Agriculture Reference
In-Depth Information
at a depth of 36-53 cm. Over all soils and depths in the Walker and Adams data set‚
organic phosphorus was highly correlated with sulphur (
r =
0.86) and nitrogen (
r
= 0.86).
However‚ there also appeared to be separate relationships between organic carbon and
phosphorus at different depths in the profile.
The individual components of the organic phosphorus fraction of soils are poorly
known; most (10-50
per cent
.) occurs as inositol phosphates‚ probably of microbial
origin while phospholipids and nucleic acids account for 1-5 and 0.2-2.5
per cent
.‚
respectively. As considered earlier‚ from 2-20
per cent.
of the organic phosphorus in soils
may be contained within the microbial biomass (Brookes
et al
.‚ 1984).
As indicated above‚ much of the inorganic phosphorus in soils is occluded in minerals
or occurs in insoluble forms unavailable to higher plants. The phosphorus available to
these plants appears to be largely taken up in inorganic form from the soil solution‚
although a small proportion is also taken up in low molecular weight organic forms.
The soil solution inorganic phosphorus is maintained by the mineralisation of organic
phosphorus from soil organic matter and decaying plant residues and through the turnover
of soil microbial populations. This may be augmented by the slow solubilisation of some
inorganic forms (McBride‚ 1994). However‚ in most soils‚ much of the inorganic
phosphorus added as fertiliser quickly becomes unavailable through fixation processes.
Phosphorus fixation refers to its removal from solution through chemisorption and
precipitation processes (McBride‚ 1994) and its subsequent inclusion in insoluble
inorganic forms‚ very largely unavailable to plants. The forms in which phosphorus is
fixed differ with pH‚ although the forms present at any pH value may overlap those
present at adjacent values. At very acid values‚ fixation mainly occurs through chemical
reaction with soluble Fe‚ Al and Mn while at slightly higher pH‚ fixation occurs on
the surfaces of the hydrous oxides of these elements. Solubility‚ and thus availability
to organisms‚ is greatest at slightly acid to neutral pH values while at alkaline values‚
phosphorus is increasingly fixed as calcium phosphates (Brady and Weil‚ 1996).
Certain forms of soil phosphorus may be preferentially available to some soil
micro-organisms over the higher plants. Fungi can grow towards sources of phosphorus
and can also utilise a wider range of forms including condensed phosphates‚ insoluble
phosphates and organic forms. Some fungi‚ together with certain bacteria and actino-
mycetes are able to solubilise otherwise insoluble phosphates through the secretion of
organic acids and phosphatase enzymes (Jennings‚ 1989‚ 1995).
The phosphorus in soils that is available to plants is particularly difficult to measure
in a direct way. A range of methods has been developed which involve leaching the soil
with various chemical extractants. While some of these may provide agronomically-
useful indexes of availability under restricted circumstances‚ the variety of chemical
forms of soil phosphorus‚ the different plant rooting strategies and the complexities
involved in defining the interactions with mycorrhizal and associated organisms suggest
that a simple universal chemical assay is unlikely to be found. Because of this‚ approaches
that determine foliar phosphorus concentrations (and sometimes those of other elements)
are often used‚ particularly in tree cropping situations.
