Agriculture Reference
In-Depth Information
Table 3.11
Forms of Phosphorus Fertilizer Suitable for Vineyards
Phosphorus
content (%)
Compound
Chemical formula
Comments
Single superphosphate
(SSP)
Ca(H
2
PO
4
)
2
.H
2
O,
CaSO
4
.2H
2
O
9
80%-90% water soluble;
remainder mostly soluble in
neutral ammonium citrate;
supplies P and S
Triple superphosphate
(TSP)
Ca(H
2
PO
4
)
2
.H
2
O
19-21
80% water soluble; 15%
citrate soluble; only traces
of S
Monoammonium
phosphate(MAP)
NH
4
H
2
PO
4
21-26
>90% water soluble, supplies
N and P; suitable for
fertigation
Diammonium
phosphate (DAP)
(NH
4
)
2
HPO
4
20-23
80% water soluble and 20%
citrate soluble, supplies
N and P; suitable for
fertigation
Rock phosphate
Ca
10
(PO
4
)
6
(F,OH)
2
with variable
SiO
2
, CaCO
3
,
and Fe, Al oxide
impurities
6-18
Water insoluble; citrate
solubility depends on
whether the rock is classed
as “soft” or “hard”
limestone, is useful for counteracting acidity in the subsoil caused by excess
exchangeable Al
3+
(see box 3.6). Gypsum is also used to improve soil struc-
ture, especially in sodic soils, as discussed in “Leaching, Salinity, and Sodicity
Control,” chapter 4.
Magnesiumiscontainedindolomite,alimestoneinwhichpartoftheCahasbeen
replacedbyMg.MagnesiumcanalsobesuppliedasEpsomsalts(MgSO
4
.7H
2
O),
which are soluble and can be applied as a foliar spray. Because many Australian soils
havehighsubsoilconcentrationsofexchangeableMg
2+
, an analysis of the 0- to 15-cm
depthonlymaybemisleading;petioleanalysisisthebestindicatorofMgdeiciency.
Table3.12showsthecommonMgandKfertilizersforvineyards.Although
potassium sulfate (K
2
SO
4
) is more expensive than potassium chloride (KCl),
it is preferred in salt-affected soils in which Cl toxicity is a potential problem.
Potassium sulfate, like KNO
3
, also has the advantage of supplying another major
nutrient. These three K compounds are soluble and can be applied by fertigation.
Sulfur
Sulfur as elemental sulfur (S) is supplied in sprays for powdery mildew. Sulfur
also occurs as the SO
4
2−
ion in fertilizers such as gypsum (14% S), single super-
phosphate (11% S), (NH
4
)
2
SO
4
(24% S), and K
2
SO
4
(18% S). Additional
sulfate-S comes from the atmosphere in rain or as dry deposition, especially in
