Agriculture Reference
In-Depth Information
Box 5.3
Atmospheric Inputs of Nitrogen by Processes Other Than N 2 Fixation
Various oxides of N (mainly NO and NO 2 ) are formed during the
combustion of fossil fuels in motor vehicles. Some are also formed by forest fires
and lightning discharges. The oxides react with hydroxyl free radicals in the air to
form nitric acid, which contributes to acid rain . NH 3 volatilizes from manures,
rotting vegetation, and soil, especially after the application of fertilizers such as
urea (section 5.4.1.3). Some NH 3 is absorbed through the stomata in plant leaves,
while the remainder is dissolved in rain or forms salts, such as ammonium sulfate
(NH 4 ) 2 SO 4 according to the reaction
2NH 3
H 2 SO 4
(NH 4 ) 2 SO 4 (B5.3.1)
Sulfuric acid (H 2 SO 4 ) is formed from S oxides (see box 5.5). (NH 4 ) 2 SO 4
forms an aerosol that is deposited on soil and plant surfaces as dry deposition
(which also includes any NH 3 and SO 2 uptake through leaf stomata). The total N
input from the atmosphere is 5-60 kg/ha/yr, depending on the extent of air
pollution.
the lowest density to 500 kg N/ha at densities of 5000/ha or more. The amount
of N removed in the fruit is between 1.2 and 2.4 kg N/t grapes, with the norm
being ca. 1.5 kg N/t.
N from the atmosphere enters the soil-plant system in several forms. Fixa-
tion of N 2 gas by legume cover crops is discussed in section 4.2.2.1. Other forms—
the oxides of N (NO x )—come down in rain or as dry deposition (box 5.3). The
average annual N input from the air in industrialized parts of Europe is 20-30
kg/ha, which can make a significant contribution to the N requirement of vines.
The other major N input to the soil is through fertilizer and manure, al-
though leguminous cover crops are becoming more important, as part of an over-
all drive to improve soil quality (chapter 7). Manures are discussed in section 5.7,
but the main forms of N fertilizers are discussed here.
Forms of N Fertilizers
The more important water-soluble N fertilizers used in viticulture are given in
table 5.6. Some of these, such as Nitram and urea, supply a single macronutrient
(N). Others, such as KNO 3 and (NH 4 ) 2 SO 4 , supply more than one macronutri-
ent and are called multinutrient fertilizers. Mixed fertilizers , which may be solid or
liquid, are made by mixing single or multinutrient fertilizers, and they are usually
identified by their N:P:K ratio. For example, the mixed fertilizer “Horticulture
Special” has a composition of 10-3-8, meaning 10% N, 3% P, and 8% K. In
the trade, fertilizer analyses are sometimes given as the ratio of N:P 2 O 5 :K 2 O. For
simplicity, in this topic analyses are given as percentages of the elements, unless
otherwise stated.
Slow-release fertilizers , such as ureaform (comprising ureaformaldehyde poly-
mers, 21-38% N), IBDU (isobutylidene diurea, 32 % N), and SCU (sulfur-coated
urea, 37-40% N), are also available. They are sparingly soluble N compounds,
developed to avoid the occurrence of high, localized concentrations of mineral N
in the soil, which predispose to N loss (section 5.4.1.2). Slow-release fertilizers in-
clude both synthetic and natural organic fertilizers (e.g., blood and bone, hoof,
5.4.1.1
 
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