Fibrophos Handbook 2015

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13
Phosphate in Fibrophos 3
The plant is only able to take up
phosphate from the soil in solution.
The amount of phosphate in
solution at any one time is small
due to its tendency to form
complex organic and inorganic
forms. There are therefore three
‘pools’ of phosphate in the soil:
1) Soluble phosphate.
2) Phosphate present in the labile pool
and adsorbed onto clay minerals.
3) Phosphate not readily available.
All three pools exist in equilibrium.
Inputs to the system tend to come
from fertiliser, rain, manures and
plant residues. These inputs distribute
the equilibrium temporarily.
Movement of phosphate to plant
roots via diffusion is some 100 times
slower than that of potash. In ideal
conditions, roots ‘explore’ more of
the soil giving them access to larger
amounts of phosphate. The ability of
the plant to perform can be restricted
when conditions are less favourable
such as poor soil structure, excess
water or low soil temperatures or
when the plant is small, soon after
germination. Low temperatures
compound the problem as the
ability of phosphate to diffuse
through the soil is also reduced
and plant growth can be slowed.
In addition, in high alkali or acid
grassland soils, access to phosphate
is further restricted by formation of
organic or inorganic compounds.
Typically, the phosphate in grazed
grassland accumulates in the top
5cmwith little downward movement
to the roots where it is needed.
For these reasons it is essential to
ensure that the soil’s reserves are
kept well ‘topped up’ and in reseeds
and arable situations, replacement
phosphate applications are well
incorporated in the soil.