Phosphorous does not exist in soils in its
simple elemental form. Phosphorous
exists as inorganic or organic, with organic being chemically bonded to a
carbon compound.
Ortho
A single
phosphate molecule is called orthophosphate. Orthophosphate is a negatively
charged anion and is the form of phosphorus absorbed through the root and taken
up by the plant.
Poly
“Poly”
refers to multiple phosphate molecules linked in a chain. Each linkage of
phosphate molecules has a name depending on its length, although polyphosphate
is the general term that includes all of these linked molecules, no matter what
length. The size or length of the chain is not fixed and can be of various
lengths. Thus “polyphosphate” is simply strings of “orthophosphate” ions of
various lengths.
This
phosphorous form is made utilising Ammonia products, therefore a valuable
amount of nitrogen is supplied and the end product is referred to as Ammonium
Polyphosphate ( APP). The most common APP fertilizer is
16-23-0. Liquid 16-23-0 contains 65 to 70% polyphosphate and 30 to 35%
orthophosphate. When polyphosphate is applied to the soil, moisture will add
water back (hydrolyse) to the polyphosphate to form orthophosphate over time.
Polyphosphates
are not absorbed by plant roots.
Why is APP helpful in P management?
The
complexity (long chains of molecules) of the polyphosphate provides a barrier
to the typical fixing and immobilisation of phosphorous that we see in soils.
Polyphosphate
fertilizers offer the advantage of a high nutrient content in a clear,
crystal-free fluid that is stable under a wide temperature range and has a long
storage life. Some small amounts of other nutrients may be mixed with
polyphosphate fertilizers, making it a good carrier for micronutrients that may
be needed by plants
Fluids are
convenient for farmers since they can be easily blended with many other
nutrients and chemicals and each drop of fluid is exactly the same. For most
situations, the decision to use dry or fluid fertilizers is based on the price
of nutrients, fertilizer handling preferences, and field practices.
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