Agriculture Reference
In-Depth Information
Similar concepts of improving the quality of plant products by correcting an
uneven supply that is due to varying soil conditions via site-specific control of the
nitrogen supply are conceivable for many crops,
e.g.
potatoes, sugar-beets and
vegetables.
9.4.9.4
Control by a Sufficiency Index
This method for the control of site-specific nitrogen application via reflectance
sensing was - starting with maize - developed by Holland and Schepers (
2010
). It
is based on defining normalized reflectance indices by using a generalized plant
growth function. The premise is that the plant growth function depends on the nitro-
gen supply. The
normalized reflectance indices
are given by a
sufficiency index
that represents the ratio of the locally sensed reflectance index to the same index that
stands for plants of the same crop that have no nitrogen limitation. Hence:
locally sensed reflectanceindexofcrop
reflec
sufficiencyindex
=
tanceindexof non N limitedcrop
−
For suitable reflectance indices that can define the sufficiency index see Table
9.5
.
The corresponding
generalized plant growth function
is assumed to be a sec-
ond order polynomial with downward cavity, thus a quadratic function of the nitro-
gen that was applied (Fig.
9.38
). The basic concept is that the sufficiency index is a
surrogate for the ability of the crop to grow, thus for its vigor, and that the nitrogen
rate given should be related to it.
It is essential to differentiate between on the one hand the nitrogen supply of a
crop in the past that provides the sufficiency index (Fig.
9.38
) and on the other hand
the nitrogen rate that should be applied. Both nitrogen parameters need to be
detected on a site-specific basis. The first parameter, the nitrogen supply of the past,
is directly represented by the sufficiency index. The second parameter, the nitrogen
to be applied, must be derived from the sufficiency index.
Holland and Schepers (
2010
) provide a mathematical deduction of the nitrogen
rate that should be applied. The result of this mathematical deduction is that the site-
specific nitrogen rate to be applied can be described by a rather simple equation:
1
sufficiencyindex
delta sufficiencyindex
Nitrogenrateinkgper ha to be appliedN C
appl
,
=
The term in the numerator, (1 - sufficiency index), is represented by the respec-
tive vertical distance in the green area of Fig.
9.38
for a given
ex ante
nitrogen sup-
ply. And the delta of the sufficiency index is the vertical difference between the
intercept of the plant growth function and maximum of the latter. The term C stands
for factors such as
e.g.
nitrogen from mineralization of soil organic matter and
organic fertilizers or from previous legumes. This term can also take into account
the fact that the agronomic optimal nitrogen rate in practice should be replaced by
