Sensing of Crop Properties - Precision in Crop Farming - page 106

Fig. 6.4 The site-specific
significance of red and
infra-red reflection within a
heterogeneous field. The
black and gray shaded field is
bare along the lower side but
covered with plants in the
upper region . The soil line
extends within bare soil from
a wet area into a dry region.
The vegetation line goes from
bare soil into areas that are
progressively vegetated
(From Jensen 2007 , altered)

much biomass present

some biomass
present

dry bare soil

wet bare soil

red reflectance, dimensionless

result of varying water content, texture, organic matter content and iron content
(Fig. 5.23 ) and that the reflectance of vegetation too can vary (Fig. 6.3 ). Because in
most cases, the general effect of vegetation on the reflectance is much more pro-
nounced than the differences that result from varying soil- or plant properties. The
soil line and vegetation line concept as defined by the red- and near-infrared reflec-
tance in Fig. 6.4 result from this. However, the soil line and vegetation line concept
can only supply for a very rough survey in order to differentiate between bare
ground and plants.
A more detailed explanation about crop properties requires an analysis of the
canopy reflectance curve either while relying on a full spectrum approach or on the
basis of discrete wavebands . A full spectrum analysis might be reasonable if
sensing of several crop properties simultaneously is aimed at, e.g. biomass, water
and nutrients. The techniques that are needed for such multiple crop property sensing
are still in a developmental stage. This holds especially for the data processing. On
the other hand, discrete waveband sensing lends itself mainly for single or dual crop
property recording. Applications for this are in a feasible stage and dealt with in the
next section.

6.2 Defining the Reflectance by Indices

The easiest method to get information about vegetation is to create a simple ratio
of the near infrared to red reflectance, e.g. with centres of the respective wavebands
at 800 and 670 nm (Figs. 6.2 , 6.3 and 6.4 ). Such near infrared to red indices with
the formula RR 800 67/ can be used for differentiating between soil and vegeta-

tion within a field by “ green seeking ” in order to obtain spot spraying instead of