input of phosphorus in agricultural soils is phosphorus in fertilisers
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(artificial fertilisers, manure , or sewage sludge ) and may amount up to 1000 kg P ha y .
Table 6.2 lists the main fertilisers and fertiliser constituents that contain phosphorus.
Artificial fertilisers are initially very soluble, but the phosphates are increasingly adsorbed,
precipitated, or immobilised by microorganisms. Fertiliser application may cause phosphorus
in the topsoil to build up substantially over many years, to up to 2–3 times the initial
phosphorus content.
Atmospheric deposition of phosphorus is usually negligible. Wet and dry phosphorus
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deposition generally vary between 0.2 and 1.5 kg P ha y . Atmospheric inputs of
phosphorus are mainly derived from suspended soil particulates eroded by wind, though
there may be a small contribution from the burning of plant materials and fossil fuels
(Whitehead, 2000).
The main phosphorus losses are due to crop harvest and grazing, which lead to a removal
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of 5 to 40 kg P ha y . However, if grazing is accompanied by supplementary feed, there is
usually a net phosphorus input from the excreta of the grazing animals. Because phosphorus
has a great affinity for soil minerals, leaching of phosphorus rarely occurs. Nevertheless,
phosphate leaching may occur on agricultural areas where fertiliser applications are so high
that the phosphate binding capacity becomes saturated. This phenomenon has been reported
in intensively used agricultural areas of north-western Europe. In the case of a shallow water
Table 6.3 Some main types of phosphate fertilisers (source: Whitehead, 2000; Miller and Gardiner, 2004).
Fertiliser Chemical formula %P Solubility
Superphosphate Ca(H 2 PO 4 ) 2 + CaSO 4 ⋅2H 2 O 8–9 High
Triple superphosphate Ca(H 2 PO 4 ) 2 20 High
Monoammonium phosphate NH 4 H 2 PO 4 26 High
Diammonium phosphate (NH 4 ) 2 HPO 4 23 High
Dicalcium phosphate dehydrate CaHPO 4 ⋅2H 2 O 18 Low
Tricalcium phosphate Ca 3 (PO 4 ) 2 12–16 Low

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