Agriculture Reference
In-Depth Information
skeletal tissues and
ca.
80
per cent.
of body phosphorus occurs in teeth and bones
(McDonald
et al
.‚ 1988). Deficiencies in phosphorus limit the productivities of many
biological systems and phosphalic fertilisers are widely applied to agricultural systems
to compensate for this.
Concentrations of phosphorus in the tissues of higher plants range up to more than
1 % of tissue dry weight (Bowen‚ 1979) although typical values are generally much
lower than this (Tables I.14‚ I.15). Phosphorus concentrations differ substantially
between species‚ between tissues within species and with organ development. Because
phosphorus is relatively mobile within the plant‚ it is most concentrated in the biologi-
cally-active tissues to which it is actively translocated from senescing leaves. Plant tissue
concentrations also depend on the readily plant-available phosphorus present in the soil.
Within the lower plants‚ uncontaminated bryophyte tissues contain 0.07-0.2
per cent.
phosphorus while bacteria may contain much higher concentrations‚ up to 7.7
per cent.
of dry weight in laboratory cultures (Anderson and Domsch‚ 1980). Field collected
fungi‚ principally Basidiomycetales‚ typically have lower concentrations ranging from
0.10 to 1.4
per cent.
of dry weight (Beever and Burns‚ 1980).
In the higher animals‚ phosphorus concentrations differ notably between hard and soft
tissues. Bone concentrations in man range from 6 to 7
per cent.
of dry weight while those
of a range of organs vary from 0.3 to 0.9
per cent.
(Bowen‚ 1979).
While phosphorus has no gaseous phase in its biogeochemical cycle‚ dry and wet
deposition from the atmosphere may be in the range 0.07 to 1.7 kg (Newman‚
1995). Thus‚ within an ecosystem‚ phosphorus must be supplied largely through organ-
ic matter decomposition‚ turnover of the microbial biomass‚ weathering of the parent
material or‚ in agro-ecosystems‚ as fertilisers. Therefore‚ the efficient cycling of phos-
phorus assumes great importance in natural ecosystems‚ particularly in older landscapes
where soils have been weathered to depths below the zone occupied by the roots.
Phosphorus plays a particularly important part in pedogenesis (Walker and Syers‚ 1976).
A wide range of phosphorus concentrations occurs in soils‚ depending principally
on the parent materials (Walker and Adams‚ 1958) and the age of the landscape (Walker
and Syers‚ 1976) or the degree of soil development (Cross and Schlesinger‚ 1995).
For example‚ soils formed from basic igneous rock parent materials usually contain
higher concentrations of phosphorus (Norrish and Rosser‚ 1983). A factor in long term
phosphorus retention in soils and the unavailability to plants of much of that present‚
is the occurrence of other elements with which phosphorus forms insoluble associations‚
notably iron‚ aluminium‚ manganese and calcium and other carbonates. The forms in
which inorganic phosphorus occurs in soils and their different availabilities depend
much on pH (see Figure I.35‚ a n d below) and the other minerals present in the soil;
Brady and Weil (1996) discusses this from an agronomic perspective. Plant-available
forms are principally
and
(Table I.14).
