Agriculture Reference
In-Depth Information
Forms of P Fertilizer Used in Vineyards
Table 5.7
Compound
Formula
P Content (%)
Comments
Single superphosphate (SSP)
Ca(H
2
PO
4
)
2
.H
2
O
9
Made from phosphate rock
plus CaSO
4
.2H
2
O
by reaction with H
2
SO
4
;
80-90% water soluble
Triple superphosphate (TSP)
Ca(H
2
PO
4
)
2
.H
2
O
19-21
Made from phosphate rock
plus traces of S
by reaction with H
3
PO
4
;
80% water soluble
Monoammonium phosphate
NH
4
H
2
PO
4
21-26
Very high water solubility,
(MAP)
supplies N and P;
suitable for fertigation
Diammonium phosphate (DAP)
(NH
4
)
2
HPO
4
20-23
High water solubility,
supplies N and P;
suitable for fertigation
Phosphate rock
Ca
10
(PO
4
)
6
F
2
with
6-18
Insoluble mineral fluorapatite;
variable SiO
2
, CaCO
3
P content varies with
and sesquioxide
the source
impurities
Bone meal and guano
Mainly apatite
5-13
Bone meal is an animal
product; guano is
consolidated bird
droppings.
is more widely used to produce high-analysis P fertilizers (e.g.,
triple superphos-
phate
, TSP). Forms of P fertilizer and their P contents are given in table 5.7.
Reactions of Water-soluble P Fertilizers in the Soil
. The active constituent of
SSP and TSP is
monocalcium phosphate
(MCP), with the formula Ca(H
2
PO
4
)
2
.H
2
O
(table 5.7). When granules of these fertilizers are placed in soil, they take up wa-
ter until a saturated solution forms. In dry soil, this solution is very acid (pH
1.5) and concentrated in P (ca. 4 mols/L) and Ca (ca. 1.4 mols/L); in wetter soil,
the solution is less concentrated. At this very low pH, soil minerals around
the granule dissolve, releasing SiO
2
, Al, Fe, and Mn ions. These ions, together
with Ca and P from the granule, are subsequently precipitated as complex new
compounds—the
soil-fertilizer reaction products
—as the pH slowly rises. In moist
soil, granules can dissolve in 24-36 hours, leaving the reaction products and a
residue of insoluble dicalcium phosphate (CaHPO
4
) containing about 20% of the
original P. The conversion of water-soluble fertilizer P to less soluble compounds
in the soil contributes to the process of “P fixation,” whereby P becomes less avail-
able to plants.
There are two important points to note about SSP and TSP dissolution in
soil:
1. Any localized acidity is neutralized by the reaction with soil minerals, so
water-soluble P fertilizers are not inherently acidifying.
2. The speed of the reaction means that a plant feeds not so much on the
fertilizer itself, but on the fertilizer reaction products. These are metastable
compounds that revert slowly to more stable (but less soluble) products,
such as amorphous AlPO
4
and FePO
4
in acid soils or octacalcium
phosphate and hydroxyapatite in neutral to alkaline soils.
