Agriculture Reference
In-Depth Information
cross-section of a soil, which e.g. is disclosed by a cultivating tool. Whatever the
situation is, an important difference to volume sensing is that the signals are based
on two dimensions and not on three dimensions in distance. And this generally
means that - ceteris paribus - the signals come from less soil and hence might be
less representative.
Yet what matters really: is it the amount of soil or is it the respective geometrical
place within the soil?
When it comes to sensing of water , the depth of the water carrying soil layer
from the surface is important for crop growth. Sensing the top-surface of a field does
not supply any indication about the depth where a soil stores water. Immediately
after a short rain, just 2 mm of the top-surface may be wetted. Following a longterm
precipitation, 100 mm down from the top-surface might store water. Yet the signals
from the top-surface might be the same. So these signals hardly help.
However, the situation for water sensing can be different if instead of the hori-
zontal top-surface a vertical cross-section within the soil is scanned. This requires
some scraping or plowing aside of soil that can be done during soil cultivation or
during sowing. Thus an adequate sensing perspective for the control of sowing-,
cultivation- or irrigation operations might be created.
It can be relevant that the topsoil is occasionally mixed when cultivations take
place. Because of this mixing, the soil constituents that respectively are at the sur-
face will vary. The soil constituents change their position within the cultivation
depth. So even if only constituents that just were at the surface were recorded during
the mapping, previous mixing within the topsoil provides actually for some volume
sensing. This is especially the case when the sensing of the same field is repeated
several times in the course of some years and thus maps of the means are obtained.
These maps of the means then are estimates of the situation within the volume of the
cultivated topsoil although the sensing occurred in two-dimensional patterns.
However, this cultivation induced change from top-surface dimensions to volume
dimensions does not make sense with transient soil properties such as water content.
This method solely is well suited for soil properties that not at all or hardly all
change over time. Thus obtainable maps of the means could be reasonable for tex-
ture, organic matter content and cation-exchange-capacity.
The situation is different when site-specific signals about soil conditions for the
application of pre-emergence herbicides are needed. The application rate for these
herbicides should be adapted to the site-specific organic matter content of the soil
top-surface. Because the pre-emergence herbicides are partly absorbed by the
organic matter of the soil surface and this makes them ineffective. In order to make
up for the growing absorbance with increasing organic matter content, it is generally
recommended to adapt the rates of application to this (Fig. 5.22 ).
This does not necessarily imply a higher burden to the environment. Because higher
organic matter contents in soils also result in faster decomposition of herbicides (Hance
1973 ). So if a higher herbicide rate is absorbed, it is also faster decomposed.
Because these herbicides mainly get in contact with the top-surface of the soil,
on-the-go information about the organic matter within this top-layer is precisely
what is needed for controlling the application.
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