Agriculture Reference
In-Depth Information
Table 9.8
Efficiency of mineral nitrogen use expressed in kg grain per kg N
Total mineral N
used in kg per ha
Efficiency of min. N use
in kg grain per kg N
Mean yield
in t/ha
Crop
Unif. rate
Var. rate
Unif. rate
Var. rate
Griepentrog and Kyhn (
2000
)
, experiments in Schleswig-Holstein, Germany
W. wheat
11.0
201
149
54
74
W. barley
9.5
191
147
50
64
W. wheat
10.8
206
131
53
81
Havrankova et al. (
2008
)
, experiments in Bedfordshire, United Kingdom
W. wheat
7.5
221
205
34
37
W. wheat
7.3
221
205
33
36
Thoele and Ehlert (
2010
)
, mean of 13 experiments in Eastern Germany
W. cereals 6.2 156 138 43 49
Average efficiency of mineral N use 44.5 56.8
The results are based on experiments in which the yield level with both site-specific and uniform
application of nitrogen is approximately the same. However, the site-specific variable rate applica-
tion is limited to the second dressing and does not apply to any first dressing
nitrogen fertilizer. Thriwakala et al. (
1998
) dealt with this question on the basis of a
simulation model for whole fields with varying fertility. The latter was defined on
the basis of the
a priori
given nitrogen supply,
e.g.
from mineralization of organic
matter. The result was that for whole fields with a high mean fertility the site-speciic
application control should be programmed for higher yield averages. And for fields
with a low fertility it should be targeted for lower mean fertilizer expenditures. The
problem is that the fertility as defined above does not only depend on soil properties
but on weather as well.
The
efficiency of nitrogen use
is a very important criterion for the environmen-
tal impact of plant production. This is not only because inefficient nitrogen use
causes higher fertilizer costs per unit of the product. In humid areas, inefficient
nitrogen use by crops also results in more nitrate that seeps into groundwaters
(Schepers et al.
1995
; Schepers
2008
). This side-effect of nitrogen fertilization has
generated the most serious environmental problem that is caused by agrochemicals,
namely too high nitrate contents in drinking waters. In the human digestive system,
most nitrate is reduced to nitrite. This holds especially for babies. The nitrite in turn
lowers the ability of the red blood cells to carry oxygen. So a serious disease due to
the lack of oxygen may develop, called “blue baby syndrome” or methemoglobin-
emia. As a safeguard, government directives have set
limits for drinking water
in
large parts of the world. For many countries, the limit is 50 mg of NO
3
−
ions per one
liter of water, which too meets the proposal by the World Health Organization of the
United Nations (WHO
2006
). The USA set the limit at 10 mg nitrate N per one liter
of water (EPA
2011
). This corresponds to 45 mg of NO
3
−
ions per liter water. So
there is rather good agreement in the objective.
However, the actual situation is that despite small recent improvements, these
limits quite often are exceeded (Nolan and Stoner
2000
; Sprague et al.
2009
,
