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Phosphate in Fibrophos
3
Phosphate Availability
in Fibrophos
For convenience, availability of
phosphate in fertilisers is typically
indicated by the solubility not just
in water but in other solutions
prepared in laboratories to simulate
soil conditions around the root hairs.
Neutral ammonium citrate and 2%
citric acid are common examples.
It has been commonly thought that
water soluble phosphate is the most
efficient and cost effective way to
apply phosphate to growing crops.
This information has been widely
preached by the major fertiliser
manufacturers who have been keen to
promote their water soluble products.
While it is certainly true that water
soluble phosphate is readily available
to the plant when it is first applied
and is vital when indices are very
low, the truth is the phosphate very
rapidly forms compounds which
render the phosphate less available
within a very short space of time.
This process begins as soon as
phosphate is applied. Some will
revert from the soluble form to the
labile pool and some to the non-
labile pool. Note that some of the
phosphate in the non-labile pool
can remain locked there for many,
many years.
Unless soil phosphate levels are
found to be below index 1 (England
and Wales), there seems to be little
justification in paying a premium
for water soluble phosphate when
cheaper forms of phosphate such
as Fibrophos are equally beneficial
to the plant.
‘It is inefficient to apply large
applications of water-soluble
phosphate at one time. If this is
done, the phosphate that is not
taken up by the crop will move
into the labile and then non-labile
forms so losing its availability.’
Dr Ian Richards,
Soil Agronomist
It has been estimated by the
Macaulay Land Use Research
Institute that in broad terms, while
only 15% of applied water soluble
phosphate will be available in the
first year of application, only 1-2%
will be available in subsequent
years. Work at ADAS Bridgets has
suggested that the higher the level
of P in the soil, the greater the rate
of fixation to an unavailable form.
These immobile reserves can
become mobile by the activity
of microbes in the soil and these
microbes are motivated by good
balanced healthy soil encouraged
by the correct nutrient balance of
trace elements and well structured
and aerated soils. The comparative
success of calcified seaweeds
and the impressive results from
Fibrophos trials support this theory.