Agriculture Reference
In-Depth Information
There is on the one hand a
spatially
highly variable nitrate supply within the
field, yet in addition on the other hand also a
temporally
rather fast changing situ-
ation. At harvest time, the nitrate supply is rather low - probably because of the
previous uptake by the crop - and it is fairly uniform. Some weeks later in late
autumn, the nitrate content in some parts of the field is much higher. Any similarity
in the spatial pattern of the two maps is difficult to detect. The temporal increase in
the nitrate content is probably due to mineralization of crop residues. But this
increase varies very much spatially. This can result from locally changing amounts
of crop residues or from varying conditions for the decomposition. Nitrogen uptake
by bacteria that decompose straw may be important. In case much rainfall had taken
place in autumn, the nitrate content at the second date probably would have been
lower because of leaching. In short, the situation for soil nitrate N is very difficult to
predict, spatially as well as temporally.
An adequate response to the spatial problem is site-specific sensing. The best
solution for the temporal problem, however, is sensing and controlling in real-time.
Up to now, this is not possible with remote sensing. Hence doing this in a proximal
way from vehicles and in an on-the-go operational mode is the logical consequence.
Yet an important point is that using ion-selective electrodes allows for on-the-go
operations, but not in real-time. This is because presently all ion-selective elec-
trodes (Figs.
9.4
and
9.6
) rely on intermittent operations with an average time of
about 10 s between successive signals. And if a spreader or sprayer of farm chemi-
cals is to be controlled on-the-go with sensing during the same operation, the
time
span
before a signal has its effect on the distribution of the fertilizer is even longer
than these 10 s. Because the actuator - the device that steadily readjusts the spreader
or sprayer based on the signals - in addition needs a fraction of this time interval of
10 s to get effective. So the time interval of 10 s between the signals to get effective
can be taken as an absolute lower limit.
Yet spreaders or sprayers operate with travel speeds between 2 m/s (= 7.2 km/h)
and about 4 m/s (= 14.4 km/h). The minimum time interval of 10 s hence corre-
sponds to a distance in the direction of travel of 20-40 m. This range is much longer
than the conventional distance between the front of a tractor - where the samples
might be taken - and the distributing devices of spreaders or sprayers. Of course,
these limitations do not hold if mapping takes place and maps are used for control-
ling subsequent operations.
9.2.4
General Prerequisites and Prospects
The
first prerequisite
for sensing by ion-selective nutrients is that the respective
ion must exhibit an electric potential in a solution or in a slurry. This potential of
either cations or of anions depends - among others - on the well known
Hofmeister
series
, which ranks the ions according to their hydrophilic or conversely their
hydrophobic properties. Within the respective Hofmeister series for anions or for
cations, the nitrate anion as well as the hydrogen- and potassium cations are located
rather centrally. Thus these ions do not present unusual situations.
