Actually, this should be
titled "Phosphate Fertilizers" since phosphorus
(P) is
commonly available only as a part of the compound with
oxygen called phosphate which has the chemical formula of
P2O5 .
One of the challenges of phosphorus
fertilizers is their
level of water solubility. The way nutrients are
absorbed by plants is when they are in water solution.
So, the more soluble the nutrient, the easier it is to
move it into the plant. However, solubility also means
that the nutrient is likely to leach through the soil
faster also. Of the three nutrients commonly found in
commercial fertilizers,
nitrogen
is the most water
soluble, followed by
potassium
(potash) with phosphorus
(phosphate) being the least soluble.
Phosphorus is strongly
attracted to clay particles in the soil. Each year, only
a small percentage of the phosphorus applied will be
used during that season. For this reason, it will build
up over time in such soils so periodic
soil tests should
be taken. After several years of adding phosphorus to
the soil, you may find that you do not have to use any
for several years.