Agriculture Reference
In-Depth Information
amount and timing of fertilizer N are well matched to plant demand, the fraction
of fertilizer N remaining in
mineral
form in the soil when the vines shut down in
autumn should be small. Any NO
3
remaining is vulnerable to leaching during
the winter. Some of the fertilizer N is also converted into labile organic N forms,
which are readily mineralized the following spring. In some German vineyards
where fertilizer inputs have been very high, there is evidence of unacceptably high
N losses (
100 kg N/ha/yr) by leaching to groundwater (Schaller 1991). The en-
vironmental impact of high N losses is discussed in chapter 7.
Phosphorus and Sulfur
The amount of P and S removed per tonne of harvested grapes averages about 0.3
and 0.2 kg, respectively. P is important for metabolic activity (ATP synthesis) and
healthy bud development; S is important for synthesis of some of the amino acids.
In soils with adequate organic matter (
2% C), mineralization of organic
forms of P and S provides a major input, as discussed in section 4.4. P concen-
trations in rain are very low, and more P is deposited by dry deposition (mainly
dust during dry weather). The rate of deposition is ca. 0.1-0.5 kg P/ha/yr. The
amount of S deposited from the atmosphere is much higher, from
5 to 70
kg/ha/yr. Atmospheric inputs of S are discussed in box 5.5.
5.4.2
5.4.2.1
Forms of P Fertilizers
Phosphate fertilizers consist of water-soluble
orthophosphates
and polymerized or-
thophosphates, called
polyphosphates
, and water-insoluble
mineral
and
organic phos-
phates
. The natural rock phosphates, consisting of minerals of the apatite type with
CaCO
3
, SiO
2
, and other impurities, are the raw material from which water-
soluble P fertilizers are made by treatment with acid. Traditionally, H
2
SO
4
was
used to produce
single superphosphate
(
SSP
), but now phosphoric acid (H
3
PO
4
)
Box 5.5
Forms of S Input from the Atmosphere
Sulfur is emitted into the air as SO
2
from the burning of fossil fuels. Near
coasts, small amounts of methyl sulfides and H
2
S are released from marine
sediments. Several processes are involved in the deposition of atmospheric S on soil
and vegetation:
•SO
2
is directly absorbed through leaf stomata.
•SO
2
is oxidized to sulfur trioxide (SO
3
), with the SO
3
reacting with water to
form sulfuric acid (H
2
SO
4
).
• H
2
SO
4
dissolves in rain (hence the term
acid rain
), or is neutralized by NH
3
gas to form (NH
4
)
2
SO
4
, which is dissolved in rain, or is deposited “dry” (box
5.3). The NH
4
ion contributes to soil acidification through nitrification.
Because of the adverse effect of acid deposition on natural ecosystems,
industry has been required to reduce S emissions from fossil fuels. Over northwest
Europe, for example, total S emissions have fallen by ca. 50% since 1970.
Currrently, atmospheric inputs are in the range of 10-30 kg S/ha/yr and are likely
to fall to 5-10 kg S/ha/yr. The contribution of S to the total acid deposition from
the air has fallen from two-thirds to one-half.
