Site-Specific Fertilizing - Precision in Crop Farming - page 206

9.3 Fertilizing Based on Reflectance of Soils

9.3.1 General Remarks

Using signals that are derived from optical reflectance and not from the
electrochemical potential in soils has distinct advantages for real-time control of
fertilizing. Because these signals are almost immediately available . The reflec-
tance signals are transferred from the soil to a sensor with the speed of the light.
Instead of a signal interval of about 10 s, as with ion-selective electrodes, several
signals within 1 s can easily be recorded by a sensor. In practice, some time for
processing of the signal and for actuating the control of a spreader or a sprayer must
be considered. Yet the operating speed of modern computers and actuators is such
that process control within real-time for site-specific fertilizing or spraying is fea-
sible and for some applications already is state of the art. This is an important crite-
rion when the situation for the supply of crops varies not only spatially but temporally
as well. A prime example for this is the supply of crops with nitrogen, which
requires rather fast control responses.
However, the situation for sensing via reflectance in case of soil nutrients is not
the same as with natural soil properties such as texture (sand, clay, loam), organic
matter and water. This is - among others - because the respective amounts differ
substantially. In rough gravimetric terms, the constituents that stand for the natural
soil properties mentioned above account for many tons per ha, whereas the macro-
nutrients at stake can easily be expressed in kg per ha. This is at least the situation
if the macronutrients that are available to crops are considered. With micronutrients,
the amounts required might be even below 1 kg per ha.
The coefficients of determination between optical reflectances and various soil
properties in Table 9.2 are based on full spectra regression analyses of soil samples
collected either in several areas of the United States (Chang et al. 2001 ; Lee et al. 2009 )
or in the Zheijang province of China (He et al. 2007 ). The natural soil properties listed
are rather well correlated to the spectral data. This is in line with some results in Sect.
5.3 . About the same applies to the properties that are related to soil pH. But most of the
soil nutrients listed in Table 9.2 show a rather poor correlation to the reflectance.
An important point in this respect is, which soil constituents can be regarded as
nutrients that are available for plants. A prerequisite is that the respective nutrients
are either solvable in water or at least in a chemical extractant (Table 9.2 ) that
approximately removes from soil samples the plant available nutrients and hence is
used in the laboratories as a standard.
The total N in soils cannot be regarded as being plant available since generally a
large part of it is fixed in organic matter. The mineralization of this part can take
years or even decades. Most of the mineral N is taken up by crops as nitrate ions,
however, plants may also absorb ammonium ions. Up to now, soil sensing methods
concentrate on recording the nitrate ions.
The rather discouraging results for spectroscopic sensing of soil nutrients as
presented in Table 9.2 should not be regarded as a proof of perpetual failure.