Agriculture Reference
In-Depth Information
The positional lag error for
spraying
of liquid fertilizers (not shown in Fig.
9.35
)
corresponds approximately to the situation for centrifugal spreaders. This is because
for sprayers the time-lags or response-times are on a low level as well and hence any
positive positional lag errors that exist can be removed by adjusting the control
devices for longer response times.
9.4.9
Sensed Signals and the Control of Nitrogen Application
9.4.9.1
Agronomic Background
The site-specific signals that indicate the crop properties cannot be used directly for
the control of the nitrogen application. These signals just provide information about
the chlorophyll content of the leaves, the biomass or the leaf-area-index. The
property-substitutes must be converted into fertilizer application rates.
In early
growing stages
of crops, the general consensus is that when crop prop-
erties indicate a low nitrogen supply, the application rate should be increased and
vice-versa
. With sensing by reflectance indices this means that when
e.g.
the red
edge inflection point or the red edge ratio (Table
9.5
) go up, the application rate
should go down. This basic relation is rather easily implemented into the control-
algorithm, however, there is much more needed. The site-specific indications that
modern crop sensors supply do not at all spare the need for a detailed agronomic
knowledge about the reasonable use of nitrogen fertilizer by taking into account
• the crop species and varieties
• the soil properties
• the growth stages
• the water supply
• the effects of rotations, crop-residues, manure and nitrogen-mineralization
• the use of the product (food, feed, fiber
etc
.)
• the costs of fertilizers and product prices.
The list is not complete and just should demonstrate that about every situation in
a field is a unique one. In order to simplify the search for the best application rate,
it has been proposed to use integrating factors. One such
integrating factor
is the
final crop yield. Several of the items listed above affect the crop yield. And if this
yield were known precisely at the time of fertilizing, taking this into account would
simplify the control needs definitely.
The problem is that the final crop yield cannot be predicted precisely enough at
the time of fertilizing. This is because the final crop yield is affected by the weather
in the growth stages that still lie ahead. The uncertainties in the weather-forecasting
beyond a few days do not allow a precise prediction of the expected final yield.
So in view of the abundance of factors that have to be considered, it might be
asked how generally the rates for in-season nitrogen application are defined. The
usual procedure in practical farming is that the farmer or his consultant inspects and
